comparative biogeography of the arid lands of central mexico

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LSU Doctoral Dissertations Graduate School

2011

Comparative biogeography of the arid lands of central Mexico Comparative biogeography of the arid lands of central Mexico

Jesus Abraham Fernandez Louisiana State University and Agricultural and Mechanical College

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COMPARATIVE BIOGEOGRAPHY OF THE ARID LANDS OF CENTRAL MEXICO

A Dissertation

Submitted to the Graduate Faculty of the

Louisiana State University and Agricultural and Mechanical College

in partial fulfillment of the requirements for the degree of

Doctor of Philosophy

in

The Department of Biological Sciences

by Jesús Abraham Fernández Fernández

Licenciado en Biología, Universidad Autónoma de Tlaxcala, 2000 Maestría en Ciencias, Universidad Nacional Autónoma de México, 2006

December 2011

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DEDICATION

This work is dedicated to the memories of my family and friends: Julia Mendoza

(R.I.P.), Carmelo Fernández (R.I.P.), Heriberta Fernández (R.I.P.), Esperanza Fernández,

Carmen Fernández, Virginia Fernández, Adriana Fernández, Pastor Fernández (R.I.P.),

Raúl Fernández (R.I.P.), Erasmo Fernández and Miguel Fernández (R.I.P.); Federico

Bahena Romero (R.I.P.); Federico García (R.I.P.), Cinthia Campusano, and Florencia

García.

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ACKNOWLEDGEMENTS

I want to thank my Advisor Mark Hafner, for all his help and guidance. Mark

gave me all the patience, time and training necessary to succed in my field of research.

The members of my committee, Chris Austin, Robb Brumfield, Chris Green, and Mike

Hellberg were unvaluable providing advise, and always having time to give a word of

support.

The day I went to the field with the Hafner brothers and collaborators, I knew my

life would be different, I want to express my gratitude to Mark, Darcey, John and Dave

Hafner, and Jessica Light and Brett Riddle for all their help and support since the days in

Mexico, to the last of the Louisiana days.

I’ve had the pleasure of making friends and interact on a daily basis with great

staff, faculty, and graduate students in the Museum of Natural Science and the

Department of Biological Sciences, special thanks to Steve Cardiff, Prosanta

Chakrabarty, Elizabeth Derryberry, Donna Dittman, Tammie Jackson, Prissy Milligan,

Tom Moore, Fred Sheldon, Adrienne Steele, Richard Stevens, and Bill Wischusen.

The following persons kindly donated tissues and specimens for use in this study:

Joe Cook, University of New Mexico, Museum of Southwestern Biology; Roxana Acosta

and Juan Morrone, Museo de Zoología “Alfonso L. Herrera,” Facultad de Ciencias,

Universidad Nacional Autónoma de México (U.N.A.M.); Fernando Cervantes,Yolanda

Hortelano, and Julieta Vargas of the Colección Nacional de Mamíferos, Instituto de

Biología, UNAM; Mark Hafner and Jessica Light of the Louisiana State University,

Museum of Natural Science; Brett Riddle and Stacy Mantooth, University of Nevada, Las

Vegas; Dave Hafner, New Mexico Museum of Natural History; Duke Rogers, Monty L.

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Bean Life Science Museum, Brigham Young University; José Ramírez-Pulido and Noé.

González, Universidad Autónoma Metropolitana-Iztapalapa; Juan Carlos López Vidal,

Colección de Mamíferos, Escuela Nacional de Ciencias Biológicas, Instituto Politécnico

Nacional; María del Carmen Corona V., Departamento de Agrobiología, Universidad

Autónoma de Tlaxcala; Robert Baker and Heath Garner, Museum of Texas Tech

University; Fred Stangl, Midwestern State University; Veronica Rosas of Universidad de

Guadalajara; and Robert Fisher, National Museum of Natural History.

The following persons kindly helped with laboratory protocols: Marjorie Matocq

and C. R. Feldman; L. Márquez V., Automatic Sequencer Facility, Instituto de Biología,

U.N.A.M., Posgrado de Ciencias Biológicas, U.N.A.M.; and Roxana Acosta, Jorge

Falcón, and Juan Carlos Windfield kindly helped during fieldwork in México.

Thanks to Fernando Cervantes, Dave Hafner, and Mark Hafner, and B. Lim for

their critical evaluations of manuscripts., and the anonymous reviewers for their critical

evaluation of manuscripts.

Thank you very much to all the graduate students that generously gave their

friendship, this tie will last for the rest of our lives: Nati Aristizabal, Gustavo Bravo, Reid

Brennan, Clare Brown, Laura Brown, Tony Chow, Santiago Claramunt, Andres Cuervo,

Adriana Dantin, Valerie Derouen, Sandra Galeano, Amber Gates, Dency Gawin, Richard

Gibbons, Heather and Nathan Jackson, Mike Harvey, Sarah Hird, Dan Lane, James

Maley, Verity Mathis, John McCormack, Caleb McMahan, John McVay, Fabi Mendoza

Luciano Naka, Lori Patrick, Whitney Pilcher, Carlos Prada, Noha Reid, Eric Rittmeyer,

Maria Sagot, César Sánchez, Sebas Tello, Yalma Vargas and Amanda Zellmer.

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TABLE OF CONTENTS

DEDICATION ............................................................................................................... ii ACKNOWLEDGEMENTS .......................................................................................... iii LIST OF TABLES ....................................................................................................... vii LIST OF FIGURES .................................................................................................... viii ABSTRACT .................................................................................................................... x CHAPTER 1. INTRODUCTION ..................................................................................... 1 1.1 Literature Cited ........................................................................................10 CHAPTER 2. MOLECULAR PHYLOGENETICS AND BIOGEOGRAPHY OF A RARE

ENDEMIC: NELSON’S WOODRAT (NEOTOMA LEUCODON NELSONI) IN THE

ORIENTAL BASIN OF MEXICO. 2.1 Introduction ........................................................................................... 16 2.2 Materials and Methods .......................................................................... 19 2.3 Results ................................................................................................... 24 2.4 Discussion ............................................................................................. 29 2.5 Literature Cited ..................................................................................... 36 CHAPTER 3. PHYLOGENETICS AND BIOGEOGRAPHY OF THE MICROENDEMIC

RODENT XEROSPERMOPHILUS PEROTENSIS (PEROTE GROUND SQUIRREL) IN THE ORIENTAL BASIN OF MEXICO. 3.1 Introduction............................................................................................ 43 3.2 Materials and Methods .......................................................................... 47 3.3 Results ................................................................................................... 51 3.4 Discussion ............................................................................................. 56 3.5 Literature Cited ..................................................................................... 59 CHAPTER 4. MOLECULAR PHYLOGENETICS AND TEMPORAL CONTEXT FOR THE DIVERSIFICATION OF THE SOUTHERN ROCK DEERMOUSE, PEROMYSCUS

DIFFICILIS (RODENTIA: CRICETIDAE). 4.1 Introduction ........................................................................................... 64 4.2 Materials and Methods .......................................................................... 67 4.3 Results ................................................................................................... 71 4.4 Discussion ............................................................................................. 74 4.5 Literature Cited ..................................................................................... 81 CHAPTER 5. MOLECULAR SYSTEMATICS AND BIOGEOGRAPHY OF THE

MEXICAN ENDEMIC KANGAROO RAT, DIPODOMYS PHILLIPSII

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(RODENTIA: HETEROMYIDAE). 5.1 Introduction ........................................................................................... 88 5.2 Materials and Methods .......................................................................... 91 5.3 Results ................................................................................................... 97 5.4 Discussion ........................................................................................... 102 5.5 Literature Cited ................................................................................... 109 CHAPTER 6. CONCLUSIONS ................................................................................ 116 APPENDIX 3.1 ........................................................................................................... 122 APPENDIX 4.1 ........................................................................................................... 127 APPENDIX 4.2 ........................................................................................................... 131 APPENDIX 5.1 ........................................................................................................... 134 APPENDIX 5.2 ........................................................................................................... 140 VITA ........................................................................................................................... 142

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LIST OF TABLES

Table 2.1.—Cytochrome-b sequence divergence values (Kimura 2-parameter model) between and within species of the Neotoma micropus and N. floridana species group ........................................................................................................... 26 Table 3.1.—Mean percent cytochrome-b sequence divergence values (Kimura 2-parameter model) among species of the genus Xerospermophilus and Cynomys .................................................................................................................. 53 Table 4.1.—Percent sequence divergence in the cytochrome-b gene (Kimura 2-parameter model) among 8 taxa of Peromyscus, including the 5 currently recognized subspecies of P. difficilis ...................................................................... 72 Table 5.1.—Mean percent cytochrome-b sequence divergence values (Kimura 2-parameter model) and ranges (in parentheses) between and within the 2 major clades of Dipodomys phillipsii, D. elator, and D. merriami ................................. 101 Table 6.1.— Overview of taxonomic conclusion, divergence values between clades of interest, and mean estimates of divergence time for clades of interest (in million of years) ................................................................................................................ 121

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LIST OF FIGURES

Fig. 1.1.—Map of Mexico showing main geographic features discussed in this dissertation ....................................................................................................... 3 Fig. 1.2.—Map of Central Mexico showing the Oriental Basin and surrounding volcanoes .......................................................................................................... 5 Fig. 2.1.—Geographical distribution of the Neotoma micropus species group ................17 Fig. 2.2.—Map of central Mexico showing the known geographic distribution of Nelson’s woodrat, Neotoma leucodon nelsoni) ............................................. 20 Fig. 2.3.—Cladogram showing the placement of Nelson’s woodrat, N. leucodon

nelsoni, within the genus Neotoma ................................................................. 27 Fig. 2.4.—Cladogram showing placement of Nelson’s woodrat, N. leucodon nelsoni,

within the genus Neotoma .............................................................................. 28 Fig. 2.5.—Phylogeny and chronogram of selected species of the genus Neotoma (and

outgroups), and estimated divergence times calculated from *BEAST analysis ........................................................................................................... 30 Fig. 3.1.—Geographical distribution of pygmy ground squirrels of the genus

Xerospermophilus .......................................................................................... 44 Fig. 3.2.—Relationships among major clades of Xerospermophilis based on maximum

likelihood and Bayesian analyses of Cytochrome-b sequences ..................... 52 Fig. 3.3.—Relationships among major clades of Xerospermophilus based on a maximum

likelihood analysis of Cytb, 12S, GHR, and IRBP sequences ....................... 55 Fig. 4.1.—Geographical distribution of Peromyscus difficilis and P. nasutus in Mexico

and the United States ..................................................................................... 65 Fig. 4.2.—Phylogenetic relationships among the 5 subspecies of P. difficilis, P. nasutus,

and allied taxa based on a maximum likelihood (ML) analysis of cytochrome-b sequences .................................................................................................... 73

Fig. 4.3.—Phylogenetic relationships among selected specimens of P. difficilis and P.

nasutus based on a partitioned maximum likelihood (ML) analysis of 2 mitochondrial genes and 2 nuclear genes ...................................................... 75

Fig. 4.4.—Estimates of divergence times in P. difficilis, P. felipensis, and close relatives calculated from *BEAST analysis ................................................... 78

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Fig. 5.1.—Map of central Mexico showing the geographic distribution of Phillips’ kangaroo rat, Dipodomys phillipsii ................................................................ 89

Fig. 5.2.—Phylogram showing placement of Phillips’ kangaroo rat, D. phillipsii, within the genus Dipodomys ........................................................................... 98 Fig. 5.3.—Phylogram showing placement of Phillips’ kangaroo rat, D. phillipsii, within

allied species of Dipodomys ......................................................................... 100

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ABSTRACT

Most biogeographic studies on the Mexican biota have assumed that the dramatic

climate cycles of the Pleistocene epoch and the prominence of the Trans-Mexico

Volcanic Belt (TMVB) have played major roles in the origin and diversification of

species. Here I studied the pylogenetics and biogeography of four codistributed rodent

species. In each case, I have generated a phylogenetic hypothesis for the taxon and allied

species using two mitochondrial (Cytochrome-b and 12S), and two nuclear genes (GHR

and IRBP), I recommended appropriate taxonomic changes, and generated a temporal

framework to identify events that may have produced the phylogenetic pattern.

Nelson’s woodrat Neotoma nelsoni and the Perote ground squirrel

Xerospermophilus perotensis, were confirmed as having their closest relatives in the

Mexican Plateau. My findings also confirmed that N. nelsoni and X. perotensis are

genetically well-differentiated from their sister taxa. Genetic distances in combination

with low levels of morphological differentiation suggest that they should be recognized

only at the subspecific level as N. leucodon nelsoni and X. spilosoma perotensis.

Molecular estimates of divergence times suggested that N. l. nelsoni and X. s. perotensis

diverged from their sister taxa to the north during early Pleistocene times.

The rock mouse Peromyscus difficilis was divided into two well-supported clades,

a northern clade including the subspecies P. d. difficilis and P. d. petricola, and a

southern clade containing the subspecies amplus, felipensis, and saxicola. Molecular-

based estimates of divergence times suggested that separation of these clades occurred in

the Pleistocene.

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My study of the Phillips’ kangaroo rat, Dipodomys phillipsii, revealed a

biogeographic pattern different from that seen for other taxa. D. phillipsii was divided

into two well-supported clades: one distributed on the Mexican Plateau, and a southern

clade in the TMVB. Several lines of evidence supported my decision to return the

Mexican Plateau clade of D. phillipsii to full species status as D. ornatus. My study

showed that D. phillipsii, D. ornatus, D. elator, and D. merriami form a well-supported

clade of kangaroo rats, but I was unable to resolve relationships among these four

species. My molecular-based analyses of divergence times suggests that D. phillipsii, D.

ornatus, D. elator, and D. merriami diverged in mid-Pliocene times, probably in or near

the Mexican Plateau. Unlike the Pleistocene divergence dates reported in previous

chapters this Pliocence divergence suggests that the morphotectonic processes that gave

rise to the Trans-Mexico Volcanic Belt may have influenced early diversification in

Mexican species of Dipodomys.

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CHAPTER 1

INTRODUCTION

Some of the oldest questions in biology focus on the origin of biological diversity.

How do new species evolve? How do geography, geology, and climate influence the rate

and timing of biological diversification? Do geographic barriers and environmental

fluctuations affect all organisms equally? One of the disciplines that addresses these

kinds of questions is biogeography. Biogeography is the study of the causes of and

limitations on the geographic distribution of organisms through space and time (Nelson

and Platnick 1981).

From a classical point of view, biogeography has been split into the subfields of

historical biogeography and ecological biogeography. Historical biogeographers study

the effect of past geographic phenomena on the distributional patterns and diversification

of species, usually at or above the species level. In contrast, ecological biogeographers

focus on the effects of climate fluctuations on the early diversification (incipient

speciation) of populations, emphasizing demographic parameters at and below the

species level (Nelson and Platnick 1981).

Populations of a species are not distributed evenly in space and time, and their

distributional ranges can expand or contract directly or indirectly by environmental

fluctuations, geological events, and other changes in a dynamic landscape. Through long

periods of time, those changes may result in disjunct populations, and the combined

affects of natural selection and genetic drift may result in differentiation between these

isolated populations and their source populations. When differentiated populations of

multiple species are exclusive to a particular geographic area, that area is known as an

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area of endemism. Other definitions of “areas of endemism” focus on the area’s

geographic delimitation by natural barriers or the distributional congruence of several

species (Harold and Mooi 1994; Hausdorf 2002; Platnick 1991). The concept of

endemism is central to biogeography and biological conservation and areas with high

numbers of endemic species, or “biodiversity hotspots,” commonly are included in

protected area networks (Myers et al. 2000).

México is widely known as a biologically megadiverse country and a biodiversity

hotspot (Lamoreaux et al. 2006). The diverse Mexican biota is the historical product of

complex interactions between an ever-changing topography and myriad ecological and

environmental factors (Velasco de Leon et al. 2007). In central Mexico, the Mexican

Plateau and the highlands and arid valleys of the Trans-México Volcanic Belt (TMVB)

are home to one of the most diverse biotas in the world (Fig. 1.1; Cartron et al. 2005;

Luna et al. 2007).

The Mexican Plateau is bounded by the Sierra Madre Occidental to the west, the

Sierra Madre Oriental to the east, the Chihuahuan desert and the Sierra Zacatecas to the

north, and the TMVB to the south (Fig. 1.1). It has the form of a parallelogram covering

85,300 km2, with elevations ranging from 1,000 to 3,300 m. The climate varies from arid

and hot to semiarid and temperate, and the vegetation is similar to that of nearby deserts,

with xeric shrublands in the plains and pine-oak forests in the surrounding mountains

(Ferrusquía-Villafranca et al. 2005). Recent studies of rodent populations in the Mexican

Plateau have revealed complex patterns of diversification and endemism (Fernández et al.

2012; Neiswenter and Riddle 2010).

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Fig. 1.1.—Map of Mexico showing main geographic features discussed in this dissertation (modified from a map produced by CONABIO (Comisión Nacional para el Conocimiento y Uso de la Biodiversidad; http://www.conabio.gob.mx/).

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Among the mountains at the southeastern edge of the TMVB lies the Oriental

Basin (Cuenca Oriental; Fig. 1.2). By any definition of “area of endemism,” this

semiarid, endorheic (closed drainage) basin, which covers portions of the states of

Puebla, Tlaxcala, and Veracruz, is an important area of endemism for arid-adapted

organisms in North America. This small (ca. 5,000 km2) basin characterized by alkaline

grasslands, bunch grasses, and aridland scrubs in the valleys and coniferous forests in the

surrounding mountains (Valdéz and Ceballos 1997), is thought to be the southernmost

extension of the Chihuahuan desert (Shreve 1942). The Oriental Basin supports several

endemic relict taxa of plants and animals, including at least four endemic mammals: the

Oriental Basin pocket gopher (Cratogeomys fulvescens), Nelson’s woodrat (N. nelsoni),

the Perote deermouse (Peromyscus bullatus), and the Perote ground squirrel

(Xerospermophilus perotensis; Best and Ceballos 1995; González-Ruíz and Álvarez-

Castaneda 2005; González-Ruíz et al. 2006; Hafner et al. 2005).

Past studies of biogeography and areas of endemism in Mexico have used a wide

variety of approaches. Some researchers have made biogeographical inferences based on

examination of distributional records analyzed with clustering algorithms, whereas others

have used geographic information system (GIS) algorithms, often augmented by

phylogenetic and phylogeographic information. These latter approaches allow the

researcher to infer evolutionary relationships, estimate current and potential distributions,

and model the potential effects of climatic and geological changes on the generation of

diversity (Escalante et al. 2002, 2004; Marshall and Liebherr 2000).

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Fig. 1.2—Map of Central Mexico showing the Oriental Basin and surrounding volcanoes: A) Iztaccíhuatl; B) Popocatépetl; C) Malinche; D) Pico de Orizaba; E) Cofre de Perote (modified from CONABIO maps). The Oriental Basin straddles the Mexican states of Puebla, Tlaxcala, and Veracruz.

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Distribution-based studies.—Over the past 20 years, many studies of the Mexican

flora and fauna have used panbiogeographic (or track) methodology. Panbiogeography

emphasizes the spatial or geographic dimension of biodiversity and often is used in

combination with Parsimony Analyses of Endemicity (PAE) and GIS (Balleza et al 2005;

Escalante et al 2002, 2005; Luna et al 2004). Panbiogeography is a phenetic approach

that uses knowledge of animal and plant distributions to draw inferences about the

patterns and processes responsible for centers of diversity or areas of endemism (Craw et

al. 1999). Many Mexican organisms have been studied using this approach, including

aquatic organisms, insects, plants, mammals, and a combination of taxa from different

groups (Andrés-Hernández et al. 2006; Corona and Morrone 2005; Corona et al. 2007;

Escalante et al. 2003, 2004, 2007; Huidrobo et al. 2006; Katinas et al. 2004; Morrone

2004; Morrone and Escalante 2002; Morrone and Gutierrez 2005; Morrone and Marquez

2001; Torres and Luna 2006).

Cladistic biogeography.—Many researchers have studied the biogeography of

Mexican organisms using cladistic biogeographic methods. These methods first

reconstructs the phylogenetic relationships of multiple sets of taxa, then use this

information to infer the relationships among the areas they occupy. Cladistic

biogeography often is used in combination with Brooks Parsimony Analysis (BPA), an

approach that has been used to study fishes, birds, or combinations of several taxa

(Domínguez et al. 2006; Marshall and Liebherr 2000; Zink et al. 2000).

Phylogenetic studies.—Most systematic studies of Mexican taxa attempt to relate

the phylogenetic patterns revealed in the study to current or past geographic barriers and

climate. The major findings of this approach include discovery of geographic clades

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congruent with historical and cryptic geographic barriers in lineages of birds and

mammals (Demastes et al. 2003; Edwards et al. 2001; Edwards and Bradley 2002;

García-Moreno et al. 2004; Hafner et al. 2004, 2005; Peppers and Bradley 2000; Peppers

et al. 2002).

Phylogeographic studies.—Studies relating the genetic architecture and spatial

distribution of conspecific populations or closely related species are increasingly frequent

in the literature (Avise 2009). Most phylogeographic studies describe population

parameters, haplotype distributions related to geographic distance, patterns of expansion,

retraction, and migration among populations, but also study species limits, timing of

diversification, and the impact of vicariance and dispersal on phylogeographic patterns.

Examples of the phylogeographic approach include many studies of vertebrate

organisms, including fishes and amphibians (e.g., Doadrio and Domínguez 2004;

Mulcahy and Mendelson 2000).

