variation in sexual reproduction in orchids and its ... interactions 2013/pollination/required...

Biological Journal of the Linnean Society, 2005, 84, 1–54. With 5 figures

© 2005 The Linnean Society of London, Biological Journal of the Linnean Society, 2005, 84, 1–54 1

Blackwell Science, LtdOxford, UKBIJBiological Journal of the Linnean Society0024-4066The Linnean Society of London, 2005? 2005841154Original Article

EVOLUTIONARY PROCESSES IN ORCHIDSR. L. TREMBLAY ET AL.

*Corresponding author. E-mail: [email protected]†Current address: HDR Engineering Inc., 2202 N. West Shore Blvd. Suite 250, Tampa, FL 33607, USA

Variation in sexual reproduction in orchids and its evolutionary consequences: a spasmodic journey to diversification

RAYMOND L. TREMBLAY1*, JAMES D. ACKERMAN2, JESS K. ZIMMERMAN3 and RICARDO N. CALVO4†

1Department of Biology, 100 carr. 908, University of Puerto Rico-Humacao, Humacao, Puerto Rico00971–43002Department of Biology, PO Box 23360, University of Puerto Rico-Rio Piedras, San Juan, Puerto Rico 00931–33603Institute for Tropical Ecosystem Studies, University of Puerto Rico, PO Box 21910, San Juan, Puerto Rico 00931–19104Department of Biology, University of Miami, Coral Gables, FL 33124, USA

Received 8 April 2003; accepted for publication 1 April 2004

The great taxonomic diversity of the Orchidaceae is often attributed to adaptive radiation for specific pollinatorsdriven by selection for outcrossing. However, when one looks beyond the product to the process, the evidence for selec-tion is less than overwhelming. We explore this problem by discussing relevant aspects of orchid biology and askingwhich aspects of reproduction explain the intricate pollination mechanisms and diversification of this family. Wereaffirm that orchids are primarily pollination limited, the severity of which is affected by resource constraints. Fruitset is higher in temperate than in tropical species, and in species which offer pollinator rewards than those that donot. Reproductive success is skewed towards few individuals in a population and effective population sizes are oftensmall. Population structure, reproductive success and gene flow among populations suggest that in many situationsgenetic drift may be as important as selection in fostering genetic and morphological variation in this family.Although there is some evidence for a gradualist model of evolutionary change, we believe that the great diversityin this family is largely a consequence of sequential and rapid interplay between drift and natural selection. © 2005The Linnean Society of London, Biological Journal of the Linnean Society, 2005, 84, 1–54.

ADDITIONAL KEYWORDS: cost of reproduction – fruit set – gene flow – genetic drift – natural selection –Orchidaceae – pollinator limitations – resource limitation – speciation.

INTRODUCTION

Early in the history of evolutionary biology, orchidshad a prominent role in providing evidence for naturalselection. Their unusual pollination mechanismsattracted the attention of Darwin (1877), who arguedthat they offer strong evidence both for natural selec-tion and for the advantages of cross-pollination. Sincethen, much effort has been devoted to describing

orchid pollination mechanisms (e.g. van der Pijl &Dodson, 1966; van der Cingel, 1995). These reportscontribute to Darwin’s arguments, although it is notoften stated explicitly. Most agree that there is a linkbetween orchid pollination systems and orchid diver-sity, the distinction between cause and effect is oftennot clear. In this review we argue that the predomi-nance of pollination limitation has had a significanteffect on the evolution of the family and propose mech-anisms by which orchids may have diversified.

Natural selection should favour levels of reproduc-tive effort that yield optimal fruit and seed set. Manyhermaphroditic plants produce far more flowers than

2 R. L. TREMBLAY ET AL.

© 2005 The Linnean Society of London, Biological Journal of the Linnean Society, 2005, 84, 1–54

fruits; orchids are superlative examples of this phe-nomenon. Low fruit-to-flower ratios in many plantsare believed to be the result of a paucity of resourcesavailable for fruit development. This assumptionforms the basis for the resource-limitation hypothesiswhereby consistently more flowers are pollinated thanfruits are matured (Stephenson, 1981; Lee, 1988).

Regulation of maternal investment occurs throughabortion of flowers and immature fruits (Lloyd, 1980;Stephenson, 1981) which may also be a mechanism forregulating seed quality (e.g. Lee & Bazzaz, 1982;Bookman, 1984; Stephenson & Winsor, 1986). Thehypothesis that resources are the ultimate limitingfactor in angiosperm reproduction has gained wide-spread acceptance because: (a) levels of fruit matura-tion remain unchanged following supplementarypollination, and (b) experimental reduction of resourceavailability causes elevated levels of fruit abortion(Stephenson, 1980, 1981; Bawa & Beach, 1981; Will-son & Burley, 1983). Thus, according to this view, vari-ation in reproductive success should be closely tied tothe severity of resource constraints.

Flowers that fail to become fruits are not alwayswasted as they may function to enhance plant fitnessthrough pollen donation (Willson & Rathcke, 1974).For example, in many milkweeds (Asclepias) fruit pro-duction is poorly correlated with the number of flowersin an inflorescence, but the amount of pollen removedby pollinators, an index of male fitness, is strongly cor-related with inflorescence size (Willson & Price, 1977;Bell, 1985; Queller, 1985). In fact, some researchersregard the corolla as primarily a ‘male’ organ (Bell,1985) because pollinator attractants influence fitnessthrough pollen donation to a much greater extent thanthrough seed production (Stanton, Snow & Handel,1986). However, some evidence suggests that largerinflorescences do not always result in proportionallyhigher male fitness (Campbell, 1989).

A common thread in these arguments is Bateman’sPrinciple (Bateman, 1948), which assigns the twoaspects of sexual selection (Darwin, 1871), intrasexualcompetition and mate choice, to the individual sexes.Noting the asymmetry in resource investment in off-spring between males and females, this principlestates that: (1) for males, reproduction is limited byaccess to mates, so that they must compete for oppor-tunities to mate with females, and (2) for females,reproduction is limited by resources and they shouldtherefore exercise a choice of mates to sire their rela-tively costly offspring.

Although Bateman (1948) and others mentioned thepossibility that sexual selection operated in plants aswell as in animals, it was much later before patternsof pollination and fruit maturation in plants wereinterpreted in this context (Janzen, 1977; Charnov,1979, 1982; Willson, 1979; Stephenson & Bertin,

1983). Despite the theoretical neatness and evidencefor resource constraints, variation in reproductive suc-cess in a number of species was found instead to becaused by low levels of pollination (Bierzychudek,1981a; Garwood & Horvitz, 1985; Hainsworth, Wolf &Mercier, 1985; Burd, 1994, and references therein),from which the pollinator limitation hypothesisemerged.

The evolutionary and ecological consequences of pol-linator limitation are likely to differ from those ofresource limitation. If reproduction is pollen limited,Bateman’s Principle is inapplicable (Stephenson &Bertin, 1983). Pollen-limited female reproduction isequivalent to saying that females are limited by accessto mates and therefore the potential for selective matechoice is reduced under these conditions (Willson &Burley, 1983). In fact, it would seem that any degree ofselectivity, not just that related to sexual selection, isof dubious value when the probability of a flowerreceiving pollen becomes small. While males may stillcompete amongst themselves for mates under pollen-limited reproduction (as do females), the intensity ofmale-male competition is restricted (Stephenson &Bertin, 1983; Tremblay, 1994).