Comparative biogeography and phylogeography.—This relatively new approach,

which involves the study of codistributed taxa, has been applied to few Mexican species

to date. The main objective of this approach is to explore the effects of climate and

geology on codistributed taxa to reveal common patterns and processes that may explain

a shared evolutionary history (Avise 2000; Arbogast and Kenagy 2001). This approach

has been used in studies of birds, fishes, mammals, and a mixed group of species in the

TMVB and in the Chihuahuan, Sonoran, and Peninsular deserts of Mexico (Marshall and

Lienherr 2000; Mateos 2005; Riddle and Hafner 2004; Zink 2002; Zink et al. 2000). This

approach also has been implemented in conjunction with approximate Bayesian

computation in studies of birds and pitvipers and colubrid snakes in the southern Mexican

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highlands, the Peninsular Desert, and the Isthmus of Tehuantepec (Barber and Klicka

2010; Castoe et al. 2009; Daza et al. 2010; Leache et al. 2007).

My dissertation research uses the comparative phylogeographic approach to

determine if a set of codistributed mammalian taxa show the same phylogenetic patterns

and, if so, whether these patterns are the result from the same climatic and geologic

events. Same topologies not necessarily imply common history for a set of codistributed

taxa because geographic barriers may produce pseudocongruence in the form of soft and

hard allopatric distributions. The first one implying an organismal response to

environmental variations, the second implies external factors limiting distributions.

(Pyron and Burbrink 2010). Well-supported phylogenetic inferences coupled with robust

estimates of divergence times in codistributed species should be a powerful analytical

tool to reveal underlying causes of lineage diversification (Bermingham and Moritz 1998;

Hickerson et al. 2006).

Several arid-adapted rodents, including Nelson’s woodrat (Neotoma nelsoni), the

Perote ground squirrel (Xerospermophilus perotensis), the rock mouse (Peromyscus

difficilis), and Phillips’ kangaroo rat (Dipodomys phillipsii), show almost identical

distributions in the Mexican Plateau and Oriental Basin of central Mexico. Not only are

these species codistributed in their general ecological requirements, but their subspecific

taxonomic boundaries are also very similar, and these boundaries are thought to result

from effects of the TMVB. My study will test the hypothesis that the timing of major

divergence events in the four rodent species listed above is coincident with the timing of

major topographical shifts in the TMVB.

Neotoma nelsoni is a member of the rodent family Cricetidae (New World rats and

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mice) and Xerospermophilus perotensis is a member of the Sciuridae (squirrels, marmots,

and their relatives). Both species are endemic to the Oriental Basin and both are listed as

threatened by the Mexican government. The putative sister species of both taxa are

found in the Mexican Plateau to the north. Both species show slight, qualitative

morphological differences when compared to their sister species, but scarcity of museum

specimens and collecting restrictions by the Mexican Government have, until now,

prevented a thorough study of their species status using modern systematic tools. Prior to

this study, nothing was known about their biogeographic history.

Dipodomys phillipsii belongs to the rodent family Heteromyidae (kangaroo rats,

pocket mice, and their allies), and Peromyscus difficilis is another cricetid rodent. Both

are Mexican endemics with similar distributions. Populations of both species occur in the

arid and semi-arid plains and low hills in the Oriental Basin and the Mexican Plateau,

however P. difficilis has disjunct populations at higher elevations in the pine forests of

the TMVB. Both species are divided into several subspecies that can be distinguished

primarily by where they occur, plus a few qualitative morphological characters, such as

body size and fur coloration. Until this study, neither species has been studied using

modern systematic techniques and analyses.

My survey of mammals of the Oriental Basin and Mexican Plateau was carried out

between 2006 and 2010 and, thanks to a collecting permit granted by the Mexican

government to F. Cervantes on my behalf, I was able to obtain multiple samples,

including fresh tissues, for each of the four study taxa. Using the tissues I collected plus

other tissues generously donated to me by colleagues in multiple museums in Mexico and

the U.S., I had the opportunity to examine, for the first time, phylogenetic and

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phylogeographic relationships among the different populations and subspecies of the four

study taxa using mitochondrial and nuclear markers analyzed using maximum likelihood

and Bayesian approaches. Use of molecular markers under a coalescent framework also

allowed me to estimate divergence dates within each taxon, which placed the

evolutionary history of these Mexican endemics into a larger, historical-biogeographical

context and provided the opportunity to test the role of geographic barriers and climate

shifts in generating the pattern of evolutionary relationships we see today in these

species.

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ARBOGAST, B. S., AND G. J. KENAGY. 2001. Comparative phylogeography as an

integrative approach to historical biogeography. Journal of Biogeography 28:819–825.

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CHAPTER 2 MOLECULAR PHYLOGENETICS AND BIOGEOGRAPHY OF A RARE ENDEMIC:

NELSON’S WOODRAT (NEOTOMA LEUCODON NELSONI) IN THE ORIENTAL BASIN

OF MEXICO

2.1 INTRODUCTION

Neotoma nelsoni (Nelson’s woodrat) is a rare, monotypic, and endemic Mexican

mammal, with only a handful of specimens housed in museum collections worldwide

(González-Ruíz et al. 2006, Hall 1981). This species has been collected at only four

localities in the states of Puebla and Veracruz (Acosta and Fernández 2009; Falcón-Ordaz

et al. 2010; Goldman 1905; González-Christen et al. 2002; González-Ruíz et al. 2006),

and >200 kilometers separate N. nelsoni from the nearest population of its putative sister

species, N. leucodon, in the state of Hidalgo, Mexico (Fig. 2.1; González-Ruíz et al.

2006).

Neotoma nelsoni was originally described by E. A. Goldman in 1905 based on 11

specimens (the holotype and 10 paratypes) collected in the vicinity of Perote, Veracruz.

Goldman differentiated N. nelsoni from N. leucodon by presence of a palatine bone with

a short posterior spine, nasals that are more wedge-shaped and pointed posteriorly, and a

tail that is indistinctly bicolored and nearly unicolored near the tip (Goldman 1905;

González-Ruíz et al. 2006). The morphological differentiation and geographic isolation

of N. nelsoni led Goldman (1905) to recognize it as a distinct species, presumably related

to the much more widespread N. leucodon.

Hall and Genoways (1970) used morphological evidence to place N. nelsoni in the

N. albigula species group, which included N. albigula (with 14 subspecies including the

current N. leucodon), N. palatina, N. varia, and N. nelsoni. Neotoma palatina, a Mexican

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Fig. 2.1.—Geographical distribution of the Neotoma micropus species group. Asterisks show the approximate collection locality of the two samples of N. leucodon. Localities are listed in the methods section (redrawn from Edwards et al. 2001; González-Ruíz et al. 2006; and Hall 1981).

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endemic restricted to the canyon of the Río Bolaños, its tributaries, and immediately

adjacent uplands in Jalisco (Hall and Genoways 1970), is still recognized as a valid

species (Musser and Carleton 2005). Neotoma varia, also a Mexican endemic restricted

to Isla Dátil off the coast of Sonora, was considered inseparable from N. albigula by

Bogan (1997), who reduced it to subspecies status within N. albigula.

Several studies have surveyed genetic relationships in Neotoma, yet none of these,

to date, has included a specimen of N. nelsoni. Planz et al. (1996) examined

mitochondrial DNA restriction site polymorphism in Neotoma and suggested that N. a.

leucodon may represent a cryptic species within N. albigula. Edwards et al. (2001)

examined variation in the mitochondrial cytochrome b gene and, in agreement with Planz

et al. (1996), concluded that N. albigula consists of two species, N. albigula in the

southwestern United States and northwestern Mexico, and N. leucodon in the

southcentral United States and northcentral Mexico (Fig. 2.1). Edwards et al. (2001)

further determined that N. albigula and N. goldmani (endemic to the Mexican altiplano)

are allied with the N. floridana species group (which also includes N. floridana and N.

magister), whereas N. leucodon is allied with the N. micropus species group (which

includes N. micropus and, possibly, N. palatina). Building on the study by Edwards et al.

(2001), Edwards and Bradley (2002) examined cytochrome b variation in taxonomically

and geographically larger samples of Neotoma and recovered the same species groupings.

Most recently, a study of variation in four mitochondrial and four nuclear genes by

Matocq et al. (2007) confirmed the Neotoma species groups defined by Edwards et al.

(2001). Because specimens of N. nelsoni and N. palatina were not included in any of

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these molecular studies, the taxonomic status and phylogenetic position of these species

remain unclear.

During a 2007 survey of mammals of the Oriental Basin of Mexico, I trapped a

specimen of N. nelsoni in the municipality of Perote, Veracruz. This capture affords the

opportunity to examine, for the first time, phylogenetic relationships between N. nelsoni

and its congeners based on molecular markers. Use of molecular markers also facilitates

calculation of estimated divergence dates within the clade containing N. nelsoni, which

places the evolutionary history of this rare Mexican endemic into a larger, historical

biogeographical context.

2.2 MATERIALS AND METHODS

Sampling.—The female specimen of Neotoma nelsoni (LSUMZ 36663) was

trapped on June 22, 2007, in Veracruz (3 km S El Frijol Colorado, Municipality of

Perote, 19.5723, -97.3835, 2,437 m; Fig. 2.2) under the authority of Mexican collecting

permit FAUT-0002 issued to F. A. Cervantes. Traps were set in a dry plain with almost

no vegetation and in low, rocky hills dominated by yucca (Yucca sp.), agave (Agave sp.),

prickly pear (Opuntia sp.), and other cacti. The specimen of N. nelsoni was captured in

the latter habitat. All mammal specimens were handled in accordance with guidelines

approved by the American Society of Mammalogists (Kelt et al. 2010, Sikes et al. 2011).

DNA sequences generated by Edwards et al. (2001) and Matocq et al. (2007) for

14 species of Neotoma were downloaded from GenBank. The samples include one

individual per species (unless indicated otherwise), and GenBank numbers are listed in

the following order: Cytb; 12S; 16S. Taxa represented were N. albigula (DQ179707;

DQ179757; DQ179857), N. cinerea (DQ179705; DQ179755; DQ179855), N. floridana

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Fig. 2.2.—Map of central Mexico (inset modified from http://gaia.inegi. org.mx/mdm5/viewer.html) showing the known geographic distribution of Nelson’s woodrat, Neotoma leucodon nelsoni (shaded area in main map). The shaded area in the inset shows the extent of the Oriental Basin. The specimen examined in this study was collected at locality 1 in the inset (Falcón-Ordáz et al. (2010; this study). The three other known collection localities of N. leucodon nelsoni are based on reports by Goldman (1905; locality 2), González-Christen et al. (2002; locality 3), González-Ruíz et al. (2006; locality 4).

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(DQ179669; DQ179719; DQ179819), N. fuscipes (DQ179672; DQ179722; DQ179822),

N. goldmani (DQ179677; DQ179727; DQ179827), N. isthmica (DQ179678; DQ179728;

DQ179828), N. lepida (DQ179681; DQ179731; DQ179831), N. leucodon (n = 2;

DQ179665; DQ179715; DQ179815 from Texas, and DQ179689; DQ179739; DQ179839

from Durango), N. macrotis (DQ179691; DQ179741; DQ179841), N. magister

(DQ179706; DQ179756; DQ179856), N. mexicana (DQ179695; DQ179745;

DQ179845), N. micropus (DQ179668; DQ179718; DQ179818 from Texas, and

DQ179698; DQ179748; DQ179848 from New Mexico), N. picta (DQ179701;

DQ179751; DQ179851), and N. stephensi (DQ179702; DQ179752; DQ179852).

Sequences generated by Edwards et al. (2001) and Matocq et al. (2007) for Hodomys

alleni (DQ179660; DQ179710; DQ179810), Peromyscus attwateri (DQ179661;

DQ179711; DQ179811), Ototylomys phyllotis (DQ179664; DQ179714; DQ179814),

Tylomys nudicaudus (DQ179662; DQ179712; DQ179812), and Xenomys nelsoni

(DQ179663; DQ179713; DQ179813) were included as outgroups. Collection localities

for all of the above specimens are available in Edwards et al. (2001) and Matocq et al.

(2007).

Laboratory protocols.—Total genomic DNA was extracted from fresh tissue using

a commercial kit (DNeasy Blood and Tissue Kit; Qiagen Inc., Valencia, California), and

3 mitochondrial genes were sequenced: cytochrome-b (Cytb), 12S ribosomal RNA (12S),

and 16S ribosomal RNA (16S). Sequences were amplified by PCR (Saiki et al. 1988)

using the following universal primers developed for rodents: MVZ-05 and H15915 for

Cytb (Irwin et al. 1991); 12S L82 and 12S H900 for 12S (Nedbal et al. 1994); and 16Sa

and 16Sb for 16S (Matocq et al. 2007). The following PCR parameters were used to

! 22

amplify genes: initial denaturation at 95°C for 2 min, followed by 27 cycles of

denaturation at 95°C for 1 min, annealing at 49°C for 1 min, and extension at 72°C for 2

min, with a final extension at 72°C for 7 min (Mantooth et al. 2000). Amplifications

were performed in a total volume of 25 µL and 200 ng of DNA. Agarose (2%) gels were

used to visualize amplified products. PCR products were purified using ExoSAP-IT

(Affymetrix, Santa Clara, California). DNA sequencing was performed for both light and

heavy strands with a Big Dye Terminator v1.1, v3.1 in an automated 3100 Genetic

Analyzer (Applied Biosystems, Foster City, California) at the Museum of Natural

Science, Louisiana State University. Sequences generated in this analysis (Cytb, 12S, and

16S.) were submitted to GenBank.

Data analysis.—Editing and alignment of sequences and matrix manipulations

were performed in Sequencher 4.7 (Gene Codes Corporation, Ann Arbor, Michigan).

Sequences were verified manually, and authenticity of the gene was confirmed by amino

acid translation and BLAST searches in Genbank

(http://blast.ncbi.nlm.nih.gov/Blast.cgi).

To enable comparison of our results with those of previous studies, raw sequence

divergence values for the Cytb gene were corrected using the Kimura 2-parameter

substitution model (Kimura 1980) in PAUP* (Phylogenetic Analysis using Parsimony,

version 4.0b, Swofford 2003).

Phylogenetic analyses were carried out under a maximum likelihood (ML)

framework in PAUP* and PhyMl 3.0 (Guindon and Gascuel 2003). Analyses based on

Bayesian inference (BI) were conducted using MrBayes (version 3.2-cvs; Huelsenbeck

and Ronquist 2001; Ronquist and Huelsenbeck 2003). In all analyses, variable nucleotide

! 23

positions were considered unordered, discrete characters with 4 possible states (A, C, G,

and T). Best-fit models for ML and BI analyses were evaluated using the Akaike

Information Criterion and the program jModeltest 0.1.1 (Akaike 1973; Guindon and

Gascuel 2003; Posada 2008). The HKY+I+G model was selected for the Cytb data, and

the TIM2+I+G model was selected for the 12S and 16S genes. ML clade support was

assessed with 500 bootstrap replicates (Felsenstein 1985) in PAUP*, and clade support in

BI analyses was evaluated using posterior probabilities (pp).

Tree searches in the ML analyses were performed with the starting trees obtained

via 100 random, stepwise additions followed by tree-bisection-reconnection (TBR)

branch swapping. In the BI analyses, best-fit models were applied to each data partition

with unlinked parameters and allowing rate variation. The Metropolis Markov Chain

Monte Carlo analysis consisted of 2 independent runs of 10 x 106 generations in which

trees were sampled every 103 generations, resulting in 104 samples for each run. A

majority-rule consensus tree was constructed using the final 2 x 104 trees. The analysis

was stopped when the average standard deviation of split frequencies approached zero,

and convergence also was assessed using Tracer 1.5 (Rambaut and Drummond 2007).

The combined data set was analyzed in a partitioned way (genes and model parameters)

to allow for independent convergence on optimal values for each component (Ronquist

and Huelsenbeck 2003). Results from each gene were analyzed separately to evaluate

potential conflict among gene trees. Nodes were considered well supported if there was

>80% bootstrap support in ML analyses or >95% pp in BI analyses.

Analysis of divergence times.—The program *BEAST version 1.6.0 (Drummond

and Rambaut 2007) was used to estimate the time of divergence between N. nelsoni and

! 24

other members of the N. micropus species group. A Yule tree prior was used, implicitly

considering the gene tree to represent the species tree, and the HKY+I+G model was

selected as the substitution model. To improve search efficiency, monophyly of the

genus Neotoma was enforced. A relaxed clock with a lognormal distribution allowing

rate variation among sites was used. Chains were run for 27 generations, sampling the

parameter every 103 generations. Convergence statistics were checked for effective

sample sizes using Tracer version 1.5 (Rambaut and Drummond 2007). Consensus trees

were generated from the resulting 2 x 104 trees using TreeAnnotator version 1.6.0

(Rambaut and Drummond 2009) after elimination of the first 25% as burn-in.

Three fossil-based dates were used to calibrate the *BEAST analysis. The 1st

documents separation of the Neotoma lineage from ancestral cricetine stock in the upper

(late) Miocene, during a period that lasted from approximately 11.6 to 5.3 MYA (Hibbard

1966). The 2nd and 3rd fossil-based dates establish minimum ages for N. albigula and N.

mexicana, respectively, at the beginning of the Pleistocene (ca. 2.5 MYA; Álvarez 1969;

Birney 1973, 1976; Dalquest and Stangl 1984; Harris 1984; Jakway 1958; Logan and

Black 1979; Murray 1957; Schultz and Howard 1935; Van Devender et al. 1977). To

account for uncertainty in the fossil-based calibrations, the dates were modeled as

lognormal distributions rather than point calibrations (Ho and Phillips 2009).

2.3 RESULTS

The final dataset consisted of 1,140 base pairs (bp) of the Cytb gene, 413 bp of the

12S gene, and 561 bp of the 16S gene for a total of 2,114 bp. Cytb divergence values

(Table 2.1) show the N. nelsoni specimen from Veracruz, Mexico to be most similar

genetically to the specimen of N. leucodon from Durango, Mexico (3.3% corrected

! 25

sequence divergence). Surprisingly, the two specimens of N. leucodon, one from

Durango and the other from Texas, are only 5.5% genetically similar at the Cytb locus.

Neotoma nelsoni exhibits >12% sequence divergence when compared to all other species

of Neotoma in the dataset (N. albigula, N. mexicana, and N. micropus).

ML and BI analyses of the 12S gene indicated different topologies, but both trees

were poorly resolved and had universally low branch support (trees not shown). Similar

analyses of the 16S dataset recovered a moderately supported clade (75% bootstrap; 0.96

pp) that includes all members of the N. micropus species group (as defined by Matocq et

al. 2007) plus the N. nelsoni sample. An internal clade with low branch support (50%

bootstrap; <0.60 pp) grouped N. nelsoni with the two N. leucodon samples, but

relationships among the 3 taxa were unresolved (trees not shown, but are available on

request).

ML and BI analyses of the Cytb gene (Fig. 2.3) recovered topologies that were

almost identical to those reported by Matocq et al. (2007). The N. micropus species

group of Matocq et al. (2007) was recovered with strong nodal support (100% bootstrap;

1.00 pp), and N. nelsoni was included as a member of this group. Within the N. micropus

species group, N. nelsoni clustered with the two samples of N. leucodon (100% bootstrap;

1.00 pp), and within this trio of taxa, N. nelsoni grouped with the specimen of N.

leucodon from Durango (92% bootstrap; 0.99 pp) to the exclusion of the sample of N.

leucodon from Texas (Fig. 2.3).

Although trees based on the individual genes showed no topological conflicts, a

partitioned homogeneity test indicted that the genes could not be combined. As a result,

! 26

Table 2.1.—Cytochrome-b sequence divergence values (Kimura 2-parameter model) between and within species of the Neotoma micropus and N. floridana species groups.

Taxa

Cytb sequence divergence

N. nelsoni–N. leucodon (average)

4.17%

N. nelsoni–N. leucodon (Durango) 3.28%

N. nelsoni–N. leucodon (Texas) 5.07%

N. leucodon (Durango)–N. leucodon (Texas) 5.52%

N. nelsoni–N. micropus 12.83%

N. nelsoni–N. albigula 14.38%

N. nelsoni–N. mexicana 13.50%

! 27

Fig. 2.3.—Cladogram showing the placement of Nelson’s woodrat, N. leucodon

nelsoni, within the genus Neotoma and the N. micropus species group (shaded box) based on a maximum likelihood (ML) analysis of cytochrome-b sequences. A Bayesian analysis of the same data yielded a tree with identical topology. Numbers above branches indicate ML bootstrap support, and numbers below branches indicate Bayesian posterior probabilities. The specimen labeled “N. leucodon (D)” is from Durango, and “N. leucodon (T)” is from Texas.

! 28

Fig. 2.4.—Cladogram showing placement of Nelson’s woodrat, N. leucodon

nelsoni, within the genus Neotoma and the N. micropus species group (shaded box) based on a partitioned maximum likelihood (ML) analysis of 3 mitochondrial genes (Cytb, 12S, and 16S). A Bayesian analysis of the same data yielded a tree with identical topology. Numbers above branches indicate ML bootstrap support, and numbers below branches indicate Bayesian posterior probabilities.

! 29

the three sequence datasets were analyzed in a partitioned analysis. To save

computational time, only members of the N. micropus and N. floridana species groups

were included in these analyses. ML and BI analyses of the partitioned data (Fig. 2.4)

showed topologies identical to the Cytb tree (Fig. 2.3) and confirmed monophyly of both

species groups (100% bootstrap and 1.00 pp for both clades). The sister relationship

between N. nelsoni and the N. leucodon sample from Durango also was confirmed (97%

bootstrap; 1.00 pp; Fig. 2.4).

Analysis of divergence times.—Except for the position of N. stephensi (which is

not relevant to the present study), the tree obtained in the *BEAST analysis (Fig. 2.5) was

identical to the BI tree published by Matocq et al. (2007; their Fig. 4). The Tracer

analysis of *BEAST output files confirmed a high effective sample size (>3,500) for all

parameters. Although error estimates are large for most nodes, results suggest that the N.

micropus and N. floridana species groups diverged near the end of the Miocene (ca. 6.7

mya) and most speciation events within these species groups took place in Pliocene or

early Pleistocene times (ca. 4.6 to 2.0 mya; Fig. 2.5). The split between N. leucodon

(Texas sample) and the N. nelsoni + N. leucodon (Durango) clade was placed at

approximately 3 mya (late Pliocene), and divergence of N. nelsoni from N. leucodon

(Durango) is estimated to have occurred approximately 2 mya during the early

Pleistocene.

2.4 DISCUSSION

Species status of N. nelsoni.—The discovery that N. nelsoni is phylogenetically

closer to N. leucodon from Durango than the latter specimen is to N. leucodon from

Texas calls into question the monophyly of N. leucodon and the species status of N.

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Fig. 2.5.—Phylogeny and chronogram of selected species of the genus Neotoma (and outgroups), and estimated divergence times calculated from *BEAST analysis. Numbers at nodes are mean divergence dates (mya) and gray bars show the 95% credibility intervals for nodes of interest. Black circles identify calibration points based on fossil evidence. The specimen labeled “N. leucodon (D)” is from Durango, and “N.

leucodon (T)” is from Texas.

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nelsoni. Continued recognition of N. nelsoni at the species level would require elevation

of the northern clade of N. leucodon to species level (as N. warreni Merriam, 1908), but

there is no compelling evidence to warrant this action. The Cytb sequence divergence

measured between the Texas and Durango samples of N. leucodon in this study (5.5%;

Table 2.1) is within the range of values reported for conspecific populations by Bradley

and Baker (2001). The morphological characters used to distinguish N. nelsoni from N.

leucodon (modest differences in pelage quality and coloration and minor, qualitative

differences in a few cranial bones; González-Ruíz et al. 2006) are on the order of

characters normally used to distinguish among subspecies in other rodents. Given the

current absence of diagnostic morphological characters to distinguish N. nelsoni from N.

leucodon and the relatively low level of Cytb sequence divergence between the 2 forms

(3.3%; well within the range of values reported for conspecific populations by Bradley

and Baker, 2001), I take a conservative taxonomic approach and recognize N. nelsoni as a

subspecies of N. leucodon (as N. leucodon nelsoni Goldman 1905). Designation of

subspecific epitaphs for all other populations of N. leucodon must await a large-scale

assessment of geographic variation within the species.

Estimates of divergence times.—Early hypotheses about the diversification of the

North American mammalian fauna suggested that Pleistocene glacial and interglacial

cycles were major generators of species-level diversity (e.g., Findley 1969; Orr 1960).

Consistent with this widely held contention, Zimmerman and Nejtek (1977) used data

obtained by protein electrophoresis to estimate that basal divergence events leading to the

current species N. albigula, N. floridana, and N. micropus occurred between 112,000 and

155,000 years ago. Zimmerman and Nejtek (1977) implicated vegetational changes that

! 32

occurred in response to Pleistocene glaciations as the causal force driving speciation in

this clade.

Recently, new distributional, paleontological, and molecular evidence has

modified the “Pleistocene paradigm,” assigning a prominent role to geologic events that

took place during Miocene and Pliocene times (Hafner and Riddle 1997; Riddle 1995;

Riddle et al. 2000; Vrba, 1992). For example, estimates of divergence times calculated in

this study suggest that the initial split between the N. micropus and N. floridana species

groups occurred near the end of the Miocene (approximately 6.7 mya; Fig. 2.5), long

before the major climatic fluctuations of the Pleistocene. Mean estimates of divergence

events within the N. micropus and N. albigula species groups also are pre-Pleistocene,

with diversification in both groups beginning in early Pliocene (ca. 4.5 mya). The

divergence between N. leucodon nelsoni and the N. leucodon sample from Durango is the

only event postulated to have taken place during the Pleistocene (Fig. 2.5).

If, as these data suggest, major divergence events in the N. micropus and N.

floridana species groups took place during Miocene and Pliocene times, then geological

and climatic events of those times probably played major roles, either directly or

indirectly, in this diversification. The Trans-Mexico Volcanic Belt (TMVB) began to

form in the Miocene (Ferrusquía-Villafranca 2007) and is still active today. Presently, it

consists of a large, but somewhat fragmented, chain of mountains extending from the

Pacific coast to the Gulf of Mexico at the latitude of Mexico City. Fragmentation of the

TMVB through time created a large number of interior lakes, some of which persist

today, whereas others have dried up, giving rise to small pockets of alkaline desert inside

the TMVB. The Oriental Basin, home to at least one population of N. leucodon nelsoni

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and several endemic animals and plants, is one such alkaline desert near the eastern end

of the TMVB (Ferrusquía-Villafranca 2007; Morán-Zenteno 2004; Shreve 1942;

Viniegra 1992).

The TMVB is widely recognized as a generator of biological diversity in Mexico,

and the uplift of the TMVB and associated creation of interior lakes and small patches of

desert may have isolated ancestral populations of many organisms, including the common

ancestor of the N. leucodon populations now in Durango and N. leucodon nelsoni in the

Oriental Basin region well before the Pleistocene. Once isolated, these populations

would be subject to the many geologic and climatic perturbations of the late Miocene,

Pliocene, and early Pleistocene, resulting in a mixed pattern of diversification, in which

some lineages diverge well before the Pleistocene, whereas others respond to the well-

known geologic, glacial, and climate changes of the Pleistocene. It seems that the

evolutionary history of N. leucodon nelsoni may be yet another example of the isolating

force of the TMVB (Demastes et al. 2002; Douglas et al. 2010; Hewitt 2001; Mateos et

al. 2002; McCormack et al. 2008; Mulcahy and Mendelson 2000; Zink and Blackwell

1998).

Natural history and conservation status of N. leucodon nelsoni.—Given the rarity

and isolation of N. leucodon nelsoni populations, a few comments on the natural history

and conservations status of this taxon are in order. Only 14 specimens of Nelson’s

woodrat are known to date. Goldman (1905) described the species based on 11

specimens collected in 1893, and more than a century passed before 3 additional

specimens were collected, including the specimen used in this study (González-Christen

et al. 2002; González-Ruíz et al. 2006). The specimens examined by Goldman (1905)

! 34

were collected near Perote, Veracruz, but it is difficult to determine the habitat at their

site of capture because Goldman used the locality “Perote, Veracruz” for all specimens

collected in the general vicinity of Perote, which contains a wide variety of habitat types

(González-Ruíz et al. 2006). González-Christen et al. (2002) collected a single specimen

of N. nelsoni in a tropical rain forest about 30 km SE of the type locality, and González-

Ruíz et al. (2006) reported another specimen collected in a mountainous area with cloud

forests and coffee plantations approximately 40 km S of the type locality. The specimen

used in this study was collected in dry, low hills of volcanic origin surrounded by xeric

shrubs and cacti approximately 15 km W of the type locality. Given the wide variety of

habitats in which the few known specimens of N. leucodon nelsoni have been captured, it

appears that this subspecies has high environmental plasticity (as suggested by González-

Ruíz et al. 2006) and can live in a broad array of vegetational associations.

Despite its high environmental plasticity, N. leucodon nelsoni seems to have a

clear association with the Oriental Basin (“Cuenca Oriental”; Fig. 2.2), and all known

specimens of N. leucodon nelsoni have been collected in or around this xeric basin. The

Oriental Basin is a semiarid, endorheic (closed drainage) basin that extends over portions

of the Mexican states of Puebla, Tlaxcala, and Veracruz. This relatively small (ca. 5,000

km2) basin is characterized by alkaline grasslands, bunch grasses, and aridland scrub in

the valleys, and coniferous forests in the surrounding mountains (Valdéz and Ceballos

1997). The basin and surrounding areas support several endemic taxa of plants and

animals, including at least 4 mammal species exclusive to the Oriental Basin region: the

Oriental Basin pocket gopher (Cratogeomys fulvescens; Hafner et al. 2005), the Perote

ground squirrel (Xerospermophilus perotensis; Best and Ceballos 1995), the Perote

! 35

deermouse (Peromyscus bullatus; González-Ruíz and Álvarez-Castaneda 2005), and

Nelson’s woodrat (N. leucodon nelsoni; González-Ruíz et al. 2006; this study).

The specimen of N. leucodon nelsoni used in this study was collected in sympatry

with several endemic and non-endemic rodent species, including the Perote ground

squirrel, Phillip’s kangaroo rat (Dipodomys phillipsii), silky pocket mouse (Perognathus

flavus), western harvest mouse (Reithrodontomys megalotis), and rock mouse

(Peromyscus difficilis). Two species of rattlesnakes (Crotalus molossus nigrescens and

C. scutulatus salvini) were collected near the capture locality, and a third species (C.

ravus ravus) known from this region (Camarillo 1998) also may prey on woodrats and

other rodents in the community.

A new genus of nematode, Lamotheoxyuris, was described from the specimen of

N. leucodon nelsoni used in this study (Falcón-Ordaz et al. 2010), and three species of

fleas were collected from this specimen, one of them (Aniomiopsyllus perotensis) new to

science (Acosta and Fernández 2009).

Despite intensive collecting efforts by several research parties working in and

around the Oriental Basin (Hall and Dalquest 1963; González-Christen et al. 2002;

González-Ruíz et al. 2006; this study), only a few specimens of N. leucodon nelsoni are

known to science. Neotoma leucodon nelsoni is listed (as N. nelsoni) as critically

endangered by the International Union for Conservation of Nature (IUCN 2011) but,

ironically, the scarcity of information about the density and distribution of N. leucodon

nelsoni populations has caused this taxon to be excluded from the Mexican list of

endangered species (Luiselli 2002). There is little doubt that N. leucodon nelsoni

populations are threatened by continued and extensive conversion of natural habitats to

! 36

agriculture in the Oriental Basin region (González-Ruíz et al. 2006). New collecting

efforts are needed in the remaining areas of natural habitat to learn more about the

biology of N. leucodon nelsoni so as to protect it in the future.


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CHAPTER 3 PHYLOGENETICS AND BIOGEOGRAPHY OF THE MICROENDEMIC RODENT

XEROSPERMOPHILUS PEROTENSIS (PEROTE GROUND SQUIRREL) IN THE

ORIENTAL BASIN OF MEXICO

3.1 INTRODUCTION

The concept of endemism is central to the fields of biogeography and biological

conservation, and areas with high numbers of endemic species, or “biodiversity

hotspots,” often are included in protected area networks (Myers et al. 2000). In

biogeography, an area of endemism usually is defined as a region that contains the only

known occurrences of two or more taxa (i.e., multiple taxa are restricted to that region),

but other definitions of “areas of endemism” focus on the area’s geographic delimitation

by natural barriers or the distributional congruence of several species in the area (Harold

and Mooi 1994; Hausdorf 2002; Platnick 1991).

Mexico is widely known as a biologically megadiverse country and a biodiversity

hotspot (Lamoreaux et al. 2006). The diverse Mexican biota is the product of interactions

between a dynamic and complex topography and myriad ecological and historical factors

(Velasco de Leon et al. 2007). In central Mexico, the highlands and arid valleys of the

Trans-Mexico Volcanic Belt (TMVB) are home to one of the most diverse biotas in the

world (Luna et al. 2007).

Among the mountains at the southeastern edge of the TMVB lies the Oriental

Basin (Cuenca Oriental; Fig. 3.1). By any definition of “area of endemism,” this

semiarid, endorheic (closed drainage) basin, which covers portions of Puebla, Tlaxcala,

and Veracruz, is an important area of endemism in North America. This relatively small

(ca.

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Fig. 3.1.—Geographical distribution of pygmy ground squirrels of the genus Xerospermophilus (modified from Hall 1981). Numbered dots show collection localities listed in Appendix 3.1 for Xerospermophilus specimens used in this study. Locality 8 and all localities outside shaded areas are collection sites for outgroup specimens (Appendix 3.1). The shaded area near locality 11 shows the approximate extent of the Oriental Basin (Cuenca Oriental).

! 45

5,000 km2) basin characterized by alkaline grasslands, bunch grasses, and aridland scrub

in the valleys and coniferous forests in the surrounding mountains (Valdéz and Ceballos

1997) supports several endemic taxa of plants and animals, including at least four taxa of

endemic mammals: the Oriental Basin pocket gopher (Cratogeomys fulvescens), the

Perote deermouse (Peromyscus bullatus), Nelson’s woodrat (N. nelsoni), and the Perote

ground squirrel (Xerospermophilus perotensis; Best and Ceballos 1995; González-Ruíz

and Álvarez-Castaneda 2005; González-Ruíz et al. 2006; Hafner et al. 2005).

Shreve (1942) referred to the Oriental Basin as the southernmost extension of the

Chihuahuan desert, and most mammals of the Oriental Basin, including X. perotensis, are

arid-adapted species. Typically, the closest relatives of Oriental Basin endemics inhabit

the deserts of the Mexican Plateau to the north, and this appears to be the case for X.

perotensis, whose sister species is thought to be X. spilosoma (Fig. 3.1; Howell 1938).

In his classic revision of North American ground squirrels, Howell (1938)

classified all ground squirrels in the genus Citellus (later transferred to Spermophilus by

Hershkovitz 1949) and divided the genus into eight subgenera: Ammospermophilus,

Callospermophilus, Citellus, Ictidomys, Notocitellus, Otospermophilus, Poliocitellus, and

Xerospermophilus. Early morphological and chromosomal studies suggested a close

relationship between Spermophilus perotensis and S. spilosoma (Howell 1938; Uribe and

Ahumada 1990), however composition of and relationships among the subgenera of

Spermophilus were not clearly understood at that time. S. perotensis and S. spilosoma

were placed as sister taxa in the subgenus Ictidomys, along with I. tridecemlineatus, I.

mexicanus, and the more recently described I. parvidens (Harrison et al. 2003; Herron et

al. 2003).

! 46

The taxonomic status and systematic affinities of S. perotensis were not

investigated again until molecular studies by Harrison et al. (2003) and Herron et al.

(2004) confirmed the close affinity of S. perotensis with S. spilosoma. In fact, both

molecular studies (using the same specimens of S. spilosoma and based on the

cytochrome-b [Cytb] gene) showed S. spilosoma to be paraphyletic with respect to S.

perotensis, with S. s. pallescens from Mexico sister to S. perotensis, and S. s. marginatus

from Kansas sister to the S. s. pallescens + S. perotensis clade. These same studies

showed the S. perotensis + S. spilosoma clade (subgenus Ictidomys) to be sister to the S.

mohavensis + S. tereticaudus clade (subgenus Xerospermophilus), with prairie dogs

(Cynomys) sister to this group.

Helgen et al. (2009) combined new morphological data with the molecular

evidence provided by Harrison et al. (2003) and Herron et al. (2004) to elevate the

subgenus Xerospermophilus to full generic status. In Xerospermophilus, Helgen et al.

(2009) included the 4 species of pygmy ground squirrels adapted to arid and semi-arid

conditions, X. mohavensis, X. tereticaudus, X. spilosoma, and X. perotensis. In view of

the potential paraphyly of X. spilosoma (Harrison et al. 2003; Herron et al. 2003), Helgen

et al. (2009) recommended future research into species-level boundaries in the spilosoma-

perotensis complex.

Despite previous morphological and molecular studies of the systematic status of

X. perotensis and allied taxa, several uncertainties remain with respect to the species

status of X. perotensis, monophyly of X. spilosoma, and the timing of diversification

events within the genus Xerospermophilus relative to major geological and climatic

events. Each of these issues is explored in this analysis using newly acquired samples of

! 47

X. perotensis and X. spilosoma and sequence evidence from multiple mitochondrial and

nuclear genes.

3.2 MATERIAL AND METHODS

Sampling.—Tissue samples of Ictidomys mexicanus (n = 1 individual), I.

parvidens (n = 2), I. tridecemlineatus (n = 4), Xerospermophilus mohavensis (n = 2), X.

perotensis (n = 4), X. spilosoma (n = 3), and X. tereticaudus (n = 2) were either collected

in the field under the authority of Mexican collecting permit FAUT-0002 (issued to F. A.

Cervantes) or donated by museums (Appendix 3.1). In addition to the DNA sequences

generated in this study, 32 sequences were downloaded from Genbank for use in the

molecular analyses (Appendix 3.1). Outgroups in the analyses included specimens of

Urocitellus townsendii (in the Cytb analysis), Callospermophilus lateralis (in the 12S

ribosomal RNA [12S] and interphotoreceptor retinoid-binding protein [IRBP] analyses),

and Sciurus niger (in the growth hormone receptor [GHR] analysis). The collection and

processing of samples was undertaken following the guidelines of the American Society

of Mammalogists for use of wild animals in research (Kelt et al. 2010; Sikes et al. 2011).

Laboratory protocols.—Total genomic DNA was extracted from tissue using a

commercial kit (DNeasy Blood and Tissue Kit; Qiagen Inc., Valencia, California).

Portions of two nuclear genes (GHR and IRBP), and two mitochondrial genes (Cytb and

12S) were sequenced for subsequent analysis. The genes were amplified by PCR (Saiki

et al. 1988) using the following universal primers developed for rodents: GHR1f and

GHRend1f for GHR (Jansa et al. 2009); IRBP-A and IRBP-B for IRBP (Stanhope et al.

1992); MVZ-05 and H15915 for Cytb (Irwin et al. 1991); and 12S L82 and 12S H900 for

12S (Nedbal et al. 1994). PCR amplification of the GHR gene was performed under the

! 48

following parameters: initial denaturation at 94°C for 5 min followed by 34 cycles of

denaturation at 94°C for 15 sec, annealing at 60°C for 1 min, extension at 72°C for 1.5

min and 1 final extention at 72°C for 10 min. Amplification of the IRBP gene began with

initial denaturation at 95°C for 10 min followed by 27 cycles of denaturation at 95°C for

25 sec, annealing at 58°C for 20 sec, extension of 72°C for 1 min, and 1 final extension at

72°C for 10 min. Amplification of both mitochondrial genes began with initial

denaturation at 95°C for 2 min followed by 27 cycles at 95°C for 1 min, annealing at

49°C for 1 min, extension at 72°C for 2 min, and 1 final extension at 72°C for 7 min

(Mantooth et al. 2000). Amplifications were performed in a total volume of 25 µL and

200 ng of DNA. Agarose gels (2%) were used to visualize amplified products. PCR

products were purified using either Polyethylene Glycol (PEG) or ExoSAP-IT

(Affymetrix, Santa Clara, California). DNA sequencing was performed for both light and

heavy strands with a Big Dye Terminator v1.1, v3.1 in an automated 3100 Genetic

Analyzer (Applied Biosystems, Foster City, California) at the Museum of Natural

Science, Louisiana State University. Editing and alignment of sequences and matrix

manipulations were performed in Sequencher 4.7 (Gene Codes Corporation, Ann Arbor,

Michigan), MacClade (Maddison and Maddison 2000), and Mesquite (Maddison and

Maddison 2010). Sequences were verified manually, and authenticity of the gene was

confirmed by amino acid translation and BLAST searches in Genbank


Estimates of genetic divergence.—To enable comparison of my results with those

of previous studies, sequence divergence values for the Cytb gene were corrected using

the Kimura 2-parameter substitution model (Kimura 1980) in PAUP* 4.0b10

! 49

(Phylogenetic Analysis using Parsimony, version 4.0b10, Swofford 2003) and MEGA

version 5 (Tamura et al. 2011). Saturation analyses for 3rd codon positions were

performed using the methods of Griffiths (1997), and maximum likelihood (ML) analyses

were run with and without 3rd codon transitions to evaluate the affects of 3rd codon

substitutions on phylogenetic reconstruction.

Phylogenetic analyses.—Initial phylogenetic analyses were conducted using

Bayesian inference (BI) in the program MrBayes (version 3.2-cvs; Huelsenbeck and

Ronquist 2001; Ronquist and Huelsenbeck 2003) and maximum likelihood (ML) in

PAUP*. Analysis of the Cytb data included 41 specimens of 14 species for which

complete Cytb sequences were available. Phylogenetic analysis of the 12S gene included

17 specimens of 8 species, analysis of GHR included 14 specimens of 6 species, and

analysis of IRBP included 18 specimens representing 8 species (Appendix 3.1).

Following analyses of individual genes, a partitioned analysis including all 4 genes was

conducted using ML and BI frameworks. ML analyses were run in both PAUP* and

PhyMl 3.0 (Guindon and Gascuel 2003). Variable nucleotide positions were considered

unordered, discrete characters with 4 possible states (A, C, G, T). Best-fit models for ML

and BI analyses were evaluated using the Akaike Information Criterion (AIC) and the

program jModeltest 0.1.1 (Guindon and Gascuel 2003; Posada 2008). The following

models were selected for Cytb, 12S, GHR, and IRBP genes, respectively: TrN+G,

TIM3+G, TPM3UF+I+G, and TPM3UF+I. ML clade support was assessed with 100

bootstrap (bs) replicates in PAUP* and PhyMl 3.0, and clade support in the BI analyses

was evaluated using posterior probablilities (pp).