The dichotomy of resource vs. pollination limitationmay be an oversimplification. Pollination limitedplants often show effects of resource constraints, so itmay be more realistic to say that such plants areaffected by both (Montalvo & Ackerman, 1987). Infact, the theoretical model of Calvo & Horvitz (1990),often cited as demonstrating pollination limitation,showed that the degree of limitation is affected by theseverity of resource constraints. Furthermore, therecan be substantial variations from year to year(Schemske & Horvitz, 1988; Vaughton, 1991).

The theoretical consequences of resource limitation,such as female choice, are not likely to be manifestedin combination with pollination constraints. Acker-man & Montalvo (1990) and Meléndez-Ackermanet al. (2000) noted that some plants are pollen limitedwithin a season but resource limited over their life-times. Under such conditions, the evolutionary conse-quences would be an optimization between increasedinvestment in pollinator attraction (and a reduction inallocation to ovules) and various aspects of life history,primarily longevity, age to first reproduction andreproductive effort within and across years (Ackerman& Montalvo, 1990).

Orchidaceae comprise c. one fifteenth of allangiosperms. While the intricate relationshipsbetween orchid flowers and their pollinators have longreceived a great deal of attention (Darwin, 1877; vander Pijl & Dodson, 1966; van der Cingel, 1995), rela-tively little consideration has been given to the factthat orchids often exhibit low fruit-to-flower ratios(Darwin, 1877; Ackerman, 1986a; Gill, 1989; Neiland

EVOLUTIONARY PROCESSES IN ORCHIDS 3


& Wilcock, 1998). Many studies in which researchershave performed supplemental pollination clearly indi-cate that these low ratios are best explained by polli-nator limitation (see Darwin, 1877; Ackerman, 1989;Zimmerman & Aide, 1989; Calvo & Horvitz, 1990).Thus, orchids provide an excellent illustration of theevolution of reproductive strategies under pollen-limitation. In this review we discuss the ecology andevolution of reproduction in orchids, pursuing ourprincipal argument that the predominance of pollina-tion limitation explains both their intricate pollinationmechanisms as well as the diversification of thefamily.

Following an overview of the essential details oforchid reproduction, we discuss the evidence thatreproductive success (both male and female) is polli-nation limited in orchids. We then summarize globalpatterns in orchid fruit reproduction, assembling datafrom almost 200 species of orchids. We then look at thecauses of pollinator limitation, resource constraintsand other ecological factors that have been shown tolimit orchid reproduction. We conclude with a discus-sion of the relationship between variation in reproduc-tive success, evolutionary processes, and the apparenthigh rates of speciation in the Orchidaceae.

OVERVIEW OF ORCHID FLORAL BIOLOGY

Darwin (1877) produced the first treatise on orchidpollination in order to corroborate his thesis that sex-ual reproduction (and cross-pollination in particular)is fundamental to organic evolution. He thoroughlydescribed the functional floral morphology of a num-ber of orchids. These essays provide indirect evidenceof selection for floral characteristics that enhance theprobability of cross-pollination. Much of the subse-quent literature has followed suit.

Three primary features, in combination, distinguishthe flowers of orchids from those of other families:

(1) The column, the fusion of male and female organswithin a single structure located at the centre ofthe flower.

(2) Pollinia, tightly packed masses of pollen found inmost orchids, transported as a unit by pollinators(Freudenstein & Rasmussen, 1997; Pacini &Hesse, 2002); a single visit is potentially sufficientto produce a full seed complement (e.g. Montalvo& Ackerman, 1987; Proctor & Harder, 1994; Naz-arov & Gerlach, 1997).

(3) Zygomorphy, whereby a labellum is often highlymodified to serve different functions (reviewed invan der Pijl & Dodson, 1966).

The diversity of floral shapes and functional modi-fications found across the family are largely the resultof variation in these three features.

Most orchids require an external pollinating agent(Dressler, 1981). Among non-autogamous species, wefind a wide variety of pollination systems: only abioticand mammalian pollination are absent. Animalgroups that pollinate orchids include birds, moths,butterflies, a wide variety of flies, numerous bees and,to a lesser extent, wasps. Hymenopterans aloneaccount for the pollination of around 60% of the family(van der Pijl & Dodson, 1966). There is also a widerange of levels of specificity in plant–pollinator inter-actions in the family (Tremblay, 1992). For example,the European Herminium monorchis was visited andpresumably pollinated by 69 insect species, includingmembers of four different orders: Lepidoptera,Coleoptera, Diptera and Hymenoptera (Nilsson,1979a). Epipactis palustris has as many as 103 speciesof potentially effective pollinators (Nilsson, 1978a;Tremblay, 1992). Nevertheless, high pollinator speci-ficity in orchid species is much more common: about60% of orchids have only one recorded pollinator(Tremblay, 1992). This relationship has been well doc-umented in a number of tropical orchids that are vis-ited by one or a few species of euglossine bee(Ackerman, 1983; Williams & Whitten, 1983; Roubik& Ackerman, 1987).

There are perhaps three kinds of floral rewardsamong orchids. The most common type is nutritional,consumed by the pollinators or their larvae. Nearly allsuch species are nectariferous, although some produceoils (Vogel, 1974; Steiner, 1989) and a handful offerpollen (Gregg, 1991b; Koryan & Endress, 2001) orpseudopollen (Dodson & Frymire, 1961; Goss, 1977;Davies, Winters & Turner, 2000). The second type ispeculiar to orchids pollinated by male euglossine bees.The bees are attracted by floral fragrances that theycollect for some as yet unknown aspect of mate attrac-tion (Dressler, 1981, 1982; Williams & Whitten, 1983;Eltz et al., 1999). A third type has not been well doc-umented. Some Maxillaria species produce waxes andresins (Dondon et al., 2002) that are collected bywasps and bees (Braga, 1977; M. Whitten, pers. comm.2002) and presumably used for nest construction.

Although most orchids offer some type of reward, anunusually high number of species offer no rewardwhatsoever. About a third of all Orchidaceae aredeceptive (van der Pijl & Dodson, 1966; Ackerman,1986a; Nilsson, 1992). Deception may be achieved bythe flowers’ resemblance to larval food, or to the nec-tariferous flowers of other families, or even to femaleindividuals of the insect pollinator (reviewed in Dafni,1984; Ackerman, 1986a; Nilsson, 1992).

Other distinguishing features of orchid reproductivebiology include delayed ovule development (Wirth &Withner, 1959), the large number (20 up to as many as4 million) of dust-like seeds contained in a single cap-sule (Arditti & Ghani, 2000), and their dependence on



fungal associations for germination and seedlingestablishment (Batty et al., 2001; McKendrick et al.,2002; Rasmussen & Whigham, 2002; Selosse et al.,2002).

POLLINATION LIMITATION IN ORCHIDS

Pollination limitation of sexual reproduction in plantsmay be detected experimentally when enhanced polli-nation elevates seed or fruit set above natural levels(Burd, 1994). Here, we consider three aspects of pollenlimitation: (1) frequency - when pollinator abundanceis low, some flowers or individual plants may never bevisited; (2) quantity - even when pollinator visits arefrequent, the amount of pollen actually reaching thestigmas or ovules may be low; (3) quality - the sourceof pollen (whether it arrives from the same plant or aclose relative, or from an unrelated donor) can influ-ence fruit and seed set and even the vigour of offspring(Charlesworth & Charlesworth, 1987).