! 50

ML analyses were performed with the starting trees obtained from 100 random,

stepwise additions followed by tree-bisection-reconnection (TBR) branch swapping. In

the BI analyses, best-fit models were applied to each data partition with unlinked

parameters and allowing rate variation. The Metropolis Markov Chain Monte Carlo

analysis consisted of two independent runs of 10 x 106 generations in which trees were

sampled every 103 generations, resulting in 104 samples for each run. After discarding

the initial 10% as burn-in, a majority-rule consensus tree was constructed using the final

18 x 103 trees. The analysis was stopped when the average standard deviation of split

frequencies approached zero and convergence was reached, as determined using Tracer

version 1.5 (Rambaut and Drummond 2007). The combined data set was analyzed in a

partitioned manner (genes and model parameters) to allow for independent convergence

on optimal values for each component (Ronquist and Huelsenbeck 2003). Data partitions

included mitochondrial versus nuclear genes, Cytb versus 12S, Cytb versus IRBP, and

12S versus IRBP genes. Nodes were considered well supported if there was >80%

bootstrap support in ML analyses or >95% posterior probability in BI analyses.

Estimates of divergence times.—The program *BEAST version 1.6.0 (Bayesian

evolutionary analysis sampling trees; Drummond and Rambaut 2007) was used to

generate estimates of the timing of divergence of X. perotensis from related species.

*BEAST analyses were carried out using a Yule tree prior and implicitly considering the

Cytb gene tree to represent the species tree. The estimate was calibrated using a dated

fossil (Goodwin 1995; Harrison et al. 2003; Pizzimenti 1975) to constrain the minimum

date for separation of Cynomys from Xerospermophilus to 2.7 mya. To account for

uncertainty in the fossil-based calibration, the fossil date was modeled on a lognormal

! 51

distribution rather than a point calibration (Ho and Phillips 2009). The analysis used a

relaxed clock with a lognormal distribution allowing rate variation among sites. Chains

were run for 207 generations, sampling the parameter every 103 generations.

Convergence statistics were checked for effective sample sizes using Tracer version 1.5.

Consensus trees were generated from the resulting 20 x 103 trees using TreeAnnotator

version 1.6.0 (Rambaut and Drummond 2009) after elimination of 10% as burn-in.

3.3 RESULTS

Analyses of DNA sequences involved a total of 3,403 base pairs (bp), including

1,141 bp of Cytb, 736 bp of 12S, 910 of GHR, and 616 bp of IRBP. ML and BI analyses

using only Cytb sequences from the 41 individuals with complete Cytb sequences

recovered trees with identical branch structure (Fig. 3.2). In these trees, there is strong

support (pp = 1.00; bs = 100%) for monophyly of the genus Xerospermophilus. Within

Xerospermophilus, X. perotensis is shown to be sister to the X. spilosoma sample from

the state of San Luis Potosí on the Mexican Plateau (locality 12 in Fig. 3.1) to the

exclusion of other samples of X. spilosoma from Durango (locality 15), Kansas (14), and

New Mexico (13). X. mohavensis and X. tereticaudus also are depicted as sister taxa.

Among the many outgroups used in the analysis (listed above and in Appendix 3.1), the

genus Cynomys was found to be sister to Xerospermophilus, although this relationship

was not well supported (pp = 0.84 and bs = 79%) and therefore is not shown in Fig. 3.2.

Cytb divergence values (Table 3.1) show the taxa included in Fig. 3.2 to be well

differentiated genetically. X. perotensis is 3.6% genetically divergent from the X.

spilosoma sample from San Luis Potosí, and these two populations together show an

average Cytb divergence of 6.4% from the other samples of X. spilosoma from Durango,

! 52

Fig. 3.2.—Relationships among major clades of Xerospermophilis based on maximum likelihood and Bayesian analyses of Cytochrome-b sequences. Locality numbers (mapped in Fig. 3.1 and listed in Appendix 3.1) are indicated before taxon names. Numbers at nodes are estimated mean divergence dates (mya) calculated from *BEAST analysis. Bars show 95% credibility intervals.

! 53

Table 3.1.—Mean percent cytochrome-b sequence divergence values (Kimura 2-parameter model) among species of the genus

Xerospermophilus and Cynomys. The X. spilosoma specimen from San Luis Potosí is from locality 12 in Fig. 3.1, the specimen from

Durango is from locality 15, and the specimens from New Mexico and Kansas are from localities 13 and 14, respectively.

X. spilosoma X. spilosoma X. spilosoma

X. perotensis San Luis Potosí Durango New Mexico, Kansas X. mohavensis X. tereticaudus Cynomys

X. perotensis – 3.6 5.2 7.9 10.0 11.5 11.9

X. spilosoma – 6.0 6.4 10.6 12.0 12.0

San Luis Potosí

X. spilosoma – 5.5 9.9 11.8 11.2

Durango

X. spilosoma – 10.6 12.9 12.2

New Mexico, Kansas

X. mohavensis – 4.6 11.0

X. tereticaudus – 14.2

Cynomys –

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New Mexico, and Kansas. The X. perotensis + X. spilosoma clade (including all samples of

spilosoma) shows an average of 11.2% sequence divergence from the X. mohavensis + X.

tereticaudus clade and 11.8% divergence from specimens of the genus Cynomys. Independent BI

and ML analyses of 12S, GHR, and IRBP sequences confirmed the Cytb topology shown in Fig.

3.2, although nodal support values varied widely depending on the gene analyzed (trees not

shown but available on request). In all analyses, the genus Xerospermophilus was monophyletic

and sister to Cynomys, the sister species status of X. mohavensis + X. tereticaudus was confirmed

with strong support, and the sister relationship between X. perotensis and the X. spilosoma

sample from San Luis Potosí was recovered, but with low nodal support in the analyses of the

nuclear genes (GHR and IRBP).

BI and ML analyses of the partitioned data sets (partitioned by mitochondrial genes only,

nuclear genes only, and mitochondrial + nuclear genes) focused on relationships within

Xerospermophilus (Fig. 3.3). In all partitioned analyses, the genus Xerospermophilus was

monophyletic with high support values and the sister status of X. perotensis and X. spilosoma

(San Luis Potosí) was confirmed with high nodal support (pp > 0.97 and bs = 100%).

Mean estimates of divergence times (Fig. 3.2) ranged from a low of 0.7 mya between X.

perotensis and X. spilosoma (San Luis Potosí) to a high of 3.5 mya between the genera Cynomys

and Xerospermophilus. All estimates of divergence times within the genus Xerospermophilus

place these events in the Pleistocene, with the possible exception of the split between the X.

mohavensis + X. tereticaudus clade and the X. perotensis + X. spilosoma clade, which was

estimated at 2.7 mya with a confidence interval extending from 4.3 to 1.3 mya (Fig. 3.2).

! 55

Fig. 3.3.—Relationships among major clades of Xerospermophilus based on a maximum likelihood analysis of Cytb, 12S, GHR, and IRBP sequences partitioned by gene. A Bayesian analysis of the same data yielded identical relationships. Xerospermophilus spilosoma from Durango (locality 15) was excluded from these analyses because only Cytb sequences were available for that specimen.

! 56

3.4 DISCUSSION

This study of mitochondrial and nuclear DNA sequences confirms that X. perotensis is a

genetically well-differentiated unit within Xerospermophilus. The sister relationship between X.

perotensis and the X. spilosoma sample from San Luis Potosí (representing the subspecies X. s.

cabrerai) also is strongly supported in this study and is consistent with evidence provided by

Uribe and Ahumada (1990) who reported chromosomal similarities between X. perotensis and X.

s. cabrerai and interpreted this as an indicator of a close phylogenetic relationship between these

taxa.

Species status of X. perotensis.–Since its original description by Merriam (1893), X.

perotensis has been considered a valid species. However, recent morphological and molecular

studies have questioned its species status, and some authors have suggested that X. perotensis is

best regarded as a subspecies of X. spilosoma (Harrison et al. 2003; Helgen et al. 2009; Herron et

al. 2004). The present study confirms that continued recognition of X. perotensis at the species

level renders X. spilosoma paraphyletic (Figs 3.2 and 3.3). Paraphyly of X. spilosoma could be

resolved taxonomically by recognizing multiple species within X. spilosoma, but unless one

recognizes species based solely on degree of genetic divergence, no evidence is available at this

time suggesting that X. spilosoma is a composite of multiple cryptic species. It could also be

argued that X. perotensis and X. spilosoma populations from San Luis Potosí (X. s. cabrerai)

should be combined into a single species. However, again, there is no evidence for species-level

divergence between X. s. cabrerai and other subspecies of X. spilosoma except for the relatively

large Cytb distances measured between the subspecies examined in this study (5.2–7.9%; Table

3.1). Synonymization of X. s. cabrerai with X. perotensis still would require recognition of

multiple species within X. spilosoma to maintain monophyletic taxa.

! 57

Sister species within the sciurid genera Cynomys and Marmota show Cytb divergence

values ranging from 1.2% to 7.7% (Harrison et al. 2003; Steppan et al. 1999), so the divergence

value calculated between X. perotensis and X. s. cabrerai in this study (3.6%) lies within this

range but is less than the “high genetic divergence” value of 5% suggested by Baker and Bradley

(2006:654) to signal possible cryptic species. Morphologically, X. perotensis differs from X.

spilosoma in ways that are usually used to distinguish among subspecies of rodents. For

example, X. perotensis resembles X. spilosoma pallescens of the northern Mexican Plateau and

Sierra Madre Oriental, except that X. perotensis is larger overall, has a shorter tail, is more

yellowish dorsally, and has smaller and less conspicuous buffy spots (Best and Ceballos 1995).

The skull of X. perotensis is similar to that of X. spilosoma spilosoma (found in southern

Durango, Zacatecas, and parts of nearby states), except that the skull of X. perotensis is larger,

has a relatively narrower and higher brain case, has auditory bullae that are broader and more

flattened, and molariform teeth that are heavier than those of X. s. spilosoma (Best and Ceballos

1995; Hafner and Yates 1983; Helgen et al. 2009; Uribe et al. 1978).

Considering the absence of morphological or chromosomal evidence supporting the

species status of X. perotensis, I herein take the conservative route and recognize perotensis as a

subspecies of X. spilosoma (as X. s. perotensis). The relatively high divergence values measured

between the subspecies of X. spilosoma in this study (Table 3.1) may signal presence of multiple

cryptic species, but recognition of additional species must await a thorough study of geographic

variation throughout the range of X. spilosoma.

Biogeography of Xerospermophilus on the Mexican Plateau and in the Oriental Basin.–

Shreve (1942) recognized that the Mexican Plateau, the Oriental Basin, and other isolated arid

and semi-arid regions of central Mexico were relicts of a once continuous southern extension of

! 58

the Chihuahuan Desert. More recent research suggests that the arid and semi-arid lands of central

Mexico were continuous until mid-late Miocene, when rise of the TMVB began to act as a barrier

between populations of arid-adapted species (Ferrusquía et al. 2005; Ferrusquía and González

2005). Hoffmann and Jones (1970) examined present day distributions of several mammal

species in Mexico and suggested that many prairie and desert species may have reached their

southernmost distributions during Pleistocene times, with subsequent range contractions leaving

isolated populations in the south. They suggested that Cynomys mexicanus was one such

peripheral isolate of the once more widespread species, C. ludovicianus (Hoffmann and Jones

1970). The Perote ground squirrel, X. s. perotensis, isolated in the Oriental Basin of central

Mexico, may be another example of this phenomenon.

Uribe and Ahumada (1990) speculated that X. spilosoma stock was once widespread

throughout the highlands of northern Mexico and were able to disperse southward because of

continuous, dry habitats found in inter-montane valleys. Subsequent tectonic or climatic events,

or a combination of both, during the Pleistocene fragmented the once continuous grassland

habitat in central Mexico, leaving the present day patches of arid and semi-arid habitats,

including the Oriental Basin (Ferrusquía et al. 2005; Ferrusquía and González 2005; Hoffman and

Jones 1970; Pizzimenti 1975).

The results of this study underscore the importance of three under-studied biogeographic

regions of Mexico: the Oriental Basin (inhabited by X. s. perotensis), the Mexican Plateau (X. s.

cabrerai), and the Bolsón de Mapimí (X. s. pallescens). The origin of the Oriental Basin is

closely linked with the volcanic activity that gave raise to alkaline lakes and the rain shadow

effect that caused isolated pockets of arid and semi-arid land in the TMVB (Caballero et al. 2003;

Morán-Zenteno 1994). The Mexican Plateau formed as a result of uplifting of the Sierra Madre

! 59

Oriental, Sierra Madre Occidental, and TMVB, which created a dry, table land in the rain shadow

of these large mountain ranges. Recent phylogenetic and biogeographic studies are beginning to

show the importance of the Mexican Plateau as a center of evolutionary divergence in many

rodent taxa (Fernandez et al. 2012; Neiswenter and Riddle 2010). Finally, the Bolsón de Mapimí,

a closed desert basin located north of the Sierra de Zacatecas, formed during the Wisconsinan

glacial period of the Pleistocene and acted as a refugium for many desert organisms (Elias 1992),

including the ancestors of X. s. pallescens.


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CHAPTER 4 MOLECULAR PHYLOGENETICS AND TEMPORAL CONTEXT FOR THE DIVERSIFICATION OF

THE SOUTHERN ROCK DEERMOUSE, PEROMYSCUS DIFFICILIS (RODENTIA: CRICETIDAE)

4.1 INTRODUCTION

Although most studies of desert mammals of North America have focused on species

confined to the Chihuahuan, Peninsular, or Sonoran deserts, recent studies have begun to explore

temporal aspects of species-level diversification in mammals whose distribution extends

southward from the Mexican Plateau into the arid lands associated with the Trans-Mexico

Volcanic Belt (TMVB; Fernández et al. in press; Neiswenter and Riddle 2010). This study

focuses on one such species, the southern rock deermouse (Peromyscus difficilis), whose

distribution in a wide variety of habitats (Fernández et al. 2010) affords an unusual opportunity to

explore the effects of geology and climate on timing of phyletic diversification in a North

American rodent species.

Peromyscus difficilis is endemic to Mexico and is found throughout the Mexican Plateau,

Sierra Madre Oriental, and southward into north central Oaxaca (Fig. 4.1; Fernández et al. 2010;

Hall 1981). Current taxonomy divides P. difficilis into five subspecies (Fernández et al. 2010;

Hall 1981; Hoffmeister and de la Torre 1961; Fig. 4.1). Habitats occupied by P. d. amplus, P. d.

difficilis, P. d. petricola, and P. d. saxicola include dry rocky areas, and oak-pine forests at higher

elevations (2,000–2,400 m) and dry rocky hillsides with scattered juniper trees at lower elevations

(1,500–1,900 m; Fernández et al. 2010). In contrast, P. d. felipensis is found in wet coniferous

forests at high elevations (2,500–3,500 m) in mountains surrounding the Valley of

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Fig. 4.1—Geographical distribution of Peromyscus difficilis and P. nasutus in Mexico and the United States (redrawn from Fernández et al. 2010 and Hall 1981). Numbered dots show collection localities listed in Appendix 4.1 for P. nasutus and the five currently recognized subspecies of P. difficilis.

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Mexico and in mountains near the city of Oaxaca (Goodwin 1954; Müdespacher-Ziehl et al.

2005; Navarro-Frías et al. 2007; Villa-Ramírez 1953).

Hoffmeister and de la Torre (1961) examined morphological variation in P. difficilis

throughout its range and recognized eight subspecies, the current five plus three subspecies

presently assigned to P. nasutus (P. nasutus nasutus, P. n. griseus, and P. n. penicillatus; Musser

and Carleton 2005). Zimmerman et al. (1975, 1978) studied allelic differentiation at 23 protein

loci in populations of P. difficilis from Zacatecas and Colorado and elevated P. nasutus to species

status (supported by Avise et al. 1979; Carleton 1989). More recent studies of Peromyscus

relationships based on the mitochondrial cytochrome-b gene (Durish et al. 2004; Bradley et al.

2007) have reaffirmed the species status of P. difficilis and P. nasutus.

Phylogeographic relationships among the subspecies of P. difficilis are not well

understood. Whereas all populations examined to date have a chromosomal diploid number of

48, variation in fundamental number (FN) within the species (Arellano-Meneses et al. 2000;

Müdespacher-Ziehl et al. 2005; Robbins and Baker 1981; Zimmerman et al. 1975) suggests

possible presence of genetic variation that remains undiscovered.

In this study, phylogeographic relationships among the subspecies of P. difficilis and

between P. difficilis and its close relatives are examined using nuclear and mitochondrial DNA

sequences analyzed by maximum likelihood and Bayesian approaches. Estimates of divergence

times based on Bayesian analyses of genetic data affords an unusual opportunity to test the role of

geographic barriers and climate shifts in generating the pattern of evolutionary relationships we

see today in this species.

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Sampling, amplification, and sequencing.—Two mitochondrial genes (cytochrome-b,

Cytb, and 12S ribosomal RNA, 12S) and two nuclear genes (growth hormone receptor, GHR, and

interphotoreceptor retinoid-binding protein, IRBP) were sequenced for this analysis. In the initial

phase of the analysis, Cytb sequences were generated for P. d. amplus (n = 12 individuals), P. d.

difficilis (n = 6), P. d. felipensis (n = 3), P. d. petricola (n = 4), P. d. saxicola (n = 3), and P.

nasutus (n = 5). Following analysis of the Cytb data, representatives of each major clade were

sequenced for 12S, GHR, and IRBP. Sequences for the 12S gene were generated for P. d. amplus

(n = 4), P. d. difficilis (n = 4), P. d. felipensis (n = 1), P. d. petricola (n = 3), P. d. saxicola (n =

1), and P. nasutus (n = 3). GHR sequences were obtained for P. d. amplus (n = 6), P. d. difficilis

(n = 1), P. d. felipensis (n = 1), P. d. petricola (n = 4), P. d. saxicola (n = 1), and P. nasutus (n =

2). Finally, IRBP sequences were generated for P. d. amplus (n = 6), P. d. difficilis (n = 2), P. d.

felipensis (n = 1), P. d. petricola (n = 4), P. d. saxicola (n = 1), and P. nasutus (n = 2). Most of

the tissue samples were collected in the field by the author following the guidelines for research

on wild mammals approved by the American Society of Mammalogists (Kelt et al. 2010; Sikes et

al. 2011). The remainder of the tissue samples were generously donated by museums (Appendix

4.1), and 22 additional sequences (including outgroup sequences for P. attwateri, P. nasutus, P.

truei, and additional outgroups listed in Appendix 4.2) were downloaded from GenBank.

Extractions of genomic DNA were performed using a commercial kit (DNeasy Blood and

Tissue Kit; Qiagen Inc., Valencia, California). Mitochondrial and nuclear genes were amplified

by polymerase chain reaction using the following universal primers developed for rodents: MVZ-

05 and H15915 for Cytb (Irwin et al. 1991); 12S L82 and 12S H900 for 12S (Nedbal et al. 1994);

GHR1f and GHRend1f for GHR (Jansa et al. 2009); and IRBP-A and IRBP-B for IRBP

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(Stanhope et al. 1992). Thermal-cycling parameters for both mitochondrial genes were: initial

denaturation at 95°C for 2 min, 27 cycles of denaturation at 95°C for 1 min, annealing at 49°C for

1 min, extension at 72°C for 2 min, and final extension at 72°C for 7 min. Amplification

parameters for GHR were: denaturation at 94°C for 5 min, 34 cycles of denaturation at 94°C for

15 sec, annealing at 60°C for 1 min, extension at 72°C for 1.5 min, and final extension for 10 min

at 72°C. Parameters for IRBP were: denaturation at 95°C for 10 min, 27 cycles of denaturation at

95°C for 25 sec, annealing at 58°C for 20 sec, extension at 72°C for 1 min, and final extension

for 10 min at 72°C. Agarose gels (2%) were used to visualize amplified products. Cleaning of

the amplified products was performed with Polyethylene Glycol (PEG) and DNA sequencing was

performed for both light and heavy strands using Big Dye Terminator v1.1, v3.1 (Applied

Biosystems, Foster City, California) in an automated 3100 Genetic Analyzer (Applied

Biosystems) at the Museum of Natural Science, Louisiana State University.

Data Analysis.—Editing and alignment of sequences were done with Sequencher 4.7

(Gene Codes Corporation, Ann Arbor, Michigan). The authenticity of each gene was confirmed

by amino acid translation and by BLAST searches in GenBank

(http://blast.ncbi.nlm.nih.gov/Blast.cgi). To enable comparison of our results with those of

previous studies, average uncorrected sequence divergence values for the Cytb gene were

corrected using the Kimura 2-parameter substitution model (Kimura 1980) in PAUP*

(Phylogenetic Analysis using Parsimony, version 4.0b, Swofford 2003). This model has been

used widely in studies of Cytb variation in mammals (Bradley and Baker 2001; Honeycutt et al.

1995).

Phylogenies were estimated using the maximum likelihood (ML) algorithms in PAUP*

4.0b (Swofford 2003) and PhyMl 3.0 (Guindon and Gascuel 2003) and the Bayesian Inference

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(BI) approach in MrBayes (version 3.2-cvs; Huelsenbeck and Ronquist 2001). Best-fit models

for all analyses were selected using the Akaike Information Criterion (AIC; Akaike 1973) and the

programs jModeltest 0.1.1 (Guindon and Gascuel 2003; Posada 2008), MrModeltest (version 2.2;

Nylander 2004), and Modeltest (version 3.7; Posada and Buckley 2004; Posada and Crandall

1998). The TrN+G model was selected for Cytb, TIM2+G for 12S, TPM1uf+G for GHR,

TPM3uf+I+G for IRBP, and TPM1uf+G for the partitioned analyses. Clade support was assessed

with 500 bootstrap replicates (Felsenstein 1985) in PAUP* 4.0b, and clade support in BI analyses

was evaluated using posterior probabilities.