To test for pollen limitation on fruit and seed set onemust increase pollen availability to flowers in natu-rally occurring populations. Supplemental pollinationis best done by using all the flowers on an individual.This reveals whether or not the plant becomesstressed by eliminating the possibility that it shuntsresources away from less intensely pollinated flowers

in favour of the experimentally pollinated ones[Stephenson, 1981; but see Zimmerman & Pyke(1988)].

The source of pollen used in supplemental pollina-tion can be problematic because of the potential forinbreeding and outbreeding depression of fruit andseed set (Waser & Price, 1991). We distinguishbetween cross- and self-pollination because rese-archers rarely note how supplementary crossed pollenwas collected (e.g. at what distance from targetplants).

EVIDENCE FROM THE LITERATURE

Data for this analysis and that presented in subse-quent sections were gleaned from an exhaustivereview of the literature (including Biological Abstractsand the Science Citation Index). Data were availablefor 15 species of non-autogamous orchids in whichresearchers tested for pollination limitation of fruit setby cross-pollinating all the flowers on individualplants growing in field populations (Table 1), compar-ing natural levels of fruit set with those obtained fromsupplemental cross-pollination. As researchers rarelystated which plants were used for pollen sources,these data do not control for variation in pollen qualityother than that self-pollen was not used. However, the

Table 1. Natural fruit set and hand cross-pollination of non-autogamous orchids. In all studies included here, theexperiments were performed by cross-pollinating all flowers on individual plants in field populations. Differences betweennatural and experimental groups are statistically significant (Wilcoxon signed rank test, tied Z-value 3.180, P = 0.002)

Natural fruit Cross-pollinationSpecies set (%) fruit set (%) References

Aspasia principissa Rchb. f. 8.4–10.6 61.2 Zimmerman & Aide, 1989Calopogon tuberosus (L.) Britton,

Sterns & Poggenb.12.4–39.9 83.0 Firmage & Cole, 1988

Calypso bulbosa var. occidentalis (L.)Oakes

11 100 Ackerman, 198121–48 50–100 Alexandersson & Ågren, 1996

Cyclopogon cranichoides (Griseb.) Schltr. 26.4–61.6 96.5–97.5 Calvo, 1990aCypripedium acaule Ait. 0–12.9 75.0–100 Davis, 1986; Gill, 1989; Primack &

Hall, 1990; O’Connell & Johnston,1998

Dendrobium toressae (Bailey) Dockrill 19.0 88.0 Bartareau, 1994Encyclia cordigera (Humb., Bonpl. &

Kunth) Dressler6.8 85.8 Janzen et al., 1980

Epidendrum ciliare L. 4.5–15.0 32.9–49.2 Ackerman & Montalvo, 1990Isotria vertcilata Muhl. ex Willd. 10.0 25.0 Mehrhoff, 1983Orchis boryi Rchb. f. 26.3–50.2 98.8 Gumbert & Kunze, 2001Platanthera blephariglottis (Willd.) Lindl. 30.7–80.6 98.6 Cole & Firmage, 1984Stelis argentata Lindl. 2.2 29.2 Christensen, 1992Tipularia discolor (Pursh) Nutt. 18.0–25.0 47.0–89.0 Snow & Whigham, 1989Tolumnia variegata (Sw.) Braem 4.0 88.0 Ackerman & Montero Oliver, 1985Mean (SE) 23.1 (6.6) 71.2 (8.5)



evidence for pollination limitation of fruit set isunequivocal. In all cases studied, orchids given sup-plemental pollination produced higher levels of fruitset than those pollinated naturally (Wilcoxon sign-rank test, Z = 3.180, P = 0.002). These differences wereoften quite substantial. Levels of fruit set between twoand ten times those seen with natural pollination

were common when plants were hand pollinated. Inthe case of Tolumnia variegata, hand pollinations pro-duced a 50-fold increase compared with natural polli-nation (Ackerman & Montero Oliver, 1985; Calvo,1993).

In studies on 42 additional orchid species (Table 2),researchers performed experimental hand pollination

Table 2. Additional studies of non-autogamous orchids that suggest pollen limitation of fruit set. These studies includethose in which not all flowers on an individual were experimentally pollinated or in which hand-pollination was notperformed on field populations

Species Reference

Aspidogyne argentea (Vell.) Garay Singer & Sazima, 2001bAspidogyne longicornu (Cogn.) Garay Singer & Sazima, 2001bAerangis ellisii (Rchb. f.) Schltr. Nilsson & Rabakonandrianina, 1988Brassavola nodosa (L.) Lindl. Schemske, 1980Catasetum viridiflavum Hook. Zimmerman, Roubik & Ackerman,1989Cleistes divaricata (L.) Ames Gregg, 1989Comparettia falcata Poepp. & Endl. Rodríguez-Robles, Meléndez & Ackerman, 1992Dactylorhiza sambucina (L.) Soó Nilsson, 1980D. incarnata (L.) Soó M. T. Kuitunen, pers. comm.Disa uniflora Berg Johnson & Bond, 1992Diuris maculata R. Br. Beardsell et al., 1986Goodyera oblongifolia Raf. Ackerman, 1975; Kallunki, 1976Ionopsis utricularioides (Sw.) Lindl. Montalvo & Ackerman, 1987Isotria verticillata Muhl. ex Willd. Mehrhoff, 1983Liparis lilifolia (L.) Rich. ex Lindl. Whigham & O’Neil, 1991Listera cordata (L.) R. Br. Ackerman & Mesler, 1979L. ovata (L.) R. Br. Nilsson, 1981Malaxis massonii Ridl. Aragón & Ackerman, 2001Myrosmodes cochleare Garay Berry & Calvo, 1991Orchis collina Sol. ex Russ Dafni & Ivri, 1979O. coriophora L. Dafni & Ivri, 1979O. laxifolia Lam. A. Fritz, pers. comm.O. mascula L. Nilsson, 1983bO. morio L. Nilsson, 1984O. spectabilis (L.) Raf. Dieringer, 1982O. spitzelli Saut. ex Koch Fritz, 1990Paphiopedilum virens (Rchb. f.) Pfitz. Atwood, 1985P. volonteanum (Sand.) Stein Atwood, 1985Platanthera bifolia (L.) Rich. Nilsson, 1983aP. chlorantha (Custer) Rchb. Nilsson, 1983aP. ciliaris (L.) Lindl. Robertson & Wyatt, 1990P. okuboi Makino Inoue, 1985P. stricta Lindl. Patt et al., 1989Prescottia densiflora Lindl. Singer & Sazima, 2001aP. plantaginea Lindl. Singer & Sazima, 2001aP. stachyodes Lindl. Singer & Sazima, 2001aPogonia japonica Rchb. f. Matsui, Ushimaru & Fujita, 2001Prosthechea cochleata (L.) W. E. Higgins J. K. Zimmerman & J. D. Ackerman, unpubl. dataPsychilis krugii (Bello) Sauleda Ackerman, 1989Schomburgkia tibicinis Bateman Rico-Gray & Thien, 1987Thelymitra epipactoides F. Muell. Cropper & Calder, 1990Thelymitra antennifera (Lindl.) Hook. f. Dafni & Calder, 1987



without specifying whether: (1) it was applied to allthe flowers on an individual; (2) they had used plantsnot growing under normal field conditions, or (3) theyhad used pollen from the same plant. While thesestudies are not unequivocal demonstrations of polli-nation limitation, all but one indicate that fruit set inthese orchids may be pollen-limited in the field. Thesingle exception is Inoue’s (1985) study of Platantheramandarinorum spp. hachijoensis, in which naturallevels of fruit set often approached 100% and couldnot therefore be raised through supplementalpollination.

While representing only a tiny proportion of the20 000 or so species, these results were obtained fromtaxa across the taxonomic spectrum of the family andstrongly suggest that pollination limitation of fruit setis a common characteristic of non-autogamousorchids.

GLOBAL PATTERNS OF FRUIT SET

In addition to summarizing data on pollination limi-tation, we compiled a larger data set on patterns offruit set (Table 3). This was done in order to comparelevels of fruit set among temperate and tropical local-ities, compare different pollinator types, explore thecontrasts between deceptive and rewarding plants andto contrast different inflorescence sizes. The globalpatterns of fruit set provide a framework for discus-sion of the ecological and evolutionary implications oforchid reproduction.

Data on natural levels of fruit set and results ofhand pollination were compiled from the literature for216 non-autogamous species (92 genera). These spe-cies are representative of the diversity of geographicaldistribution, habitat and pollination systems in theOrchidaceae. There are species from all continentswhere orchids are known, including 123 temperateand 93 tropical species. Both terrestrial and epiphyticspecies, as well as rewarding (N = 84) and deceptive(N = 132) species were considered. Most pollinatorgroups are also included.

In compiling the data, we had to accommodate dif-ferences among studies in scope, sample size, numberof populations and years, and ways of reporting fruitset. For each species, one value of natural and exper-imental fruit set was sought. For studies that includedmore than one population and/or year, and for speciesthat have been studied by more than one author, themedian of all reported fruit set values was used. Fruitset was reported either as the ratio between the totalnumber of fruit and the total number of flowers in thesample or as the average of all individuals in the sam-ple. For hand pollination, the largest value reportedwas included in order to denote the highest fruit setachievable.

Because data on inflorescence size are not consis-tently reported, we assigned each species a qualitativeindex according to the range of sizes most frequentlyrepresented. Species that had an average of less thanten flowers per inflorescence were assigned a value of0, and species with more than ten flowers had a valueof 1. In this paper, inflorescence size is broadly definedas the total number of flowers produced throughoutthe reproductive season. To facilitate comparison, pol-linators were grouped in broad categories: bees,wasps, moths, butterflies, ants, flies, birds, beetles andgeneralist.

A three-way non-parametric ANOVA with interac-tion (Iman, 1974; Zar, 1996) was used to determinewhether latitude (tropical and temperate), presence orabsence of pollinator reward, and inflorescence size(less than vs. more than ten flowers/inflorescence),had a significant effect on median fruit set. Valueswere ranked before analysis.

Pollinator reward and latitude had a significanteffect on median fruit set (Table 4). Median fruit set intemperate species (34.6 ± 2.3; N = 123) is more thantwice that of tropical species (17.0 ± 2.1; N = 91);deceptively pollinated species have a per cent fruit set(20.7 ± 1.7; N = 130) half that of rewarding species(37.1 ± 3.2; N = 84). Inflorescence size and none of theinteractions were significant (Table 4). In general,most species have low fruit set (Fig. 1).

FRUITING FAILURE OF INDIVIDUAL PLANTS

Species vary in the percentage of flowering plants thatfail to set fruit (Table 3). Sample size for this analysisis limited because few surveys have reported fruitingfailure (temperate N = 18, tropical N = 18; deceptiveN = 28, rewarding N = 8). A two-way non-parametricANOVA with interaction of geographical area and pol-lination mechanism was performed on the ranked

Figure 1. Frequency distribution of median fruit set of216 orchids using data from Table 3.

0

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ora

Lin

dl.

–55

.697

.71

–8

Ste

iner

, 199

8C

hlo

raea

lam

ella

te L

indl

.–

15.6

76.0

1–

4L

ehn

ebac

h &

Riv

eros

, 200

3C

leis

tes

div

aric

ata

(L.)

Am

es–

57.9

95.0

040

.01

Gre

gg, 1

989,

199

1a, b

Cor

ysan

thes

tri

loba

Hoo

k. f

.–

2.5

–0

–0

Fit

zger

ald,

(ci

ted

in D

arw

in, 1

877)

Cyc

lopo

gon

cra

nic

hoi

des

(G

rise

b.)

Sch

ltr.

–39

.0–

0–

–C

alvo

, 199

0bC

ypri

ped

ium

aca

ule

Ait

.13

.15.

410

0.0

096

.81

Pri

mac

k &

Hal

l, 19

90; G

ill,

1989

;D

avis

, 198

6; O

’Con

nel

l &

Joh

nst

on, 1

998

Cyp

ripe

diu

m c

alce

olu

s L

.–

10.5

–0

93.0

1N

ilss

on, 1

979b

; Ku

ll &

Ku

ll, 1

991;

Ku

ll, 1

998

–33

.096

.00

–1

Bli

von

a, 2

002

Cyp

ripe

diu

m c

alif

orn

icu

m A

. Gra

y–

76.0

–0

7.2

1K

ippi

ng,

197

1C

ypri

ped

ium

can

did

um

Mu

hl.

ex W

illd

.–

11.6

–0

––

Car

roll

, Mil

ler

& W

hit

son

, 198

4;C

urt

is, 1

954

Cyp

ripe

diu

m f

asci

cula

tum

Kel

logg

ex

S.W

atso

n–

47.0

–0

–2

Kip

pin

g, 1

971

–28

.963

.20

–2

Lip

ow, B

ern

har

dt &

Van

ce, 2

002

Cyp

ripe

diu

m m

acra

nth

os S

w. v

ar.

rebu

nen

se (

Ku

do)

Miy

abe

& K

udo

Sit

e 1

–8.

325

.00

91.7

1S

ite

2–

1.2

–0

98.8

1S

ugi

ura

et

al.,

2001



Cyp

ripe

diu

m m

onta

nu

m D

ougl

asex

Lin

dl.

–62

.5–

0–

1C

olem

an, 1

995

Cyp

ripe

diu

m r

egin

ae W

alte

r–

15.9

100.

00

–3

Pro

ctor

, 199

8–

29.0

–0

–3

K. B

. Gre

gg, u

npu

bl. d

ata

Dac

tylo

rhiz

a fu

sch

ii (

Dru

ce)

Ver

m.

–39

.1–

0–

1D

afn

i &

Woo

dell

, 198

6; N

eila

nd

& W

ilco

ck,

1998

; Wai

te, H

opki

ns

& H

itch

ings

, 199

1D

acty

lorh

iza

inca

rnat

a (L

.) S

oó–

42.0

–1

–1

Lam

mi

& K

uit

un

en, 1

995

Mat

tila

& K

uit

un

en, 2

000

Dac

tylo

rhiz

a la

ppon

ica

(Lae

st. e

x H

artm

an)

Soó

–16

.3–

1–

1N

eila

nd

& W

ilco

ck, 1

998

Dac

tylo

rhiz

a m

acu

lata

(L

.) S

oó–

34.5

–1

–1

Nei

lan

d &

Wil

cock

, 199

8D

acty

lorh

iza

purp

ure

lla

(T. &

T. A

.S

teph

enso

n)

Soó

–32

.6–

––

1N

eila

nd

& W

ilco

ck, 1

998

Dac

tylo

rhiz

a sa

mbu

cin

a (L

.) S

oó–

8.0

100.