Tree searches in ML analyses were performed with the starting trees obtained from 100

random stepwise additions followed by tree-bisection-reconnection (TBR) branch swapping. In

BI analyses, best-fit models were applied to each data partition with unlinked parameters and

allowing rate variation. The Metropolis Markov Chain Monte Carlo analysis consisted of two

independent runs of 10 x 106 generations in which trees were sampled every 103 generations,

resulting in 104 samples for each run. The first 103 trees of each run were discarded as burn-in,

and a majority-rule consensus tree was constructed using the final 1.8 x 103 trees. The analysis

was stopped when the average standard deviation of split frequencies approached zero, and

convergence also was assessed using Tracer 1.5 (Rambaut and Drummond 2007). The combined

data set was analyzed in a partitioned manner to allow independent convergence on optimal

values for each fragment (Ronquist and Huelsenbeck 2003).

Each gene fragment was analyzed separately then compared visually to evaluate potential

conflict among resulting gene trees. Nodes were considered well supported if there was > 85%

bootstrap support or significant Bayesian posterior probabilities > 0.90. Because analyses of

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individual data sets showed no conflicting clades that were well supported, all genes were

combined and analyzed in a single matrix partitioned by gene.

Analysis of divergence times.—The program *BEAST version 1.6.0 (Drummond and

Rambaut 2007) was used to estimate the times of divergence among P. difficilis, P. nasutus, P.

truei, and P. attwateri. Several representative species from clade VI of the tribe

Reithrodontomyini (Miller and Engstrom 2008) were included in the *BEAST analysis, as

follows: Habromys ixtlani, Megadontomys thomasi, Neotomodon alstoni, Onychomys arenicola,

O. leucogaster, Osgoodomys banderanus, Peromyscus crinitus, P. eremicus, P. levipes, P.

maniculatus, P. melanophrys, and P. melanotis (Appendix 4.2). Isthmomys pirrensis and

Reithrodontomys sumichrasti were included as outgroups. A Yule tree prior was used, implicitly

considering the gene tree to represent the species tree, and the HKY+I+G model was selected as

the substitution model. A relaxed clock with a lognormal distribution allowing rate variation

among sites was used. Chains were run for 107 generations, sampling the parameter every 103

generations. Convergence statistics were checked for effective sample sizes using Tracer version

1.5 (Rambaut and Drummond 2007). Consensus trees were generated from the resulting trees

using TreeAnnotator version 1.6.0 (Rambaut and Drummond 2009) after elimination of 25% as

burn-in.

Two fossil-based dates were used to calibrate the *BEAST analysis. The first documents

the separation of Onychomys from the main stock of Peromyscus in the early Pliocene (ca. 5.3 to

3.6 mya; Korth 1994). The second sets the minimum age for P. eremicus in late Pleistocene (ca.

126,000 to 5,000 years ago; Martin 1968). To account for uncertainty in the fossil-based

calibrations, the fossil-based dates were modeled as lognormal distributions rather than point

calibrations (Ho and Phillips 2009).

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4.3 RESULTS

Phylogenetic analyses.—A total of 1,140 base pairs (bp) of Cytb, 820 bp of 12S, 654 bp of

GHR, and 775 bp of IRBP were sequenced, yielding 3,389 bp for use in analyses. Cytb

divergence values (Table 4.1) show two well differentiated units within P. difficilis, one

distributed in the Sierra Madre Oriental and Mexican Plateau (the subspecies petricola and

difficilis, respectively; Fig. 4.1) and the other occupying the eastern end of the TMVB southward

into Oaxaca (saxicola, amplus, and felipensis, respectively). These clades show an average Cytb

divergence of 6.7%, whereas within-clade divergence is only 0.7% within the northern clade and

1.2% within the southern clade. Average Cytb divergence between P. nasutus and the northern

clade of P. difficilis is 6.85%, and divergence between P. nasutus and the southern clade of P.

difficilis is 7.87%. Finally, P. attwateri shows an average divergence of 7.9% from the P.

nasutus + P. difficilis clade, and P. truei shows an average of 13.2% Cytb divergence from all

other taxa included in Table 4.1.

ML and BI analyses of the Cytb sequences yielded very similar trees that showed only

minor differences near the tips of the branches. The ML tree (Fig. 4.2; BI tree available on

request) showed subdivision of P. difficilis into the same northern and southern clades evident

from the distance data (Table 4.1). These two clades were linked strongly with P. nasutus (100%

bootstrap support and 1.0 posterior probability), but relationships among these three lineages

were unresolved (Fig. 4.2). ML and BI analyses of 12S generated trees that were concordant with

the Cytb tree, but with lower branch support values (trees not shown but available on

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Table 4.1. Percent sequence divergence in the cytochrome-b gene (Kimura 2-parameter model) among eight taxa of

Peromyscus, including the five currently recognized subspecies of P. difficilis. Geographic distributions of the two major

clades within P. difficilis are indicated. TMVB = Trans-Mexico Volcanic Belt.

Mexican Plateau and Eastern end of TMVB

Sierra Madre Oriental southward into Oaxaca

P. truei P. attwateri P. nasutus P. d. petricola P. d. difficilis P. d. saxicola P. d. amplus P. d. felipensis

P. truei – 13.2 13.0 12.6 13.2 13.4 13.6 13.1

P. attwateri – 7.9 7.4 7.2 8.1 8.6 8.2

P. nasutus – 6.9 6.8 7.6 7.7 8.3

P. d. petricola – 0.7 6.8 6.7 6.3

P. d. difficilis – 6.9 6.8 6.7

P. d. saxicola – 1.5 1.6

P. d. amplus – 0.4

P. d. felipensis –

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Fig. 4.2.—Phylogenetic relationships among the 5 subspecies of P. difficilis

(sensu Durish et al. 2004), P. nasutus, and allied taxa based on a maximum likelihood (ML) analysis of cytochrome-b sequences. A Bayesian analysis of the same data yielded a tree with identical topology. Locality numbers are indicated for ingroup taxa and refer to mapped localities in Fig. 4.1 and localities listed in Appendix 4.1. Numbers above branches indicate ML bootstrap support, and numbers below branches indicate Bayesian posterior probabilities. The subspecies amplus, felipensis, and saxicola, formerly assigned to P. difficilis, are recognized as junior synonyms of P. felipensis (Merriam 1898) in this study.

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request). Finally, ML and BI analyses of the two nuclear genes yielded trees with well-

supported resolution only at the node defining the northern clade of P. difficilis and at the

node defining the southern clade of P. difficilis (trees not shown but available on request).

ML and BI analyses of the combined data set (partitioned by gene) yielded

identical and well-resolved trees (Fig. 4.3). Again I observed the 3 clades evident in the

Cytb analysis (northern P. difficilis, southern P. difficilis, and P. nasutus; Fig. 4.2), but

the trichotomy that was unresolved in the Cytb analysis is now resolved to show P.

nasutus sister to the northern clade of P. difficilis (90% bootstrap support and 0.94

posterior probability).

Estimates of divergence time.—The tree obtained in the *BEAST analysis (Fig.

4.4) showed the same northern and southern P. difficilis clades evident in the Cytb (Fig.

4.2) and 4-gene (Fig. 4.3) analyses. However, the *BEAST tree differed from the other

trees in showing P. nasutus sister to P. attwateri. The Tracer analysis in *BEAST

confirmed a high effective sample size (> 3,000) for all parameters. All mean estimates

of divergence time for the P. attwateri, P. difficilis, and P. nasutus clades were < 1.0 mya

(Pleistocene). The split between the northern and southern clades of P. difficilis was

estimated at approximately 0.7 mya.

4.4 DISCUSSION

P. difficilis is composed of two genetically well-differentiated lineages, one found

in the Sierra Madre Oriental and Mexican Plateau (the subspecies petricola and difficilis,

respectively; Fig. 4.1) and the other occupying the eastern end of the TMVB southward

into Oaxaca (saxicola, amplus, and felipensis, respectively). This subdivision of P.

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Fig. 4.3.—Phylogenetic relationships among selected specimens of P. difficilis and P. nasutus based on a partitioned maximum likelihood (ML) analysis of two mitochondrial genes (Cytb and 12S) and 2 nuclear genes (GHR and IRBP). A Bayesian analysis of the same data yielded a tree with identical topology. Locality numbers are indicated for ingroup taxa and refer to mapped localities in Fig. 4.1 and localities listed in Appendix 4.1. Numbers above branches indicate ML bootstrap support, and numbers below branches indicate Bayesian posterior probabilities. The subspecies amplus, felipensis, and saxicola, formerly assigned to P. difficilis, are recognized as junior synonyms of P. felipensis (Merriam 1898) in this study. Original subspecies assignments are indicated in parentheses: a = amplus, d = difficilis, f = felipensis, p = petricola, s = saxicola.

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difficilis into two divergent clades is consistent with the findings of Durish et al. (2004),

although they did not include samples of P. d. petricola or P. d. felipensis in their study.

The 4-gene partitioned analysis (Fig. 4.3) shows a sister relationship between the

northern clade of P. difficilis and P. nasutus, with the southern clade of P. difficilis

outside this group.

Specimens from Durango originally identified as P. difficilis by Avise et al.

(1979) and Durish et al. (2004) were represented by the specimen from locality 5 in this

study (Fig. 4.1). Durish et al. (2004) reported that their specimens from Durango were

more closely related to P. nasutus than to P. difficilis and my results concur in showing

strong evidence (Fig. 4.2) that these specimens from Durango represent a southward

range extension of P. nasutus into the Sierra Madre Occidental of Mexico. This finding

also is consistent with the chromosomal findings of Robbins and Baker (1981), who

reported a P. nasutus-like karyotype from Peromyscus specimens from Durango. A

detailed reassessment of the distributions of P. difficilis and P. nasutus in northern

Mexico is needed.

Average Cytb genetic distances between the three major clades of Peromyscus

identified in this analysis (P. nasutus and the northern and southern clades of P. difficilis)

range from 6.7% to 7.9% (Table 4.1) and compare favorably with Cytb distances

measured between sister species of Peromyscus reported in previous studies (5.8 –7.7%;

Baker and Bradley 2006).

Biogeographical considerations.— Recent distributional, paleontological, and

molecular studies of Mexican mammals have shown that most phyletic diversification in

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desert-dwelling lineages took place during Miocene and Pliocene times (Hafner and

Riddle 1997; Riddle 1995; Riddle et al. 2000; Vrba, 1992). In contrast, phyletic

diversification in lineages of mammals inhabiting the highlands of the TMVB, the Sierra

Madre Oriental, and the Sierra Madre Occidental seems to have been influenced more

strongly by Pleistocene climate cycles (Findley 1969; León-Paniagua et al. 2007; Orr

1960). Because populations of mammals currently recognized as P. difficilis inhabit both

xeric habitats at mid-elevations and mesic habitats at higher elevations in Mexico, it was

impossible to predict prior to this study whether phyletic diversification in P. difficilis

was influenced more by Pleistocene or pre-Pleistocene climatic and geologic events.

The divergence times estimated in this study (Fig. 4.4) suggest that the two

lineages of P. difficilis diverged during the Pleistocene. If so, the major mountain chains

of northern Mexico (TMVB, Sierra Madre Oriental, and Sierra Madre Occidental),

instead of acting as a barrier to range expansion in rodents, may have acted as a dispersal

corridor connecting the three mountain ranges during warm interglacial cycles. Repeated

expansion and contraction of the geographic range of P. difficilis may have resulted in

isolation of the southern lineage in the eastern TMVB promoting the genetic

differentiation we see today between the northern and southern clades of P. difficilis. The

same or a similar process has been hypothesized to explain phyletic diversification in

other peromyscine taxa of Mexico, including Reithrodontomys, Habromys, the P. aztecus

species group, and the P. maniculatus complex (Arellano et al 2005; Avise et al. 1979;

Bradley et al. 2004a, 2004b; Dawson 2005; Hibbard 1968; Kalkvik et al. 2011, León-

Paniagua et al. 2007; Sullivan et al. 1997, 2000).

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Fig. 4.4.—Estimates of divergence times in P. difficilis, P. felipensis, and close relatives calculated from *BEAST analysis. Numbers at nodes are mean divergence dates, and bars show 95% credibility intervals. Additional outgroup taxa (listed in the text) were included in the analysis, which used two dated fossils to calibrate the tree.

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Taxonomic conclusions.—P. difficilis was originally described by Allen (1891)

from a specimen taken in Sierra de Valparaiso, Zacatecas. Seven years later, P. felipensis

was described (Merriam 1898) from specimens collected in Cerro San Felipe, near

Oaxaca City, and in 1909, Osgood placed felipensis in synonymy under P. difficilis.

Herein, I return P. felipensis to full species status based on phylogenetic analysis of

mitochondrial and nuclear genes, estimates of divergence time from its sister lineage

(most likely P. nasutus), geographic distribution, and morphology. Synonymies of P.

difficilis and P. felipensis follow.

Peromyscus difficilis (J. A. Allen, 1891)

Zacatecan deermouse

Vesperimus difficilis J. A. Allen, 1891:518. Type locality “Sierra de Valparaiso,

Zacatecas, Mexico.”

[Peromyscus] difficilis Trouessart, 1897:518. First use of current name combination.

P. d. petricola Hoffmeister and de la Torre, 1959:167–168. Type locality “12 mi. E. San

Antonio de las Alazanas, 9000 ft., Coahuila, Mexico.”

Geographic range.— Peromyscus difficilis is endemic to Mexico and is found

throughout the Sierra Madre Oriental and adjacent mountain ranges from southwestern

Chihuahua and southeastern Coahuila, southward through the low hills of Durango and

Zacatecas. This species probably occurs in the Sierra Madre Occidental as well. Its

range continues southward from Zacatecas into parts of San Luis Potosi and mountainous

regions of Guanajuato (Fig. 4.1; Fernández et al. 2010; Hall 1981; Hoffmeister and de la

Torre 1961; Osgood 1909).

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Description.—Size medium to large for Peromyscus; total length 230 mm in adult

males (range 223–237 mm); 237 mm in adult females (range 227–250 mm); tail length

128.6–132.5 mm. Fur dull and dark dorsally with little or no ochraceous, and heavily

overlaid with black. Auditory bullae greatly inflated; brain case and interorbital region

broad; skull, nasals, and toothrow long (Hoffmeister and de la Torre 1961; Fernández et

al. 2010).

Comments.—The currently recognized subspecies P. d. difficilis and P. d.

petricola were not recognized as monophyletic clades in this analysis (Fig. 4.2), so until

additional research on subspecific variation within P. difficilis is completed, no formal

subspecies are recognized in this study.

Peromyscus felipensis Merriam, 1898

Southern rock deermouse

P. felipensis Merriam, 1898:122–123. Type locality “Cerro San Felipe, Oaxaca, Mexico

(alt. 10,200 ft).”

P. amplus Osgood, 1904:62–63. Type locality “[San Juan Bautista] Coixtlahuaca,

Oaxaca, Mexico.”

P. difficilis saxicola Hoffmeister and de la Torre, 1959:168–169. Type locality

“Cadereyta, 2100 meters, Querétaro, México.”

Geographic range.— P. felipensis occurs in Querétaro, northern Hidalgo, the

mountains of southern Hidalgo, northern México state, Tlaxcala, Puebla, westcentral

Veracruz, and northcentral Oaxaca (Goodwin 1954; Hall 1981; Hoffmeister and de la

Torre 1961; Osgood 1904, 1909; Villa-Ramírez 1953; Fig. 4.1).

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Description.—Size medium to large for Peromyscus; total length of 10 adult

topotypes (including both sexes) 241.5 mm (range 225–248 mm); tail length 118–132

mm. Fur glossy and unicolored dorsally; dorsal coloration ochraceous, brownish or

reddish in northern populations to blackish in the southernmost populations. Southern

populations of P. felipensis (the former subspecies P. d. amplus and P. d. felipensis) have

a narrower brain case and interorbital region compared to P. difficilis; northern

populations (the former P. d. saxicola) are smaller in overall size than southern

populations (Hoffmeister and de la Torre 1959, 1961; Fernández et al. 2010).

Comments.— The currently recognized subspecies P. d. felipensis, P. d. amplus,

and P. d. saxicola were not recognized as monophyletic clades in this analysis (Fig. 4.2),

so until additional research on subspecific variation within P. felipensis is completed, no

formal subspecies are recognized in this study.


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CHAPTER 5 MOLECULAR SYSTEMATICS AND BIOGEOGRAPHY OF THE MEXICAN ENDEMIC

KANGAROO RAT, DIPODOMYS PHILLIPSII (RODENTIA: HETEROMYIDAE) JESÚS A. FERNÁNDEZ*, FERNANDO A. CERVANTES, AND MARK S. HAFNER

Department of Biological Sciences and Museum of Natural Science, Louisiana State

University, Baton Rouge, Louisiana 70803 (JAF, MSH)

Departamento de Zoología, Instituto de Biología, Universidad Nacional Autónoma de

México, Apartado Postal 70-245, México, D. F., 04510, México (FAC, JAF)

5.1 INTRODUCTION

Phillips’ kangaroo rat (Dipodomys phillipsii) is a threatened, endemic rodent of

Mexico (Luiselli 2002). Only a few taxonomic studies (all based on morphology) have

focused on this species, and the systematic status of the four currently recognized

subspecies is unknown. Dipodomys phillipsii is a medium sized kangaroo rat (total

length 230–304 mm) with four toes on the hind limbs, a relatively short tail with black

stripes united at the distal one-third of the tail, and a whitish tuft on the tip of the tail

(Hall 1981; Jones and Genoways 1975; Schmidly et al. 1993). This species is distributed

along a narrow band of dry, semi-desert habitat extending from southeastern Durango to

the Tehuacán–Cuicatlán Valley of southern Puebla and northern Oaxaca (McMahon

1979; Schmidly et al. 1993; Fig. 5.1). The current taxonomy of D. phillipsii, which is

based on morphometric and qualitative morphological traits, is as follows: D. p. phillipsii

is found mainly in the Valley of México, in the Distrito Federal, and in the states of

México and Hidalgo; D. p. ornatus is known from Durango, Zacatecas, Aguascalientes,

San Luis

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Fig. 5.1.—Map of central Mexico modified from Hall (1981) and Jones and Genoways (1975) showing the geographic distribution, sampling localities, and current subspecies of Phillips’ kangaroo rat, Dipodomys phillipsii, in Mexico. Localities are listed in Appendix 5.1.

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Potosí, Querétaro, Guanajuato, and Jalisco; D. p. perotensis is found in eastern Tlaxcala,

central-western Veracruz, and Puebla; and D. p. oaxacae is known from southern Puebla

and northern Oaxaca (Hall 1981; Jones and Genoways 1975; Wilson and Reeder 2005;

Fig. 5.1).

Dipodomys phillipsii was described by Gray (1841) based on one specimen from

near Real del Monte, Hidalgo. Some 50 years later, Merriam (1894) collected specimens

in the northern portion of the species range (Berriozábal, Zacatecas) and in the Oriental

Basin (Cuenca Oriental) in central Mexico (Perote, Veracruz) and described D. ornatus

and D. perotensis, respectively. Merriam (1894) emphasized differences in size and fur

color between these taxa, and he noted that the skulls were very similar, showing only

variation in proportions. Davis (1944) compared individuals of D. phillipsii and D.

perotensis and was unable to see the differences that Merriam (1894) reported. As a

result, Davis (1944) suggested that D. perotensis be regarded as a subspecies of D.

phillipsii. Hooper (1947) trapped specimens of D. phillipsii in Teotitlán, Oaxaca, and

described D. p. oaxacae based also on size and fur color, recognizing the cranial

similarities between his specimens and those of the geographically nearest subspecies, D.

p. perotensis.

Genoways and Jones (1971) analyzed specimens from several populations

throughout the range of D. phillipsii and D. ornatus and suggested that a suite of

morphological characters, including mastoid breadth, maxillar breadth, interorbital

breadth, body size, and pelage coloration could be used to distinguish among the

subspecies of D. phillipsii. They reported low levels of within-population variation in

cranial dimensions, but high levels of within-population variation in pelage coloration.

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Because they could find no consistent morphological difference between D. ornatus and

D. phillipsii, they reduced D. ornatus to subspecific status under D. phillipsii. Genoways

and Jones (1971) generated a phenogram based on a morphological distance matrix but

found no congruence between the phenogram and the geographic locations of their

samples.

Evolutionary relationships among the geographic subunits of D. phillipsii have

remained controversial since publication of the study by Genoways and Jones (1971). In

this study we use mitochondrial and nuclear DNA sequence data to test species

monophyly and assess phylogenetic relationships within D. phillipsii. To place our

results in a larger, historical biogeographical context, we estimate the time of major

divergence events within D. phillipsii and among its close relatives.


Sampling, amplification, and sequencing.—Samples of D. phillipsii tissue or ear

clips were either collected in the field under the authority of the Mexican collecting

permit FAUT-0002 issued to FAC or donated to us by museums (Appendix 5.1). We

sequenced specimens from 17 localities in northern and central Mexico, including six

localities of D. p. ornatus (n = 7 individuals), one locality of D. p. phillipsii (n = 1); eight

localities of D. p. perotensis (n = 8), and two localities of D. p. oaxacae (n = 5; Fig. 5.1;

Appendix 5.1). Outgroup sequences for D. agilis, D. californicus, D. compactus, D.

deserti, D. elator, D. heermanni, D. merriami, D. microps, D. nelsoni, D. ordii, D.

panamintinus, D. spectabilis, and Heteromys irroratus (use of Heteromys follows the

taxonomy of Hafner et al. 2007) were obtained from Genbank or generated for this study

(Appendix 5.1). The collection and processing of samples were undertaken following the

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guidelines of the American Society of Mammalogists for use of wild animals in research

(Kelt et al. 2010; Sikes et al. 2011).

Total genomic DNA was extracted from tissue using a standard phenol-

chloroform protocol (Darbre 2001; Saunders and Parkes 1999) and a commercial kit

(DNeasy Blood and Tissue Kit; Qiagen Inc., Valencia, California). Two nuclear and two

mitochondrial genes were sequenced. The nuclear genes code for the growth hormone

receptor (GHR) and the interphotoreceptor retinoid-binding protein (IRBP), and the

mitochondrial genes code for a subunit of the respiratory chain protein Cytochrome-b

(Cytb) and the 12S ribosomal RNA gene (12S). Sequences were amplified by PCR (Saiki

et al. 1988) using universal primers developed for rodents (Appendix 5.2). The following

PCR parameters were used to amplify both mitochondrial genes from fresh tissue

samples: initial denaturation at 95°C for 2 min, followed by 27 cycles of denaturation at

95°C for 1 min, annealing at 49°C for 1 min, and extension at 72°C for 2 min, with a

final extension at 72°C for 7 min (Mantooth et al. 2000). PCR parameters for

mitochondrial genes obtained from dry skin clips were: denaturation at 94°C for 1 min,

followed by 35 cycles at 94°C for 1 min, 50°C for 1 min, and 72°C for 1 min, with a final

extension at 72°C for 5 min. PCR parameters for the nuclear gene GHR were:

denaturation at 94°C for 5 min, followed by 34 cycles at 94°C for 15 s, 60°C for 1 min,

and 72°C for 1.5 min, with a final extension at 72°C for 10 min. PCR amplification of

the IRBP gene was performed as follows: denaturation at 95°C for 10 min, followed by

27 cycles at 95°C for 25 s, 58°C for 20 s, and 72°C for 1 min, with a final extension at

72°C for 10 min. Amplifications were performed with 200 ng of DNA in a total volume

of 25 µL. Agarose (2%) gels were used to visualize amplified products. PCR products

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were purified using a QIAquick PCR Purification Kit (Qiagen Inc.) with either

Polyethylene Glycol or ExoSAP-IT (Affymetrix, Santa Clara, California). DNA

sequencing was performed for both light and heavy strands with a Big Dye Terminator

v1.1, v3.1 in an automated 3100 Genetic Analyzer (Applied Biosystems, Foster City,

California) at the Instituto de Biología, Universidad Nacional Autónoma de México and

at the Museum of Natural Science, Louisiana State University.

Distance analysis.—Editing and alignment of sequences and matrix manipulations

were performed in Sequencher 4.7 (Gene Codes Corporation, Ann Arbor, Michigan),

MacClade (Maddison and Maddison 2000), and Mesquite (Maddison and Maddison

2010). Sequences were verified manually, and authenticity of the gene was confirmed by

amino acid translation and BLAST searches in Genbank


To enable comparison of our results with those of previous studies, average

uncorrected sequence divergence values for the Cytb gene were corrected using the

Kimura 2-parameter substitution model (Kimura 1980) in PAUP* (Phylogenetic Analysis

using Parsimony, version 4.0b, Swofford 2003). This model has been used widely in

studies of Cytb variation in mammals (Bradley and Baker 2001; Honeycutt et al. 1995).

We also calculated average maximum and minimum divergence values for each of the

main clades in our study. To detect phylogenetic “noise,” saturation analyses for 3rd

codon positions were performed as described by Griffiths (1997). To explore the effects

of 3rd position substitutions in phylogenetic reconstruction, we ran maximum likelihood

(ML) analyses including all 3rd position substitutions, omitting all 3rd position

substitutions, and excluding only 3rd position transitions.

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Phylogenetic analyses.—Our preliminary phylogenetic analysis included all

specimens (n = 21 individuals of D. phillipsii) for which we had partial Cytb sequences

(1,022 base pairs, bp). To test monophyly of D. phillipsii, we included sequences from

the closely related species D. merriami (n = 2), and D. elator (n = 2) as well as one

sequence from several other species of the genus Dipodomys and Heteromys irroratus

(Appendix 5.1; Alexander and Riddle 2005; Hafner et al. 2007). We selected five

individuals from the Trans-Mexico Volcanic Belt (TMVB) clade and five individuals

from the Mexican Plateau (MP) clade and sequenced these individuals for portions of the

12S, GHR, and IRBP genes. A partition homogeneity test in PAUP* (Swofford 2003;

1,000 replicates, P = 0.68) showed no significant heterogeneity among individual data

sets, so the mitochondrial and nuclear data sets were concatenated for the remaining

analyses, which were carried out with at least five representatives of each major clade.

Variable nucleotide positions were considered unordered, discrete characters with four

possible states (A, C, G, T). Phylogenetic analyses were carried out under a ML

framework in PAUP* or PhyMl 3.0 (Guindon and Gascuel 2003). Analyses based on

Bayesian inference (BI) were conducted using MrBayes (version 3.2-cvs; Huelsenbeck

and Ronquist 2001; Ronquist and Huelsenbeck 2003). The best-fit models for ML and

BI analyses were evaluated using the Akaike Information Criterion and the programs

jModeltest 0.1.1 (Akaike 1973; Guindon and Gascuel 2003; Posada 2008), MrModeltest

(version 2.2; Nylander 2004), or Modeltest (version 3.7; Posada and Buckley 2004;

Posada and Crandall 1998). The GTR+I+G model was selected for the mitochondrial

sequences, and the HKY+G model was selected for the nuclear sequences. ML clade

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support was assessed with 500 bootstrap (Felsenstein 1985) replicates in PAUP*, and

clade support in BI analyses was evaluated using posterior probabilities.

Tree searches in the ML analyses were performed with the starting trees obtained

via 100 random, stepwise additions followed by tree-bisection-reconnection (TBR)

branch swapping. To test preliminary results that showed a paraphyletic D. phillipsii, we

conducted an additional ML analysis forcing D. phillipsii to be monophyletic and

compared this tree to the unconstrained tree using the Kishino-Hasegawa test (Kishino

and Hasegawa 1989; Schmidt 2009). In the BI analyses, best-fit models were applied to

each data partition with unlinked parameters and allowing rate variation. The Metropolis

Markov Chain Monte Carlo analysis consisted of two independent runs of 10 x 106

generations in which trees were sampled every 103 generations, resulting in 104 samples

for each run. The first 103 trees of each run were discarded as burn-in, and a majority-

rule consensus tree was constructed using the final 1.8 x 103 trees. The analysis was

stopped when the average standard deviation of split frequencies approached zero, and

convergence also was assessed using Tracer 1.5 (Rambaut and Drummond 2007). The

combined data set was analyzed as partitioned (genes and model parameters) to allow for

independent convergence on optimal values for each component (Ronquist and

Huelsenbeck 2003).

The data sets were partitioned into mitochondrial versus nuclear genes, Cytb

versus 12S genes, and GHR versus IRBP genes. Whenever possible, each gene fragment

was analyzed separately to evaluate potential conflict among gene trees. Nodes were

considered well supported if there was > 80% bootstrap support in ML analyses or > 95%

posterior probability in BI analyses.

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Divergence time analysis.—We used the program *BEAST version 1.6.0

(Drummond and Rambaut 2007) to generate an estimate of the timing of divergence of

the basal polytomy involving D. phillipsii and its close relatives. The *BEAST analyses

included one Cytb sequence for each of the ingroup and outgroup species. A Yule tree

prior was used, implicitly considering the gene tree to represent the species tree. The

absence of dated fossils of D. phillipsii prevented us from using fossil-based time

calibrations, so instead we used global substitution rates to calibrate our analyses

(Drummond et al. 2007). Substitution rates calculated exclusively for the genus

Dipodomys are not available in the literature, so we used three published estimates for

other rodent species. The first substitution rate estimate was 4.0% per 1.0 x 106 years

calculated for the split between the pocket gopher genera Pappogeomys and Cratogeomys

(DeWalt et al. 1993). Pappogeomys and Cratogeomys belong to the family Geomyidae,

which is sister to the Heteromyidae. The second rate estimate we used was 4.78% per 1.0

x 106 years generated for the more distantly related house mouse Mus musculus, and the

third was 5.23% per 1.0 x 106 years generated for the brown rat Rattus norvegicus

(Bininda-Emonds 2007). We excluded the frequently used rate of 2% per 1.0 x 106 years

because it has been shown recently to be an underestimate of actual rates (Nabholz et al.

2009). We used a relaxed clock with a lognormal distribution allowing rate variation

among sites. Chains were run for 107 generations, sampling the parameter every 103

generations. Convergence statistics were checked for effective sample sizes using Tracer

version 1.5 (Rambaut and Drummond 2007). Consensus trees were generated from the

resulting 104 trees using TreeAnnotator version 1.6.0 (Rambaut and Drummond 2009)

after elimination of 10% as burn-in.

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5.3 RESULTS

Distance and phylogenetic analyses.—We sequenced 1,022 bp of the Cytb gene,

796 bp of the 12S gene, 820 bp of GHR, and 470 bp of IRBP, yielding a total of 3,110 bp

for use in the analyses. Initial ML and BI analyses using only Cytb sequences from the

21 individuals generated trees that differed only in relationships among terminal taxa

(Fig. 5.2). In these trees, D. phillipsii is divided into two well-supported clades, the MP

clade, which contains all specimens currently assigned to D. p. ornatus (localities 1–6 in

Fig.5.1) and the TMVB clade, which contains all other D. phillipsii samples in this

analysis (localities 7–17 in Fig. 5.1). Initial analyses grouped the MP clade of D.

phillipsii with D. elator, suggesting paraphyly of D. phillipsii, but support for this group

was weak, causing us to collapse the two D. phillipsii clades plus D. elator and D.

merriami into a strongly supported four-way polytomy (Fig. 5.2). The Kishino-Hasegawa

test showed the tree with the MP clade of D. phillipsii sister to D. elator to be a slightly

better fit to the sequence data than the tree forcing D. phillipsii to be monophyletic (-lnL

= 3529.6743 and 3673.9037, respectively), but the difference was not statistically

significant (P > 0.285; 1-tailed test).

The analysis of substitutional saturation showed Cytb 3rd position transitions to be

saturated, so ML analyses were re-run twice, once excluding all 3rd position substitutions

and again excluding only 3rd position transitions. Trees resulting from both analyses

showed the same two clades of D. phillipsii (as in Fig. 5.22) but, strong bootstrap support

for relationships within the D. phillipsii + D. elator + D. merriami clade was lacking.

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Representatives of the D. phillipsii TMVB clade (five individuals from four

localities), D. phillipsii MP clade (five individuals from three localities), and two

representatives each of D.

Fig. 5.2.—Phylogram showing placement of Phillips’ kangaroo rat, D. phillipsii, within the genus Dipodomys based on a maximum likelihood analysis of Cytochrome-b sequences. Numbers before state names refer to localities mapped in Fig. 5.1. The gray dot corresponds to the divergence time of approximately 4 mya estimated using *Beast. Numbers above branches are Bayesian posterior probabilities and numbers below branches are ML bootstrap support values. Black dots represent nodes with Bayesian posterior probabilities ! 0.95 and ML bootstrap values ! 85. A Bayesian analysis of the same data yielded a tree that differed only in minor rearrangements of the terminal branches.

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D. elator and D. merriami were sequenced for the other three genes (12S, GHR,

and IRBP). ML and BI analyses of the 12S sequences showed basically the same tree as

in Fig. 5.2, with only minor differences near the tips of the branches (tree not shown).

Separate analyses of the nuclear genes showed little or no resolution at the shallow nodes

but recovered the same major clades seen in the analyses of the mitochondrial genes (Fig.

5.2). The partition homogeneity test showed no significant heterogeneity among

individual data sets, so the mitochondrial and nuclear sequences were concatenated for all

remaining analyses.

Bayesian analyses of the concatenated data set using the data partitions defined

earlier showed the same subdivision of D. phillipsii into two well-defined clades (MP and

TMVB) and the same four-way polytomy involving the two clades of D. phillipsii, D.

elator, and D. merriami (Fig. 5.3). As in the other analyses, branch suppport values were

generally higher for the basal nodes and slightly lower for some of the shallower nodes in

the trees.

Cytb divergence values (Table 5.1) show the two lineages of D. phillipsii (MP and

TMVB) to be approximately 9.8% genetically divergent. In contrast, average within-

clade divergence was only 1.5% for the MP clade and 1.2% for the TMVB clade.

Genetic distances between the two D. phillipsii clades and D. elator ranged from 11.3%

to 11.4% and distances between the two D. phillipsii clades and D. merriami ranged from

11.6% to 12.3%.

Estimates of divergence time.— Except for minor rearrangement of terminal taxa,

the tree obtained in the *BEAST analysis was identical to those generated in the ML and

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Fig. 5.3.—Phylogram showing placement of Phillips’ kangaroo rat, D. phillipsii, within allied species of Dipodomys based on a partitioned maximum likelihood (ML) analysis of two mitochondrial genes (Cytb and 12S) and two nuclear genes (GHR and IRBP). Numbers before state names refer to localities mapped in Fig. 5.1. Numbers above branches are Bayesian posterior probabilities and numbers below branches are ML bootstrap support values. A Bayesian analysis of the same data yielded a tree with identical topology.

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Table 5.1.—Mean percent cytochrome-b sequence divergence values (Kimura 2-parameter model) and ranges (in parentheses) between and within the two major clades of Dipodomys phillipsii (Trans-Mexico Volcanic Belt, n = 14; Mexican Plateau, n = 7), D. elator (n = 2), and D. merriami (n = 2).

Dipodomys phillipsii

Trans-Mexico Volcanic Belt

Mexican Plateau

D. elator

D. merriami

Trans-Mexico Volcanic Belt

1.2

(0.7–2.2)

9.8

(9.2–10.2)

11.4

(10.8–12.0)

12.3

(12.0–12.8) Mexican Plateau

1.5

(0.6–4.7)

11.3

(10.5–12.0)

11.6

(11.1–12.6)

D. elator - 12.7

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BI analyses (Fig. 5.2). The Tracer analysis in *BEAST confirmed a high effective

sample size (> 3,000) for all parameters. Estimates of divergence time (95% highest

posterior density interval) for the node that marks the separation of the two D. phillipsii

clades, D. elator, and D. merriami (gray dot in Fig. 5.2) were 2.86–6.46 mya when the

Pappogeomys–Cratogeomys rate calibration was used, 2.74–6.16 mya when the Mus

musculus calibration was used, and 2.83–6.24 mya when the Rattus norvegicus

calibration was used. The mean node age based on these three estimates ranged from

4.19 to 4.29 mya.

5.4 DISCUSSION

All phylogenetic analyses (ML, BI, and *BEAST) of the concatenated sequence

data recovered a strongly supported clade composed of four genetically well-

differentiated lineages: D. phillipsii (TMVB), D. phillipsii (MP), D. elator, and D.

merriami (Fig. 5.3). The composition of this clade is consistent with the molecular

findings of Mantooth et al. (2000) and Hafner et al. (2007), although neither of those

studies included a sample of D. phillipsii from the TMVB and Hafner et al. (2007) did

not include a sample of D. elator. Our trees (Figs. 5.2 and 5.3) show D. ordii and D.

compactus to be sister to the above-mentioned clade, which is consistent with the

findings of Mantooth et al. (2000; although that study did not include D. compactus) and

does not conflict with the findings of Hafner et al. (2007) if 1 branch (with questionable

support in that study: maximum parsimony bootstrap < 75%, ML bootstrap = 77%, and

BI posterior probability = 1.0) that lead to D. ordii and D. compactus as sister to a clade

formed by D. panamintinus, D. heermanni, D. agilis, D. microps, and D. californicus is

collapsed.

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Although phyletic resolution within the phillipsii + elator + merriami clade was

poor, certain of our analyses suggested (with only weak branch support) that the two D.

phillipsii clades may not be sister lineages. In contrast, the smallest Cytb genetic distance

between clades in Table 5.1 (9.8%) was between the two D. phillipsii clades, thus

supporting their sister status, followed by D. phillipsii (MP clade) and D. elator (11.3%).

Cytb distance calculations should be viewed with caution, however, because it is well

known that distance values can be misleading when substitution rates are heterogeneous

among lineages (Rzhetsky and Sitnikova 1996), and our test for homogeneous

substitution rates in our dataset led us to reject the assumption of clock-like behavior

(PAUP*; P = 0.0001). It also is important to note that accuracy of distance values lessen

when they exceed 10% (Jin and Nei 1990), which is the case in most of our inter-clade

comparisons in Table 5.1.

Although our results are ambiguous with respect to the question of D. phillipsii

monophyly, this question may be moot considering that the two lineages within what is

now considered D. phillipsii (MP and TMVB) approach 10% Cytb sequence divergence.

According to our estimates, divergence of these two lineages occurred approximately 4

mya, roughly contemporaneous with divergence of D. merriami and D. elator. We were

unable to resolve this four-way polytomy despite using a large number of base pairs from

multiple genes (both mitochondrial and nuclear) and multiple phylogenetic approaches.

We interpret our inability to resolve the branching sequence in this clade as resulting

from near-simultaneous and rapid divergence of these four clades such that few, if any,

molecular changes that would reveal the order in which these taxa diverged remain to be

discovered today (Lanyon 1988; Spradling et al. 2004).

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Even if future studies show the two clades of D. phillipsii to be sister lineages,

this finding would be irrelevant to the question of whether these lineages are genetically

isolated and may represent distinct species. Although we are reluctant to base species-

level designations primarily on level of Cytb divergence, the level of Cytb sequence

divergence between these lineages (9.8%) is not trivial—it is well above the mean level

of divergence (7.3%) measured between 19 pairs of sister species of rodents reported by

Baker and Bradley (2006) in their discussion of the genetic species concept.

Biogeographical considerations.—It has been proposed that the North American

mammalian biota is young by geological standards and that most species-level

diversification within mammals was largely in response to glacial and interglacial

changes during the Pleistocene (Findley 1969; Orr 1960; Schmidly et al. 1993).

However, new distributional, paleontological, and molecular evidence has shown that

most phyletic diversification of the mammals of North American deserts took place

during Miocene and Pliocene times (Hafner and Riddle 1997; Riddle 1995; Riddle et al.

2000; Vrba, 1992). Our time estimates suggest that diversification of the two lineages of

D. phillipsii also occurred prior to the major climatic fluctuations of the Pleistocene, and

we contend that the Trans-Mexico Volcanic Belt, which is widely recognized as a

generator of biological diversity, may have played a causal role in this divergence. The

long and slow rise of this complex of mountain ranges that extends nearly from the

Pacific ocean to the Gulf of Mexico has been implicated as a causal force in phyletic

diversification at multiple taxonomic levels in many organisms, frequently resulting in

genetically isolated lineages north and south of the TMVB (Devitt 2006; Douglas et al.

2010; Esteva et al. 2010; Mateos et al. 2002; McCormack et al. 2008; Mulcahy and

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Mendelson 2000; Sosa et al. 2009; Zink and Blackwell 1998). We believe that the two

lineages of D. phillipsii are yet another example of the powerful isolating force of the

TMVB.

Researchers have proposed multiple hypotheses for diversification of D. elator,

D. merriami, and D. phillipsii, most assuming that this occurred in the Mexican Plateau

and northward (Alexander and Riddle 2005; Lidicker 1960; Mantooth et al. 2000;

Morafka 1977; Savage 1960). Most of these scenarios suggest that diversification took

place in response to development of extensive desert areas in North America, coincident

with the uplifting of mountains in the United States and Mexico during middle and late

Tertiary (Ferrari et al. 2000; Ferrusquía 1998). One hypothesis (Lidicker 1960) suggests

that D. merriami originally had a wider distribution than it does today and by peripheral

isolation gave rise to D. elator in Oklahoma and Texas and to D. phillipsii in Mexico.

Another hypothesis (Mantooth et al. 2000) suggests that a D. merriami-like ancestor give

rise to D. phillipsii, which originally had a wider distribution than it does today and

eventually gave rise to D. elator in the southwestern United States. A third scenario (also

proposed by Mantooth et al. 2000) suggests that the D. merriami-like ancestor first gave

rise to D. elator, then D. elator, in turn, gave rise to D. phillipsii. Because our analysis

could not resolve the sequence of branching events in this clade, our findings cannot

contribute to this debate. However, our findings suggest that each of these hypotheses is

missing an important phyletic event: subdivision of ancestral D. phillipsii into two

distinct clades (the MP and TMVB clades), probably in response to the final uplift of the

TMVB in central Mexico.

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Taxonomic conclusions.—The Mexican Plateau clade of D. phillipsii (currently

recognized as D. p. ornatus) was originally described as a distinct species, D. ornatus, by

Merriam (1894). Genoways and Jones (1971) synonymized D. ornatus under D.

phillipsii for lack of consistent morphological differences between the two forms. If D.

ornatus and D. phillipsii are, in fact, genetically isolated species, then the evidence

presented by Genoways and Jones (1971) only confirms that they are crypic, and possibly

even sibling, species. Today, populations of the MP clade (D. p. ornatus) are separated

from populations of the TMVB clade of D. phillipsii by > 130 km, and the likelihood that

these clades come into contact, much less interbreed, is extremely low. Moreover, the

Cytb evidence suggests that these populations have been genetically isolated for

approximately four million years. We believe that the sum total of the evidence suggests

that D. ornatus should again be recognized as a full species distinct from D. phillipsii,

and below we present a key that can be used to distinguish between these

morphologically cryptic species with reasonable (77.4%) accuracy.