01

–1

Nil

sson

, 198

0; P

ette

rsso

n &

Nil

sson

, 198

3D

isa

atri

capi

lla

(Th

un

b.)

Sw

.–

64.7

–0

–2

Ste

iner

, Wh

iteh

ead

& J

ohn

son

, 199

4D

isa

biva

lvat

a (L

. f.)

Du

ran

d &

Sch

inzl

.–

56.5

–0

–2

Ste

iner

, Wh

iteh

ead

& J

ohn

son

, 199

4D

isa

ferr

ugi

nea

(T

hu

nb.

) S

w.

–69

.5–

0–

5Jo

hn

son

, 199

4D

isa

gran

difl

ora

L.

–6.

4–

0–

–D

arw

in, 1

877

Dis

a ra

cem

osa

L. f

.23

.030

.310

0.0

1–

1Jo

hn

son

et

al.,

1998

Dis

a te

nu

ifol

ia (

Th

un

b.)

Lin

der

39.6

43.3

81.1

0–

1Jo

hn

son

& S

tein

er, 1

994

Dis

a ve

nos

a S

w.

49.1

13.6

100.

00

–1

Joh

nso

n e

t al

., 19

98D

iuri

s m

acu

lata

R. B

r.36

.519

.010

0.0

1–

1B

eard

sell

et

al.,

1986

Dra

kaea

gly

ptod

on F

itz

–13

.9–

0–

2P

eaka

ll, 1

990

Gal

eari

s sp

ecta

bili

s (L

.) R

af.

–2.

8–

–D

ieri

nge

r, 19

82H

ersc

hel

ian

thes

gra

min

ifol

ia (

Spr

eng.

)D

ura

nd

& S

chin

zl.

52.3

63.0

–0

–1

Joh

nso

n, 1

993

Isot

ria

vert

icil

lata

Mu

hl.

ex W

illd

.–

5.5

43.0

094

.71

Meh

rhof

f, 19

83L

epor

ella

fim

bria

ta (

Lin

dl.)

A. S

. Geo

rge

–17

.8–

0–

6P

eaka

ll, 1

989a

; Pea

kall

, Bea

ttie

& J

ames

, 198

7L

ipar

is l

ilif

olia

(L

.) R

ich

. ex

Lin

dl.

–1.

662

.81

–3

Wh

igh

am &

O’N

eill

, 199

1O

phry

s ar

anif

era

Hu

ds.

–0.

03–

––

–D

elph

ino

in D

arw

in, 1

877

Oph

rys

bom

byli

flor

a L

ink

–21

.4–

0–

1N

eila

nd

& W

ilco

ck, 1

998

Oph

rys

inse

ctif

era

L.

–8.

7–

0–

1N

eila

nd

& W

ilco

ck, 1

998

Oph

rys

sph

egod

es M

ill.

–21

.1–

0–

1N

eila

nd

& W

ilco

ck, 1

998

Oph

rys

ten

thre

din

ifer

a W

illd

.–

55.5

–0

–1

Nei

lan

d &

Wil

cock

, 199

8

PF

S

Spe

cies

PR

open

han

dIS

FF

PG

Ref

eren

ces

Tab

le 3

.C

onti

nu

ed



Oph

rys

vern

ixia

Bro

t.–

7.6

–0

–2

Nei

lan

d &

Wil

cock

, 199

8O

rch

is b

oryi

Rch

b. f

.43

.648

.595

.81

–1

Gu

mbe

rt &

Ku

nze

, 200

1O

rch

is c

aspi

a T

rau

tv.

–48

.2–

1–

1D

afn

i, 19

83O

rch

is c

olli

na

Sol

. ex

Ru

ss–

30.0

100.

01

–1

Daf

ni

& I

vri,

1979

Orc

his

gal

ilae

a (B

orn

m. &

M. S

chu

lze)

Sch

ltr.

–30

.0–

1–

1B

ino,

Daf

ni

& M

eeu

se, 1

982

Orc

his

isr

aeli

tica

H. B

aum

ann

& D

afn

i–

38.1

–0

–1

Daf

ni

& I

vri,

1981

aO

rch

is i

tali

ca P

oir.

–14

.3–

1–

1N

eila

nd

& W

ilco

ck, 1

998

Orc

his

lax

iflor

a L

am. s

sp. p

alu

stri

s(J

acq.

) A

sch

. & G

raeb

n.

–23

.010

0.0

1–

1A

.-L

. Fri

tz, p

ers.

com

m.

Orc

his

mas

cula

L.

52.0

7.8

100.

01

50.0

1N

ilss

on 1

983b

, Joh

nso

n &

Nil

sson

, 199

9O

rch

is m

ilit

aris

L.

–13

.6–

1–

1Fa

rrel

l 19

85, S

pren

gel

(cit

ed i

n D

arw

in, 1

877)

–21

.5–

1–

1K

isse

leva

& T

imon

in, 2

001

Orc

his

mor

io L

.–

20.2

100.

01

55.0

1N

ilss

on, 1

984

Orc

his

pal

len

s L

.–

13.0

–1

–1

Vöt

h, 1

982

(cit

ed i

n v

an d

er C

inge

l, 19

95)

Orc

his

pap

ilio

nac

ea L

.–

50.0

–0

–1

Vog

el, 1

972

(cit

ed i

n D

afn

i, 19

87)

Orc

his

pu

rpu

rea

Hu

ds.

–5.

5–

0–

1N

eila

nd

& W

ilco

ck, 1

998

Orc

his

spi

tzel

ii S

aut.

ex

Koc

h–

20.4

100.

01

24.8

1F

ritz

, 199

0P

ogon

ia o

phio

glos

soid

es (

L.)

Ker

–28

.610

0.0

071

.41

Bol

and

& S

cott

, 199

1; P

roct

or, 1

998

Sep

apia

s co

rdig

era

L.

–64

.5–

0–

1N

eila

nd

& W

ilco

ck, 1

998

Ser

apia

s pa

rvifl

ora

Par

l.–

58.6

–0

–1

Nei

lan

d &

Wil

cock

, 199

8S

erap

ias

vom

erac

ea B

riq.

–63

.8–

0–

1D

afn

i, Iv

ri &

Bra

ntj

es, 1

981

Ste

ven

iell

a sa

tyro

ides

(S

teve

n)

Sch

ltr.

69.0

––

––

Naz

arov

, 199

5T

hel

ymit

ra e

pipa

ctoi

des

F. M

uel

l.–

35.0

––

–4

Cro

pper

, Cal

der

& T

omki

nso

n, 1

989

Th

elym

itra

ixi

oid

es S

w.

10.8

28.0

––

–4

Syd

es &

Cal

der,

1993

Tri

phor

a tr

ian

thop

hor

a (S

w.)

Ryd

b.–

5.0

90.0

0–

1W

illi

ams,

199

4

TE

MP

ER

AT

E, R

EW

AR

D

Aci

anth

us

sin

clai

rii

Hoo

k. f

.–

81.6

––

––

Ch

eese

man

, cit

ed i

n D

arw

in, 1

877

Are

thu

sa b

ulb

osa

L.

–5.

0–

0–

1T

hie

n &

Mar

cks,

197

2C

rem

astr

a ap

pen

dic

ula

ta D

. Don

–1.

795

.51

–1

Ch

un

g &

Ch

un

g, 2

003

var.

vari

abil

is B

lum

e–

1.8

–1

–1

Su

giu

ra, 1

996a

Cym

bid

ium

goe

rin

gii

(Rch

b. f

.) R

chb.

f.