Dipodomys ornatus Merriam, 1894

Plateau Kangaroo Rat

Dipodomys ornatus Merriam, 1894:110. Type locality “Berriozábal, Zacatecas.”

Geographic range.—This species is known only from the Mexican Plateau, where

it has been collected in the states of Aguascalientes, Durango, Guanajuato, Jalisco,

Querétaro, San Luís Potosí, and Zacatecas.

Description.—Total body length 252–302 mm; 4 toes on hind foot; dorsal

coloration pale; interorbital region narrow (57.9–60.9% of mastoid breadth; Genoways

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and Jones 1971); mastoid breadth of skull narrow relative to maxillary breadth (ratio

between mastoid breadth and maxillary breadth ! 1.08).

Dipodomys phillipsii Gray, 1841

Phillips’ Kangaroo Rat

(Synonymy under subspecies)

Description.—Total body length 230–304 mm; four toes on hind foot; dorsal

coloration variable. Populations in the Valley of Mexico (D. p. phillipsii) show dark

dorsal coloration, medium body size for the species, and broad maxillary and interorbital

regions relative to mastoid breadth (Genoways and Jones 1971). Populations in the south

(D. p. oaxacae) are small for the species (hind foot < 37 mm), with pale dorsal coloration

and narrow maxillary and interorbital breadths relative to mastoid breadth. Populations

in the Oriental Basin (D. p. perotensis) are slightly darker and larger in body size (hind

foot > 37 mm) than D. p. oaxacae and have a somewhat longer cranium than D. p.

phillipsii.

Geographic range.—Known from the arid, semi-desert regions of southwestern

Hidalgo, Mexico, Distrito Federal, Tlaxcala, west-central Veracruz, Puebla, and northern

Oaxaca.

D. phillipsii oaxacae Hooper, 1947

D. phillipsii oaxacae Hooper, 1947:48. Type locality “Teotitlán, 950 m, Oaxaca.”

Geographic range.—Known only from extreme southern Puebla and adjacent

northern Oaxaca.

D. phillipsii perotensis Merriam, 1894

D. perotensis Merriam, 1894:111. Type locality “Perote, Vera Cruz [Veracruz].”

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Geographic range.—Known only from the Oriental Basin, west-central Veracruz

(vicinity of Perote) and adjacent parts of Puebla and Tlaxcala.

D. phillipsii phillipsii Gray, 1841

Dipodomys phillipii [sic] Gray, 1841:522. Type locality "Mexico, near Real del Monte.”

Type locality determined to be in the state of Hidalgo, “in the mountains at the

extreme north end of the Valley of Mexico, about 50 miles northeast of the City

of Mexico” by Merriam (1893:91). Species name spelled phillipii by

typographical error corrected to phillipsii by same author a few months later

(Gray 1842. American Journal of Science 42:335).

Macrocolus halticus A. Wagner, 1846:172. Type locality “Mexico.”

Distribution.—Known only from the Valley of Mexico and immediately adjacent

areas of Hidalgo, Mexico, and the Distrito Federal.

KEY TO THE SPECIES AND SUBSPECIES OF D. PHILLIPSII AND D. ORNATUS

1 Hind foot length ! 37.0………………………………………………..D. p. oaxacae

Hind foot length > 37.0.. ………………………………………………………...…2

2 Middorsal pelage very dark (red reflectance < 13%) and specimen taken from vicinity

of the Valley of Mexico (Hidalgo, State of Mexico, or Distrito Federal)…D. p. phillipsii

Middorsal pelage very dark to light, but if very dark (red reflectance < 13%) then

specimen not taken from vicinity of the Valley of Mexico …………………………... 3

3 Ratio between mastoid breadth of skull and maxillary breadth of skull (mastoid

breadth divided by maxillary breadth) > 1.08………………………… D. p.

perotensis

*Ratio between mastoid breadth and maxillary breadth ! 1.08 …….... D. ornatus

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*This character is 77.4% reliable based on examination of 31 randomly

selected adult specimens of D. phillipsii perotensis (n = 17) and D. ornatus (n =

14).


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CHAPTER 6

CONCLUSIONS

A vast number of biological studies have focused on the Mexican biota, and at

first glance, one’s impression might be that several of the major biological questions have

been addressed. However, a careful eye will detect that many species in many parts of

the country have yet to be studied, and large numbers of taxa that have been studied

previously should be reexamined using modern techniques and analyses. In studies of the

Mexican biota, one cannot help but assume that the dramatic climate cycles of the

Pleistocene epoch and the prominence of the Trans-Mexico Volcanic Belt (TMVB)

played major roles in the origin and diversification of Mexican species. However, only

recently have studies been designed to clarify and test the role of Pleistocene climate

cycles and the TMVB in generating the biological diversity we see today.

The four nearly codistributed rodent species studied in this dissertation offer

excellent opportunities to investigate individual and shared patterns of evolutionary

diversification. In each case, I have generated a phylogenetic hypothesis for the taxon,

recommended appropriate taxonomic changes, as necessary, and placed the phylogeny in

a temporal framework to identify events that may have generated the phylogenetic

pattern.

Two of the study taxa, Nelson’s woodrat Neotoma nelsoni (Chapter 2) and the

Perote ground squirrel Xerospermophilus perotensis (Chapter 3), are both endemics to the

Oriental Basin in east-central Mexico. Historically, both taxa have been treated as valid

species, based mainly on a few qualitative morphological characters and the fact that they

are geographically isolated from congeneric populations. Previous studies have

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postulated that the sister species of N. nelsoni and X. perotensis occur in the Mexican

Plateau and that current populations of these taxa are remnants of once more widespread

desert-adapted forms whose distribution was fragmented by Pleistocene climatic

oscillations. It is thought that these taxa evolved in isolation in the Oriental Basin

following extinction of intermediate populations.

My analyses confirmed that the closest relatives of N. nelsoni and X. perotensis

occur in the Mexican Plateau (N. leucodon and X. spilosoma, respectively), and my

findings also confirmed that N. nelsoni and X. perotensis are genetically well-

differentiated from their sister taxa to the north. However, the Cytb genetic distances

(3.3% between N. nelsoni and N. leucodon and 3.6% between X. perotensis and X.

spilosoma; Table 6.1) are not large, and this in combination with low levels of

morphological differentiation between the Oriental Basin and Mexican Plateau

populations of these taxa suggest that they should be recognized only at the subspecific

level as N. leucodon nelsoni and X. spilosoma perotensis.

Molecular estimates of divergence times suggested that N. l. nelsoni and X. s.

perotensis diverged from their sister taxa to the north during early Pleistocene times ;

Table 6.1). Thus, climatic fluctuations during the Pleistocene may have played a role in

the early diversification of these lineages, causing repeated fragmentation of their

geographic distributions followed by extintion of geographically intermediate

populations. Because divergence of N. l. nelsoni and X. s. perotensis from their relatives

to the north following the Miocene/Pliocene uplift of the TMVB, it appears that the

TMVB played only a minor, if any, role in the diversification of these lineages. In the

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Xerospermophilus study (Chapter 3), a well-differentiated lineage of X. spilosoma was

discovered in the Bolsón de Mapimí in Durango, México.

The rock mouse Peromyscus difficilis (Chapter 4) is another Mexican endemic

found in the dry foothills of the Sierra Madre Oriental, the Mexican Plateau, the low dry

hills of the TMVB, and some apparently disjunct populations in pine forests of central

México as far south as Oaxaca. Historically, P. difficilis has been divided into five

subspecies based primarily on distribution, but also on a few qualitative morphological

characters. The results of my analyses divided this species into two well-supported

clades (ca. 6.7% Cytb divergence; Table 6.1)), a northern clade including the subspecies

P. d. difficilis and P. d. petricola, and a southern clade containing the subspecies amplus,

felipensis, and saxicola. Molecular-based estimates of divergence times suggested that

separation of these clades occurred in the Pleistocene, again (as in Chapters 2 and 3)

suggesting that glacial and interglacial cycles of the Pleistocene may have influenced

genetic differentiation in the common ancestors of these lineages. The southernmost

subspecies of P. difficilis (P. d. felipensis) was originally described as P. felipensis.

Several lines of evidence, including phylogenetic analyses of mitochondrial and nuclear

genes, estimates of divergence times, disjunct geographic distributions, and

morphological differences supported my decision to return the southern clade of P.

difficilis to full species status as P. felipensis.

My study of the endangered Phillips’ kangaroo rat, Dipodomys phillipsii (Chapter

5), revealed a biogeographic pattern different for that seen in Chapters 2–4. D. phillipsii

occurs on the Mexican Plateau and in the arid lowlands of the TMVB. Past studies

divided this taxon into several subspecies based mainly on geographic distribution, but

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also supported by several qualitative morphological characters. In my analysis, D.

phillipsii was divided into two well-supported clades (ca. 9.8% Cytb divergence; Table

6.1), one distributed on the Mexican Plateau (formerly recognized as D. ornatus), and a

southern clade in the TMVB. Several lines of evidence, including phylogenetic analyses

of mitochondrial and nuclear genes, estimates of divergence times, disjunct geographic

distributions, and morphological differences supported my decision to return the Mexican

Plateau clade of D. phillipsii to full species status as D. ornatus. My study showed that

D. phillipsii, D. ornatus, D. elator, and D. merriami form a well-supported clade of

kangaroo rats, but I was unable to resolve relationships among these four species. My

inability to resolve this polytomy using a large number of base pairs from a combination

of mitochondrial and nuclear genes analyzed in multiple ways suggests that

diversification among these four species may have occurred over a relatively short period

of time.

My molecular-based analyses of divergence times suggests that D. phillipsii, D.

ornatus, D. elator, and D. merriami diverged in mid-Pliocene times; Table 6.1), probably

in or near the Mexican Plateau. Unlike the Pleistocene divergence dates reported in

Chapters 2–4, this Pliocence divergence suggests that the morphotectonic processes that

gave rise to the Trans-Mexico Volcanic Belt may have influenced early diversification in

Mexican species of Dipodomys.

Many studies of the mammals of Mexico have focused on the rich fauna of the

tropical and semitropical regions of southern Mexico. To some extent, the desert

environments of Mexico have been ignored, especially by researchers using modern

systematic techniques and methods of analysis. By means of this dissertation, I wish to

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reaffirm and highlight the importance of Mexico’s deserts, especially the southern

extension of the deserts into central Mexico, as natural laboratories and important centers

of evolutionary diversification. I invite my colleagues to test the phylogenetic and

biogeographic hypotheses contained in this dissertation with their own studies of the flora

and fauna of central Mexico.

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Table 6.1.—Overview of taxonomic conclusion, divergence values between clades of interest, and mean estimates of divergence time for clades of interest (in million years ago).

Monophyly Divergence values

between clades (%)

Mean estimated

divergence time

(mya)

Neotoma nelsoni Yes 3.2 2.10 Pleistocene

Xerospermophilus

perotensis

Yes 3.6 0.75 Pleistocene

Peromyscus

difficilis

Yes 6.7 0.80 Pleistocene

Dipodomys

phillipsii

Yes 9.8 4.20 Pliocene

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APPENDIX 3.1

Specimens used in the analysis of X. s. perotensis relationships listed

alphabetically by taxon, locality, geographic coordinates, elevation, catalogue number,

sequenced genes, and GenBank numbers. Specimens are housed in the following

museums: Colección Nacional de Mamíferos, Instituto de Biología, Universidad

Nacional Autónoma de México (CNMA), Cornell University DNA collection (CU;

samples from CU are followed by the collector’s field number in parentheses), Los

Angeles County Museum of Natural History (LACM), Louisiana State University

Museum of Natural Science (LSUMZ), Museo de Zoología, Facultad de Ciencias,

Universidad Nacional Autónoma de México (MZFC); New Mexico Museum of Natural

History (NMMNH), and University of New Mexico Museum of Southwestern Biology

(MSB). Numbers in parentheses before localities indicate localities mapped in Fig. 3.1.

Callospermophilus lateralis

Collection locality not available. LACM 85487, 12S = AY227530, IRBP = AY227586.

Cynomys gunnisoni

United States: (1) Arizona, Apache Co., Petrified Forest National Park, 34.909, -109.806,

1,641 m, S 75 (73), Cytb = AF157923; CU 82 (WA1), Cytb = AF157930.

Cynomys leucurus

United States: (2) Utah, Uintah Co., 8 km E Jensen on Highway 40, CU 1 (EY 1138),

40.369, -109.240, 1,689 m, Cytb = AF157838; 12S = AY227528, IRBP = AY227584.

Utah, Uintah Co., 11 km NW Bonanza on Hwy 45, CU 2 (EY1137), 40.094, -109.269,

1,572 m, Cytb = AF157879.

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Cynomys ludovicianus

United States: Nebraska, Omaha Zoo CU 38 (1A), Cytb = AF157890; CU 41 (4A), Cytb

= AF157892; Collection locality not available, CU 1 (EY 1138), IRBP = AY227584.

Cynomys mexicanus

Mexico: (3) Nuevo León, Ejido El Tokio, 18 km E highway 57 on highway 58, 24.6, -

100.2, 1,891 m, CU 101 (EY 1180), Cytb = AF157841; CU 102 (EY 1181), Cytb =

AF157842.

Cynomys parvidens

United States: (4) Utah, Kane Co., Bryce Canyon National Park, 1 km from Visitor

Center, 37.58, -112.18, 2,466 m, CU 74 (BCU 1), Cytb = AF157922; Utah, Kane Co.,

Bryce Canyon National Park, 20 m from boundary, 37.58, -112.18, 2,466 m, CU 81

(BC1), Cytb = AF157929.

Ictidomys mexicanus mexicanus

Mexico: (5) México, Parque Nacional Zoquiapan, 15 km SW Rio Frío, 19.3, -98.6, 2,980

m, CU 108 (EY 1210), Cytb = AF157848.

Ictidomys parvidens

United States: (6) New Mexico, Chaves Co., Eastern NM University, Roswell, West

Wells & University Street, 33.39, -104.52, 1,097 m, MSB 135244, Cytb = sequences

submitted to GenBank, 12S = b = sequences submitted to GenBank, GHR = b =

sequences submitted to GenBank, IRBP = b = sequences submitted to GenBank;

Mexico: (7) Nuevo León, 3 km NE Apodaca, ca. 23 km NE Monterrey, 25.80, -100.16,

408 m, CU 111 (EY 1197), Cytb = AF157852; CU 112 (MVA 105), Cytb = AF157853.

Ictidomys tridecemlineatus

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United States: (8) Kansas, Finney Co., Garden City, 37.9, -100.8, 866 m, CU 13 (EY

1147), Cytb = AF157870; CU 14 (EY 1148), Cytb = AF157877; Collection locality not

available (from personal collection of R. L. Honeycutt), H 2147, 12S = U67290, IRBP =

AF287278; (9) South Dakota, Union Co., 1 mi. N, 1.5 mi. W Junction City, 42.800, -

96.815, 377 m, LSUMZ 10692, Cytb = b = sequences submitted to GenBank, 12S = b =

sequences submitted to GenBank, GHR = b = sequences submitted to GenBank, IRBP =

b = sequences submitted to GenBank; South Dakota, Clay Co., 3.5 mi. N, 3.5 mi. E

Vermillion, 42.829, -96.857, 376 m, LSUMZ 10728, Cytb = b = sequences submitted to

GenBank, 12S = b = sequences submitted to GenBank, GHR = b = sequences submitted

to GenBank, IRBP = b = sequences submitted to GenBank.

Xerospermophilus mohavensis

United States: (10) California, San Bernardino Co., 9 mi NNE Johannesburg, 35.473, -

117.589, 1,075 m, MSB 40496, Cytb = b = sequences submitted to GenBank, 12S = b =


b = sequences submitted to GenBank; MSB 40503, Cytb = b = sequences submitted to


to GenBank, IRBP = b = sequences submitted to GenBank.

Xerospermophilus perotensis

Mexico: (11) Puebla, Tepayahualco, 19.490, -97.489, 2,336 m, CNMA b = sequences

submitted to GenBank (EY 1176), Cytb = AF157948, 12S = b = sequences submitted to

GenBank, GHR = b = sequences submitted to GenBank, IRBP = b = sequences submitted

to GenBank; CNMA b = sequences submitted to GenBank (EY 1179) Cytb = AF157840,

12S = b = sequences submitted to GenBank, GHR = b = sequences submitted to

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GenBank, IRBP = b = sequences submitted to GenBank; Veracruz: Municipality of

Perote, 5 km W Perote, 19.587, -97.330, 2,400 m MZFC XXXX (JAFF2248), Cytb = b =

sequences submitted to GenBank, 12S = b = sequences submitted to GenBank, GHR = b

= sequences submitted to GenBank, IRBP = b = sequences submitted to GenBank;

Veracruz, Municipality of Perote, 3 km S El Frijol Colorado, 19.572, 97.383, 2,435 m

MZFC XXXX (JAFF2154), Cytb = b = sequences submitted to GenBank, 12S = b =


b = sequences submitted to GenBank.

Xerospermophilus spilosoma cabrerai

Mexico: (12) San Luis Potosi, 10 mi. S Villa de Ramos, 22.666, -101.953, 2,200 m,

NMMNH 3651, Cytb = b = sequences submitted to GenBank, 12S = b = sequences

submitted to GenBank, GHR = b = sequences submitted to GenBank, IRBP = b =

sequences submitted to GenBank.

Xerospermophilus spilosoma marginatus

United States: (13) New Mexico, Bernalillo Co., 8 km W Albuquerque, 35.084, -106.738,

1,631 m, LSUMZ 01, Cytb = b = sequences submitted to GenBank, 12S = b = sequences

submitted to GenBank, GHR = b = sequences submitted to GenBank, IRBP = b =

sequences submitted to GenBank; LSUMZ 05, Cytb = b = sequences submitted to


to GenBank, IRBP = b = sequences submitted to GenBank. (14) Kansas, Finney Co., 13

km S, 2 km E Holcomb, 37.872, -100.964, 893 m, CU 3 (EY1142), Cytb = AF157885;

CU 6 (EY1146), Cytb = AF157911.

Xerospermophilus spilosoma pallescens

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Mexico: (15) Durango, 4 km E Ceballos, 26.523, -104.089, 1,117 m, CU 105 (EY 1195),

Cytb = AF157845; Durango, Ejido La Flor, 20 km E Ceballos, 26.524, -103.929, 1,146

m, CU 106 (EY 1193), Cytb = AF157846.

Xerospermophilus tereticaudus neglectus

United States: (16) Arizona, Pima Co., Tucson, 32.221, -110.926, 917 m, MSB 86022,

Cytb = b = sequences submitted to GenBank, 12S = b = sequences submitted to GenBank,

GHR = b = sequences submitted to GenBank, IRBP = b = sequences submitted to

GenBank; MSB 92638, Cytb = b = sequences submitted to GenBank, 12S = b =


b = sequences submitted to GenBank; Arizona, Pima Co., 18 km W Tucson, Ryan Field,

32.223, -111.116, 735 m, CU 91 (EY 1169), Cytb = AF157940; CU 92 (EY 1167), Cytb,

= AF157941.

Poliocitellus franklinii

United States: Nebraska, Omaha Zoo CU 42 (1A), Cytb = AF157893; CU 43 (2A), Cytb

= AF157894.

Urocitellus townsendii idahoensis

Collection locality not available, CU 86 (EY 1062), Cytb = AF157949.

Urocitellus townsendii

Collection locality not available, CU 87 (EY 1064), Cytb = AF157938.

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APPENDIX 4.1

Specimens examined in the molecular analysis of the rock mouse Peromyscus

difficilis listed by taxon, state, collecting locality, geographic coordinates, elevation,

catalogue number, and GenBank numbers listed in the following order: Cytb, 12S, GHR,

and IRBP. A dash (–) indicates that a specimen was not sequenced for that gene.

Mammal collections where voucher specimens are housed are CWK = C. William

Kilpatrick; GK=Ira I. Greenbaum; TK=K. Nutt and Texas Tech University Tissue

Number. Carnegie Museum of Natural History (CM); Colección Regional Durango,

Centro Interdisciplinario de Investigacion para el Desarrollo Integral Regional-Durango,

Instituto Politécnico Nacional (CDR); Louisiana State University Museum of Natural

Science (LSUMZ); Museum of Southwestern Biology, University of New Mexico

(MSB); Texas A&M University, Texas Cooperative Wildlife Collection (TCWC); Texas

Tech University (TTU); Universidad Nacional Autónoma de México, Instituto de

Biología (CNMA); Universidad Nacional Autónoma de México, Museo de Zoología

“Alfonso L. Herrera”(MZFC); University of Vermont, Zadock Thompson Natural

History Collections (ZTNH);. Numbers in parentheses indicate localities mapped in Fig.