–0.

5–

1–

?C

hu

ng

& C

hu

ng,

200

3D

acty

lorh

iza

fuch

sii

(Dru

ce)

Soó

–53

.7–

1–

1D

afn

i &

Woo

dell

, 198

6D

isa

un

iflor

a B

erg

52.0

45.5

–0

–5

Joh

nso

n &

Bon

d, 1

992

Val

ley

–20

.069

.50

–5

Gor

ge–

71.0

78.5

0–

5E

pipa

ctis

con

sim

ilis

D. D

on–

15.5

–1

–3

Ivri

& D

afn

i ,1

977

Epi

pact

is h

elle

bori

ne

(L.)

Cra

ntz

41.0

––

1–

3P

iper

& W

aite

, 198

8

PF

S

Spe

cies

PR

open

han

dIS

FF

PG

Ref

eren

ces



Epi

pact

is p

alu

stri

s (L

.) C

ran

tz64

.062

.3–

0–

4N

ilss

on, 1

978a

Epi

pact

is t

hu

nbe

rgii

A. G

ray

–54

.7–

1–

3S

ugi

ura

, 199

6bG

ood

yera

fol

iosa

(K

un

tze)

Ben

th. e

x H

ook.

f. v

ar. m

axim

owic

zian

a M

akin

o17

.557

.1–

0–

1S

ugi

ura

& Y

amag

uch

i, 19

97

Goo

dye

ra o

blon

gifo

lia

Raf

. –

52.6

100.

01

0.0

1A

cker

man

197

5; K

allu

nki

, 197

6G

ood

yera

pro

cera

Ker

-Gaw

l.75

.092

1–

1W

ong

& S

un

, 199

9G

ood

yera

rep

ens

(L.)

R. B

r.–

68.3

–1

–1

Nei

lan

d &

Wil

cock

, 199

8G

ood

yera

rep

ens

(L.)

R. B

r. va

r.op

hio

ides

Fer

nal

d–

49.7

–1

–1

Kal

lun

ki, 1

981

Goo

dye

ra t

esse

lata

Lod

d.–

41.0

–1

–1

Kal

lun

ki, 1

981

Gym

nad

enia

con

opse

a (L

.) R

. Br.

–63

.3–

1–

–N

eila

nd

& W

ilco

ck, 1

998

Her

min

ium

mon

orch

is (

L.)

R. B

r.59

.083

.0–

1–

4N

ilss

on, 1

979a

Lip

aris

ku

mok

iri

Mae

k.–

11.2

–1

––

Oh

et

al.,

2001

Lip

aris

mak

inoa

ma

Sch

ltr.

–0.

1–

1–

–O

h e

t al

., 20

01L

ipar

is r

eflex

a (R

. Br.

) L

indl

.–

22.3

–1

–3

Wal

lace

, 197

4L

iste

ra c

ord

ata

(L.)

R. B

r.–

69.5

85.0

1–

4A

cker

man

& M

esle

r, 19

79–

20.7

100.

01

–4

Mel

énde

z-A

cker

man

& A

cker

man

, 200

1L

iste

ra o

vata

(L

.) R

. Br.

–1.

010

0.0

1–

4N

ilss

on, 1

981

Mic

roti

s pa

rvifl

ora

R. B

r.86

.096

.6–

1–

6P

eaka

ll &

Bea

ttie

, 198

9M

onad

enia

oph

ryd

ea L

indl

.94

.093

.010

0.0

1–

0Jo

hn

son

, 199

5O

rch

is c

orio

phor

a L

.–

78.0

100.

01

–1

Daf

ni

& I

vri,

1979

Orc

his

spe

ctab

ilis

L.

–5.

010

0.0

0–

1D

ieri

nge

r, 19

82O

reor

chis

pat

ens

Lin

dl.

90.7

32.9

–1

0.0

4S

ugi

ura

, Oka

jim

a &

Mae

ta, 1

997

Pla

tan

ther

a bi

foli

a (L

.) R

ich

.60

.159

.410

0.0

1–

1N

ilss

on, 1

983a

; Mat

tila

, 200

0P

lata

nth

era

blep

har

iglo

ttis

(W

illd

.)L

indl

.–

46.8

100.

01

2.0

0S

mit

h &

Sn

ow 1

976;

Col

e &

Fir

mag

e, 1

984

Pla

tan

ther

a ch

lora

nth

a (C

ust

.) R

chb.

49.3

51.8

100.

01

–1

Nil

sson

197

8b, 1

983a

Pla

tan

ther

a ci

liar

is (

L.)

Lin

dl.

27.3

–1

–0

Sm

ith

& S

now

197

6; R

ober

tson

& W

yatt

, 199

0;91

.0–

1–

0G

regg

, 199

0P

lata

nth

era

inte

gril

abia

(C

orre

ll)

Lu

er4.

913

.6–

––

–Z

ettl

er &

Fai

rey,

199

0P

lata

nth

era

lace

ra (

Mic

hx.

) G

. Don

–70

.291

.01

––

Gre

gg, 1

990

Pla

tan

ther

a m

and

arin

oru

m R

chb.

f. s

sp.

hac

hij

oen

sis

(Hon

da)

Mu

rata

81.5

90.0

–1

–0

Inou

e, 1

986b

Pla

tan

ther

a m

etab

ifol

ia F

. Mae

k.–

11.3

–1

40.0

0In

oue,

198

6aP

lata

nth

era

obtu

sata

(B

anks

ex

Pu

rsh

)L

indl

.–

14.5

–1

50.0

4T

hie

n &

Ute

ch, 1

970

PF

S

Spe

cies

PR

open

han

dIS

FF

PG

Ref

eren

ces

Tab

le 3

.C

onti

nu

ed



Pla

tan

ther

a ok

ubo

i M

akin

o–

26.5

–1

–0

Inou

e, 1

985

Pla

tan

ther

a st

rict

a L

indl

.67

.552

.098

.01

–1

Pat

t et

al.

, 198

9P

ogon

ia j

apon

ica

Rch

b. f

.17

.520

.275

.00

–1

Mat

sui,

Ush

imar

u &

Fu

jita

, 200

111

.75.

0–

0–

1U

shim

aru

& N

akat

a, 2

001

Pog

onia

oph

iogl

osso

ides

(L

.) K

er-G

awl.

–55

.010

0.0

0–

1T

hie

n &

Mar

cks

1972

; Pro

ctor

, 199

8P

raso

phyl

um

od

orat

um

R. B

r.75

.052

.0–

1–

3B

ern

har

t &

Bu

rns-

Bal

ogh

, 198

6P

raso

phyl

lum

rom

anzo

ffia

Ch

am.

–>7

5.0

–1

–1

Lar

son

& L

arso

n, 1

987

Pte

rogl

ossa

psis

ru

wen

zori

ensi

s (R

endl

e)R

olfe

–54

.8–

1–

8S

inge

r &

Coc

ucc

i, 19

97a

Sat

yriu

m b

icor

ne

Th

un

b.

20.1

84.7

100.

01

–7

Ell

is &

Joh

nso

n, 1

999

Sat

yriu

m c

orii

foli

um

Sw

.38

.041

.810

0.0

1–

7E

llis

& J

ohn

son

, 199

9S

atyr

ium

ere

ctu

m S

w.

56.0

68.2

100.