4.1. All newly generated sequences were submitted to genbank.

P. d. amplus

PUEBLA: (13) 8 mi. SE Chignahuapan, 19.753, -97.947, 8,808 ft., (ZTNHC: CWK

2770) AY376414 Cytb only; (12) 3 km S Ciudad Serdan, crossroad between Ciudad

Serdan-Esperanza, towards Santa Catarina, 18.932, -97.421, 2,536 m, CNMA 44007

Cytb, –, GHR, IRBP; (12) 1 km S Coyotepec, 19.009, -97.555, 2,435 m, CNMA 44001

Cytb, –, GHR, IRBP (12) 2 km W Guadalupe Victoria, 19.280, -97.378, 2,406 m., CNMA

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43992 Cytb, 12S, GHR, IRBP; (11) 1.5 km S Oriental, 19.352, -97.635, 2,360 m, CNMA

43966 Cytb only. TLAXCALA: (13) 18 km N, 9 km E Apizaco, 19.58, -98.051, 9,142

ft., CM55804, AY387488 Cytb only; (12) 2.5 km NW El Carmen Tequexquitla, 19.35, -

97.665, 2,378 m, CNMA 43957 Cytb, 12S, GHR, IRBP; CNMA 43960 Cytb, 12S, GHR,

IRBP; (13) Barranca Huehuetitla, 2 km NE San Ambrosio Texantla, 19.316, -98.25,

2,272 m, CNMA 44269 Cytb only; (13) Mt. Malinche, 19.265, -98.022, 10,709 ft.,

TCWC: GK 3904) AY376415 Cytb only; (13) 2 km NE Tepetitla, 19.277, -98.365, 7,293

ft., TTU 82690, AY376416 Cytb only. VERACRUZ: (10) 3 km S El Frijol Colorado,

19.572, -97.383, 2,435 m, CNMA 43978 Cytb, 12S, GHR, IRBP.

P. d. difficilis

AGUASCALIENTES: (7) 6 mi. W Rincon de Romos, 22.229, -102.413, 6,852 ft.,

(TCWC GK:4129) AY376418 Cytb only; DURANGO: (5) 50 km W Las Herreras,

25.185, -106.036, 8,526 ft., CRD 1143, AY376417 Cytb only; ZACATECAS: (6) 12.4

mi. NW, 16.2 mi. NE (by road) Sombrerete, 23.9, -103.5, 2,268 m, MSB 54457 Cytb,

12S, GHR, IRBP; (6) 13.2 mi. NW, 5.4 mi. NE (by road) Sombrerete, 23.9, -103.5, 2,214

m, MSB 55604 Cytb, 12S, –, IRBP; (6) 12.4 mi. NW, 6.2 mi. NE Sombrerete, 23.9, -

103.5, 2,010 m, MSB 55616 Cytb, 12S, –, –; MSB 57701 Cytb, 12S, –, –.

P. d. felipensis

MEXICO: (15) Highway Ocuilan de Artega-Cuernavaca, km 14, 18.978, -99.416,

7,711 ft., MZFC 5689 Cytb, –, GHR, IRBP; (14) National Park Izta-Popo-Zoquiapan, 7.3

km SE Amecameca, 19.1014, -98.6955, 2,848 m, CNMA 45643, Cytb only; CNMA

45644 Cytb, 12S, GHR, IRBP.

P. d. petricola

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COAHUILA: (3) 10 mi. E San Antonio de las Alazanas, 25.266, -100.766, 2,586

m, MSB 48201 Cytb only; (3) 13.9 mi. W San Antonio de las Alazanas, 25.266, -

100.766, 2,028 m, MSB 48177 Cytb, 12S, GHR, IRBP; NUEVO LEON: (4), Cerro

Potosi, Municipio Galeana, 24.85, -100.316, 1,655 m, CNMA 44276, Cyb, –, GHR,

IRBP; CNMA 44277 –, 12S, GHR, IRBP; M8647 Cytb, –, –, IRBP; M8667 –, 12S, GHR,

P. d. saxicola

HIDALGO: (8) 5.4 mi. SE, 3.2 mi. S Ixmiquilpan, 20.391, -99.316, 6,431 ft.,

(TCWC GK2642) AF155394 Cytb only; (9) 1.8 mi. E Jonacapa, 20.436, -99.506, 7,480

ft., (TCWC GK3076) AY376419 Cytb only. MEXICO: (11) Cerro Gordo, Santiago

Tolman, 19.743, 98.628, 2,500 m, CNMA 43035 Cytb, 12S, GHR, IRBP.

P. nasutus

NEW MEXICO: (1) Socorro Co., Sevilleta, Sepultura Canyon, 34.304, -106.613,

6,470 ft.; MSB 63666 Cytb, 12S, –, –; Cibola Co., 11 mi. S, 14 mi. W San Rafael, 34.963,

-108.128, 6,450 ft.; MSB 54841, Cytb, 12S, GHR, IRBP; MSB 54819, Cytb, 12S, GHR,

IRBP.

P. nasutus griseus

NEW MEXICO: (1) Lincoln Co., 4 mi. S Carrizozo, 33.583, -105.876, 5,965 ft.,

TTU 78401, AY155399 Cytb only.

P. nasutus nasutus

TEXAS: (2) Jeff Davis Co., Mt. Livermore Preserve, 30.750, -104.193 5,722 ft.,

TTU 78316, AY376426 Cytb only.

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Neotoma nelsoni

VERACRUZ: (10) 3 km S El Frijol Colorado, 19.572, -97.383, 2,435 m, LSUMZ

36663 Cytb, 12S, GHR, IRBP.

Peromyscus attwateri

OKLAHOMA: McIntosh Co., 3.1 mi. E Dustin, 32.270, -95.975, 821 ft., TTU

55688, AF155384 Cytb only.

Peromyscus beatae

GUERRERO: Carrizal del Bravo, 17.266, -99.733, 3,625 ft., MZFC 9364 Cytb, –,

GHR, IRBP.

Peromyscus leucopus

CONNECTICUT: (T-175) X99463 12S.

Peromyscus levipes

OAXACA: San Martín Caballero, Distrito de Teotitlán, 17.057, -96.734, 5,069 ft.,

MZFC 8726, –, –, GHR, IRBP.

Peromyscus truei truei

ARIZONA: Apache Co., 35.580, -109.575, 6,832 ft., TTU 104427, AY376433

Cytb only.

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APPENDIX 4.2

Specimens included in the *BEAST analysis of divergence times (Drummond and

Rambaut 2007). Specimens are arranged by taxon, state, collecting locality, geographic

coordinates, elevation, catalogue number, and GenBank number for Cytb sequence.

Specimens were obtained from the Angelo State Natural History Collection (ASNHC),

Brigham Young University (BYU), the Royal Ontario Museum (ROM), and other

museum collections listed in Appendix 4.1. Specimens from Appendix 4.1 used in the

*BEAST analysis include CNMA 44001, CNMA 45643, MSB 48177, MSB 54457, MSB

63666, TCWC GK3076, and TTU 104423.

Habromys ixtlani

OAXACA: Llano de la Flores, km 132 on highway from Tuxtepec–Oaxaca, 17.09,

-96.70, 5,470 ft., CNMA 29849, EF989941.

Isthmomys pirrensis

PANAMA: Darien Province, summit of Cerro Pirre, 7.933, -77.716, 396 ft., ROM

116308, EF989945.

Megadontomys thomasi

OAXACA: 11 km SW La Esperanza, 17.561, -96444, 8,147 ft., CNMA 29186,

EF989948.

Neotomodon alstoni

DISTRITO FEDERAL: 3 km S Parres, 19.137, -99.159, 9,260 ft., ASNHC 1595,

EF989950.

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Onychomys arenicola

TEXAS: Presidio Co., Hip O Ranch, 5 mi. W Marfa, 30.308, -104.10, 4,805 ft.,

ROM 114904, EF989954.

Onychomys leucogaster

TEXAS: Cameron Co., 11 mi. N Port Isabel, 26.2, -97.2, 50 ft., ASNHC 4348,

EF989958.

Osgoodomys banderanus

MICHOACAN: 8 km N La Mira, 18.107, -102.328, 322 ft., ASNHC 2664,

EF989956.

Peromyscus attwateri

OKLAHOMA: McIntosh Co., 3.1 mi. E Dustin, 32.270, -95.975, 821 ft., TTU

55688, AF155384.

Peromyscus crinitus

UTAH: Uintah County, Bitter Creek Canyon, 41.1, -111.9, BYU 16629, EF989973.

Peromyscus eremicus

SONORA: 22 km S (by road) Hermosillo, 28.889, -110.963, 755 ft., BYU 17952,

EF989975.

Peromyscus levipes

CHIAPAS: Cerro Tzontehuiz, 16.7, -92.6, 7,513 ft., ROM 97624, EF989981.

Peromyscus maniculatus

CANADA: Ontario: Kwataboahegan, Kwataboahegan River, 51.15, -80.50, 20 m,

ROM 98941, EF989983.

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Peromyscus melanophrys

Stock animal from Peromyscus Genetic Stock Center, University of South Carolina,

USC-PGSC XZ 1073, EF989989.

Peromyscus melanotis

Stock animal from Peromyscus Genetic Stock Center, University of South Carolina,

USC-PGSC 25, EF989990.

Reithrodontomys_sumichrasti

GUATEMALA: Huehuetenango: 10 km SW Santa Eulalia, 15.673, -91.528, 8,663

ft., ROM 98383, EF990023.

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APPENDIX 5.1

Specimens examined.—Species name, collection locality, geographic coordinates,

elevation, and catalogue number are listed for specimens used in this analysis of

Dipodomys phillipsii. Specimens used to develop the morphological key are indicated by

“(m)” following the catalogue number. GenBank numbers are listed for each specimen

used in the molecular analyses. Mammal collections housing voucher or tissue specimens

are ASNHC = Angelo State Natural History Collection, Angelo State University; CNMA

= Colección Nacional de Mamíferos, Instituto de Biología, Universidad Nacional

Autónoma de México; ENCB = Escuela Nacional de Ciencias Biológicas, Instituto

Politécnico Nacional; LSUMZ = Louisiana State University, Museum of Natural Science;

M = Louisiana State University, Museum of Natural Science Mammal Tissue Collection;

LVT = University of Nevada, Las Vegas; MLZ = Moore Laboratory of Zoology,

Occidental College; MVZ = Museum of Vertebrate Zoology, University of California,

Berkeley; MWSU = Midwestern State University; NMMNH = New Mexico Museum of

Natural History; TTU = Museum of Texas Tech University; UAM-I = Universidad

Autónoma Metropolitana-Iztapalapa; UATX = Universidad Autónoma de Tlaxcala; and

USNM = United States National Museum of Natural History. Numbers in parentheses

refer to localities mapped in Fig. 5.1.

D. p. oaxacae

Puebla: (15) 4 km SSW San José Axusco, 18.022, -97.232, 1,031 m, CNMA 41881, Cytb

= JN183909, 12S = JN208377, GHR = JN661650; CNMA 41882, Cytb = JN183906, 12S

= JN208375, GHR = JN661651; IRBP = JN661671; CNMA 41883, Cytb = JN183907,

12S =JN661642, GHR = JN661652, IRBP = JN661672; CNMA 41884, Cytb =

! 135

JN183908, 12S = JN208376, GHR = JN661653; (14) 2.7 km SE San José Buenavista,

18.642, -97.566, UAMI 17257, Cytb = JN183910.

D. p. ornatus

Aguascalientes: (4) 8.8 km N, Las Fraguas, 22.233, -102.083, 2,150 m, IPN 36320, Cytb

= JN183912; Durango: (1) 5.8 km N, 2.1 km E Vicente Guerrero, 23.783, -103.962,

1,937 m, CNMA 39681, Cytb = JN183913, 12S = JN208380, GHR = JN661654, IRBP =

JN661676; TTU 75585, IRBP = GU955164; (2) 2.2 km S, 2.5 km E Vicente Guerrero,

23.711, -103.957, 1930 m, CNMA 39683, Cytb = JN183914, 12S = JN208382, GHR =

JN661655, IRBP = JN661677; Durango, 24.0, -104.6 , USNM 94622 (m) and 94624 (m);

Jalisco: (5) 4 km W Guadalupe Victoria, 21.699, –101.617, 2,185 m, CNMA 43385, Cytb

= JN183915, 12S = JN208384, GHR = JN661656, IRBP = JN661678; Lagos [de

Moreno], 21.4, -101.9, USNM 78952 (m); Querétaro: Tequisquiapan, 20.5, -99.9, USNM

78430 (m) and 78431 (m); San Luis Potosi: 1 km N Arenal [de Morelos], 22.42, -101.27,

LSUMZ 5178 (m); Bledos, 21.8, -101.1, LSUMZ 5177 (m); About 1 mi. W Bledos,

21.84, -101.13, LSUMZ 4286 (m); (6) Las Cabras, 4.6 mi. NW Bledos, 21.800, -100.933,

1,820 m, LVT 2056, Cytb = AY926376; Zacatecas: (3) 2 mi. E San Jeronimo, 22.640, -

97.231, 2,350 m, CNMA 42049, Cytb = JN183917, 12S = JN208385, GHR = JN661657,

IRBP = JN661679; CNMA 42050, 12S = JN208385; CNMA 42121, Cytb = JN183918,

12S = JN208386, GHR = JN661658, IRBP = JN661680; Berriozábal, 22.5, -102.3,

USNM 79506 (m); Hacienda San Juan Capistrano, 22.6, -104.1, USNM 90804 (m); [El]

Plateado [de Joaquín Amaro], 21.9, -103.1, USNM 90808 (m); Valparaíso, 22.8, -103.6,

USNM 91933 (m) and 91945 (m); Zacatecas, 22.8, -102.6, USNM 120167 (m).

D. p. perotensis

! 136

Puebla: (10) 4.5 km S, 9.5 km San José Alchichica, 19.766, -97.316, 2,350 m, IPN

42239, Cytb = JN183899; (11) 2 km W Guadalupe Victoria, 19.280, -97.378, 2,406 m,

CNMA 43986, Cytb = JN183904, 12S = JN208381, GHR = JN661659, IRBP =

JN661670; CNMA 43987–43988 (m); (12) 3.1 km SW Veladero, 20.591, -97.600, 2,340

m, LSUMZ 36253 (m), Cytb = JN183898; (16) 11 km (by road) SW Alchichica,19.766, -

97.316, 2,449 m, LSUMZ 36244 (m), Cytb = JN183900; (13) 6.7 km E Techachalco,

19.390, -97.408, 2,500 m; UAMI 17258, Cytb = JN183901; Chalchicomula (= Ciudad

Serdán), 19.0, -97.4, USNM 53323–53328 (m) and 53333 (m); Tlaxcala: Huamantla,

19.3, -97.9, USNM 53635 (m) and 53637 (m); (8) 2.5 km NW El Carmen Tequexquitla,

19.350, -97.665, 2,378 m, CNMA 44321, Cytb = JN183902, 12S = JN208378, GHR =

JN661660, IRBP = JN661674; (17) 6 km NE Cuapiaxtla, 19.300, -97.766, 2,425 m, UAT

M0366, Cytb = JN183903, 12S = JN208379, GHR = JN661661, IRBP = JN661673;

Veracruz: (9) 3 km S El Frijol Colorado, 19.572, -97.383, 2,435 m, CNMA 43972, Cytb

= JN183905, IRBP = JN661675; CNMA 43974 (m); Perote, 19.6, -97.2, USNM 54281–

54283 (m).

D. p. phillipsii

Mexico: (7) 5 km SE Nopaltepec, 19.753, -98.670, 2,400 m, UAMI 2779, Cytb =

JN183911.

D. agilis

Baja California: 6 km S, 17 km E, Valle de la Trinidad, 31.754, -116.364, 905 m, MVZ

153957, Cytb = U65303.

! 137

D. californicus

California: Tehama County, 2.5 miles S, 0.2 miles E Paynes Creek, 40.306, -121.904,

1,960 feet, MLZ 2061, Cytb = AY926368.

D. compactus

Texas: Cameron County, 4.5 miles N, 3.6 miles E Port Isabel, 26.13, _97.16, ASNHC

4327, Cytb = AY926379.

D. deserti

Nevada: Clark County, Corn Creek Desert Wildlife Refuge, 39.55, -119.46, NMMNH

5374, Cytb = AY926381.

D. elator

Texas: Cottle County, 1.6 km N, 1.8 km E junction FM 1033 and FM 104, 34.250, -

100.050, 500 m, TTU 45633, Cytb = AF172834, JN183919, 12S = JN208373, GHR =

JN661664, IRBP = JN661688; Wichita County, 8.6 miles N Iowa Park; 34.056, -98.746,

1117 feet, MWSU17542, Cytb = JN661645, 12S = JN208374, GHR = JN661665, IRBP

= JN661687.

D. heermanni

California: San Luis Obispo County, 15.0 miles S, 8.2 miles E Simmler, 33.134, -

119.839, 2,300 feet, MLZ 1852, Cytb = AY926369.

D. merriami

Arizona: Maricopa County, 11.2 km N Gila Bend, 33.666, -111.866, 224 m, TTU 41781,

Cytb = AF173502; Texas: Brewster County, Elephant Mountain Wildlife Management

Area, 29.66, -103.35, 3562 feet, TTU 97980, IRBP = GU985162; Presidio County, Las

Palomas Wildlife Management Area, 26.40, -97.80, 33 feet, TTU 75675, Cytb =

! 138

AF172837. Coahuila: 6 mi. E Parras, 24.86, -100.18, 2,118 m, M-8688, Cytb =

JN661644, 12S = JN661641, GHR = JN661662, IRBP = JN661686. Durango: 3 mi. N

Lazaro Cardenas, 25.39, -103.98, 1,500 m, M-8708, Cytb = JN661643, 12S = JN661640,

GHR = JN661663, IRBP = JN661685.

D. microps

California: Inyo County, 6.0 miles N, 0.5 miles W Bishop, 37.450, -118.404, 4,200 feet,

MLZ 1765, Cytb = AY926385.

D. nelsoni

Coahuila: 5 km S, 16 km W General Cepeda, 25.330, -101.631, 5,599 feet, NMMNH

4703, Cytb = AY926364; Durango: 7 mi. NNW La Zarca, 25.853, -104.877, 6292 feet,

NMMNH 2472, IRBP = GQ480799. Durango: 3 mi. N Lazaro Cardenas, 25.39, -103.98,

1,500 m, M-8709, Cytb = JN661648, 12S = JN661638, GHR = JN661667, IRBP =

JN661681; M-8710 Cytb = JN661649, 12S = JN661639, GHR = JN661666, IRBP =

JN661682.

D. ordii

New Mexico: Grant County, 2.6 miles N, 1.8 miles E Redrock, 32.724, -108.707, 1,241

m, NMMNH 4377, Cytb = AF173501. Luna County, 4.0 mi. S, 9.5 mi. W Deming,

33.760, -108.780, 6,198 feet, MVZ 150772, Cytb = JN661646, 12S = JN661636, GHR =

JN661669, IRBP = JN661684; MVZ 150775, Cytb = JN661647, 12S = JN661637, GHR

= JN661668, IRBP = JN661683.

D. panamintinus

California: San Bernardino County, 8.9 miles N, 1.1 miles E Red Mountain, 35.486, -

117.595, 3,150 feet, MLZ 1879, Cytb = AY926384.

! 139

D. spectabilis

New Mexico: Hidalgo County, 6 mi. SE Portal (Cochise County, Arizona), 31.859, -

109.061, NMMNH 4399, Cytb = AF173503; Lincoln County, 4 mi. N, 3 mi. W, 33.698, -

105.927, 5285 feet, TTU 38443, IRBP = GU985165.

Heteromys irroratus

Puebla: 6 km N Tilapa, 18.648, -98.553, 1,300 m, LSUMZ 36295, Cytb = GU647037.

! 140

APPENDIX 5.2

List of primers used for the amplification of nuclear (GHR and IRBP) and

mitochondrial (Cytb and 12S) genes in the Phillips’ kangaroo rat (Dipodomys phillipsii).

Primer

Gene Sequence

Reference

MVZ-05 Cytb CGAAGCTTGATATGAAAAACCATCGTTG Irwin et al. 1991

H15915 Cytb AACTGCAGTCATCTCCGGTTTACAAGAC Irwin et al. 1991

MVZ-04 Cytb GCAGCCCCTCAGAATGATATTTGTCCTC Smith and Patton

1993

MVZ-45 Cytb ACJACHATAGCJACAGCATTCGTAGG Smith and Patton

1993

MVZ-16 Cytb AAATAGGAARTATCAYTCTGGTTTRAT Smith and Patton

1993

MVZ-17 Cytb ACCTCCTAggAgAYCCAgAHAAYT Smith and Patton

1993

MVZ-14 Cytb GGTCTTCATCTYHGGYTTACAAGAC Smith and Patton

1993

12S L82 12S CATAGACACAGAGGTTTGGTCC Nedbal et al. 1994

12S H900 12S TGACTGCAGAGGGTGACGGTGTGT Nedbal et al. 1994

GHR1f GHR GGRAARTTRGAGGAGGRGAACAC Jansa et al. 2009

GHRend1F GHR GATTTTGTTCAGTTGGTCTCTGCT Jansa et al. 2009

IRBP-A IRBP-A ATGGCCAACGTCCTCTTGGATAAC Stanhope et al.

1992

! 141

IRBP-B IRBP-B CGCAGGTCCATGATGAGGTGCTCC Stanhope et al.

1992

! 142

VITA

Jesús Abraham Fernández Fernández was born in Mexico City to Miguel Angel

and Heriberta Fernández. He grew up in the city of Santa Ana Chiautempan, Tlaxcala,

México, where he graduated from Escuela Secundaria Técnica No. 4. He received his

bachelor’s degree in biology from Universidad Autónoma de Tlaxcala in 2000 working

on bird diversity. In 2002, he began graduate school at the National University of

Mexico, where he obtained the title of Master in Science under the tutelage of Dr.

Fernando Cervantes. His thesis topic was phylogenetics and biogeography of kangaroo

rats. In August of 2006, Jesús Abraham Fernández Fernández entered the Graduate

School of Louisiana State University. Under the supervision of Dr. Mark S. Hafner,

Jesús will graduate with the degree of Doctor of Philosophy in December, 2011.

comparative biogeography of the arid lands of central mexico

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