01

–7

Ell

is &

Joh

nso

n, 1

999

Spi

ran

thes

lac

era

(Raf

.) R

af. v

ar. l

acer

a–

57.0

100

1–

1C

atli

ng,

198

2S

pira

nth

es l

uci

da

(H. H

. Eat

on)

Am

es–

0.0

100

1–

1C

atli

ng,

198

2S

pira

nth

es o

chro

leu

ca (

Ryd

b.)

Ryd

b.–

0.0

100

1–

1C

atli

ng,

198

2S

pira

nth

es r

oman

zoffi

ana

Ch

am.

–0.

010

01

–1

Cat

lin

g, 1

982

Spi

ran

thes

ver

nal

is E

nge

lm. &

Gra

y–

0.0

100

1–

1C

atli

ng,

198

2T

ipu

lari

a d

isco

lor

(Pu

rsh

) N

utt

.–

23.0

91.3

111

.00

Wh

igh

am &

McW

eth

y, 1

980;

Sn

ow &

Wh

igh

am, 1

989

TR

OP

ICA

L, D

EC

EP

TIV

E

Asp

asia

pri

nci

piss

a R

chb.

f.

58.1

9.5

60.8

077

.31

Zim

mer

man

& A

ide,

198

9B

leti

a pa

tula

Gra

ham

17.3

27.6

–1

6.3

1A

cker

man

199

5; J

. D. A

cker

man

&C

arro

mer

o, u

npu

bl. d

ata

Bra

ssav

ola

nod

osa

(L.)

Lin

dl.

38.3

13.2

67.0

082

.60

Sch

emsk

e, 1

980;

Mu

rren

& E

llis

son

199

6B

ulb

oph

yllu

m i

nvo

lutu

m B

orba

, Sem

ir &

F. B

arro

s39

.33.

850

.01

–3

Bor

ba &

Sem

ir, 1

998;

Bor

ba

& S

emir

199

9b;

Bor

ba, S

hep

pard

& S

emir

, 199

9B

ulb

oph

yllu

m i

pan

emen

se H

oeh

ne

92.3

3.8

70.0

1–

3B

orba

& S

emir

, 199

8; B

orba

&

Sem

ir 1

999b

;B

orba

, Sh

eppa

rd &

Sem

ir, 1

999

Bu

lbop

hyl

lum

war

min

gian

um

Cog

n.

–0.

033

.01

–3

Saz

ima,

197

8B

ulb

oph

yllu

m w

edd

elli

i (L

indl

.) R

chb.

f.

51.5

4.5

20.6

1–

3B

orba

& S

emir

, 199

8; B

orba

&

Sem

ir 1

999b

;B

orba

, Sh

eppa

rd &

Sem

ir, 1

999

Coc

hle

anth

es l

ipsc

ombi

ae (

Rol

fe)

Gra

y–

15.0

–0

83.0

1A

cker

man

, 198

3C

orya

nth

es e

lega

nti

um

Lin

den

& R

chb.

f.

–25

.0–

0–

1D

odso

n, 1

965

Cor

yan

thes

leu

coco

rys

Rol

fe–

0.0

–0

–1

Dod

son

, 196

5C

orya

nth

es m

acra

nth

a (H

ook.

) H

ook.

–21

.0–

0–

1D

odso

n, 1

965

Cor

yan

thes

rod

rigu

esii

Hoe

hn

e–

43.0

–0

–1

Dod

son

, 196

5C

orya

nth

es t

rifo

liat

a C

. Sch

wei

nf.

–40

.0–

0–

1D

odso

n, 1

965

Cor

ymbo

rkis

for

cipi

gera

L. O

. Wil

liam

s–

9.1

–1

––

Ack

erm

an, 1

995

Cyc

lopo

gon

cra

nic

hoi

des

(G

rise

b.)

Sch

ltr.

–29

.597

.51

38.5

–C

alvo

, 199

0aC

ymbi

die

lla

flab

ella

ta (

Th

ou.)

Rol

fe52

.25.

0–

1–

2N

ilss

on e

t al

., 19

86

PF

S

Spe

cies

PR

open

han

dIS

FF

PG

Ref

eren

ces



Cyr

topo

diu

m b

road

way

i A

mes

–1.

0–

1–

1Q

ues

nel

et

al.,

1982

Den

dro

biu

m i

nfu

nd

ibu

lum

Lin

dl.

5.5

6.5

–1

91.0

1K

jell

sson

, Ras

mu

ssen

& D

upu

y, 1

985

Den

dro

biu

m m

onop

hyl

lum

F. M

uel

l.79

.56.

6–

110

.21

Bar

tare

au, 1

995

Den

dro

biu

m s

peci

osu

m S

m.

–10

.0–

1–

4C

alde

r, A

dam

s &

Sla

ter,

1982

Den

dro

biu

m t

ores

sae

(Bai

ley)

Doc

kril

l72

.019

.088

.00

37.5

1B

arta

reau

, 199

4D

ilom

ilis

mon

tan

a (S

w.)

Su

mm

erh

.I.

Rod

rígu

ez-C

olón

& J

. D. A

cker

man

,u

npu

bl. d

ata

Sit

e 1

–6.

1–

092

.21?

Sit

e 2

–14

.3–

070

.31?

Ell

ean

thu

s cf

. bre

nes

ii–

29.0

–1

–7

B. G

rabo

wsk

i, pe

rs c

omm

.E

ncy

clia

cor

dig

era

(Hu

mb.

, Bon

pl. &

Ku

nth

) D

ress

ler

–7.

097

.00

78.0

1Ja

nze

n e

t al

., 19

80

Epi

den

dru

m c

ilia

re L

.21

.97.

749

.20

–0

Ack

erm

an &

Mon

talv

o, 1

990

Epi

den

dru

m e

xasp

erat

um

Rch

b. f

.–

2.0

–1

40.0

5C

alvo

, 199

0bIo

nop

sis

utr

icu

lari

oid

es (

Sw

.) L

indl

.–

6.1

19.1

1–

7M

onta

lvo

& A

cker

man

, 198

7L

aeli

a sp

ecio

sa (

Hu

mb.

, Bon

pl. &

Ku

nth

.)

Sch

ltr.

–14

.966

.70

–1

Her

nán

dez-

Apo

lin

ar, 1

992

Lep

anth

es c

arit

ensi

s T

rem

blay

&A

cker

man

4.3

0.0

–1

100.

03

Tre

mbl

ay, 1

997b

; Tre

mbl

ay e

t al

., 19

98

Lep

anth

es e

ltor

oen

sis

Sti

mso

n19

.11.

8–

188

.53

Tre

mbl

ay, 1

996

Lep

anth

es r

ubr

ipet

ala

Sti

mso

n11

.75.

0–

175

.93

Tre

mbl

ay, 1

996

Lep

anth

es r

upe

stri

s S

tim

son

12.6

4.9

–1

50.7

3T

rem

blay

, 199

6L

epan

thes

san

guin

ea H

ook.

–5.

9–

171

.43

Ack

erm

an &

Zim

mer

man

(ci

ted

inC

hri

sten

sen

, 199

2)L

epan

thes

wen

dla

nd

ii R

chb.

f.

–12

.0–

134

.0–

Cal

vo, 1

990b

Lep

anth

es w

ood

bury

ana

Sti

mso

n–

9.1

–0

62.5

–J.

D. A

cker

man

& J

. K. Z

imm

erm

an,

un

publ

. dat

aM

alax

is m

asso

nii

(R

idl.)

Ku

ntz

e14

.32.

429

.01

––

Ara

gón

& A

cker

man

, 200

1M

orm

odes

tu

xtle

nsi

s S

alaz

ar–

3.3

–1

30.0

1S

osa

& R

odrí

guez

-An

gulo

, 200

0M

yrm

ecop

hil

a ti

bici

nis

(B

atem

an)

Rol

fe–

2.4

92.0

069

.31

Ric

o-G

ray

& T

hie

n, 1

987

Ner

vili

a bi

cari

nat

a (B

l.) S

chlt

r.14

.6–

–0

–2

Pet

ters

son

, 198

9N

ervi

lia

hu

mil

is S

chlt

r.21

.6–

–0

–1

Pet

ters

son

, 198

9N

ervi

lia

shir

ensi

s (R

olfe

) S

chlt

r.–

10.0

–0

–2

Pet

ters

son

, 198

9N

ervi

lia

stol

zian

a S

chlt

r.25

.950

.0–

0–

2P

ette

rsso

n, 1

989

PF

S

Spe

cies

PR

open

han

dIS

FF

PG

Ref

eren

ces

Tab

le 3

.C

onti

nu

ed



On

cid

ium

alt

issi

mu

m (

Jacq

.) S

w.

–2.

0–

1–

1A

cker

man

, 199

5O

nci

diu

m a

scen

den

s L

indl

.F

ores

t20

.06.

8–

1–

1P

arra

-Tab

la e

t al

., 20

00P

asto

ral

fiel

d6.

53.

1–

1–

1P

arra

-Tab

la e

t al

., 20

00O

nci

diu

m s

tipi

tatu

m L

indl

.–

1.8

–1

–1

J. K

. Zim

mer

man

, un

publ

. dat

aP

aph

iope

dil

um

vil

losu

m (

Lin

dl.)

Ste

in–

7.8

–0

–1

Bän

zige

r, 19

96P

olys

tach

ya c

oncr

eta

(Jac

q.)

Gar

ay &

Sw

eet

24.5

10.0

–1

–1

Gos

s, 1

977

Ple

uro

thal

lis

adam

anti

nen

sis

Bra

de–

–80

.81

–3

Bor

ba, S

emir

& S

hep

her

d, 2

001;

Bor

ba &

Sem

ir 2

001

Ple

uro

thal

lis

fabi

obar

rosi

i B

orba

&S

emir

––

78.6

1–

3B

orba

, Sem

ir &

Sh

eph

erd,

200

1;

Bor

ba &

Sem

ir 2

001

Ple

uro

thal

lis

joh

ann

ensi

s B

arb.

Rod

r.–

39.0

59.0

1–

3B

orba

, Sem

ir &

Sh

eph

erd,

200

1; B

orba

&S

emir

200

1P

rost

hec

hea

coc

hle

ata

(L.)

W. E

.H

iggi

ns

–4.

654

.50

76.3

1J.

D. A

cker

man

& J

. K. Z

imm

erm

an,

un

publ

. dat

aP

sych

ilis

kru

gii

(Bel

lo)

Sau

leda

21.3

4.0

8.0

090

.0–

Ack

erm

an, 1

989

Ste

lis

arge

nta

ta L

indl

.58

.72.

229

.21

––

Ch

rist

ense

n, 1

992

Ste

lis

sp. 1

–15

.0–

0–

–C

hri

sten

sen

, 199

2S

teli

s sp

. 2–

8.0

–1

––

Ch

rist

ense

n, 1

992

Ste

lis

sp. 3

–12

.0–

0–

8C

hri

sten

sen

, 199

2S

teli

s sp

. 4–

2.0

–0

––

Ch

rist

ense

n, 1

992

Tet

ram

icra

can

alic

ula

ta (

Au

bl.)

Urb

.–

6.0

80.0

0–

–P

agán

, Mar

tín

ez &

Ack

erm

an (

cite

d in

Ack

erm

an, 1

995)

Tol

um

nia

var

iega

ta (

Sw

.) B

raem

3.9

2.3

77.8

098

.01

Ack

erm

an &

Mon

tero

Oli

ver,

198

5;A

cker

man

, Mel

énde

z-A

cker

man

&S

algu

ero-

Far

ía, 1

997;

Cal

vo, 1

990b

Van

illa

bar

bell

ata

Rch

b. f

.5.

318

.210

0.0

0–

1I.

Pan

etto

& J

. D. A

cker

man

, u

npu

bl.

data

;L

. R. N

iels

en &

J. D

. Ack

erm

an, u

npu

bl. d

ata

Van

illa

cla

vicu

lata

(W

. Wri

ght)

Sw

.17

.915

.010

0.0

0–

1I.

Pan

etto

& J

. D. A

cker

man

, u

npu

bl.

data

;L

. R. N

iels

en &

J. D

. Ack

erm

an, u

npu

bl. d

ata

Van

illa

dil

lon

ian

a C

orre

ll5.

514

.510

0.0

0–

1I.

Pan

etto

& J

. D. A

cker

man

, u

npu

bl.

data

;L

. R. N

iels

en &

J. D

. Ack

erm

an, u

npu

bl. d

ata

Van

illa

pla

nif

olia

An

drew

s–

1.0

–0

–1

Ack

erm

an, 1

995

Van

illa

poi

taei

Rch

b. f

.–

6.4

100.

00

–1

I. P

anet

to &

J. D

. Ack

erm

an, u

npu

bl. d

ata

TR

OP

ICA

L, R

EW

AR

D

Aer

angi

s el

lisi

i (R

chb.

f.)

Sch

ltr.

–22

.250

.00

14.0

0N

ilss

on &

Rab

akon

andr

ian

ina,

198

8A

ngr

aecu

m a

rach

nit

es S

chlt

r.A

spid

ogyn

e ar

gen

tea

(Vel

l.) G

aray

– –41

.011

.1– 86

.10 1

– –0 5

Nil

sson

, 198

5; N

ilss

on e

t al

., 19

85,

Sin

ger

& S

azim

a, 2

001b

PF

S

Spe

cies

PR

open

han

dIS

FF

PG

Ref

eren

ces



Asp

idog

yne

lon

gico

rnu

(C

ogn

.) G

aray

–18

.010

0.0

10.

01

Sin

ger

& S

azim

a, 2

001b

Cat

aset

um

mac

roca

rpu

m R

ich

. ex

Ku

nth

–7.

1–

0–

1C

arva

lho

& M

ach

ado,

200

2C

atas

etu

m v

irid

iflav

um

Hoo

k.–

12.0

95.8

171

.21

Zim

mer

man

, Rou

bik

& A

cker

man

, 198

9;Z

imm

erm

an, 1

991

Com

pare

ttia

fal

cata

Poe

pp. &

En

dl.

29.1

15.7

86.5

0–

7R

odrí

guez

-Rob

les,

Mel

énde

z &

Ack

erm

an,

1992

; A

cker

man

,Rod

rígu

ez-R

oble

s &

Mel

énde

z,19

94; S

algu

ero-

Far

ía &

Ack

erm

an, 1

999

Cyc

lopo

gon

con

gest

us

(Vel

l.) H

oeh

ne

–0.

098

.01

–1

Sin

ger

& S

azim

a, 1

999

Den

dro

chil

um

lon

gbra

ctea

tum

Pfi

tzer

–0.

02–

1–

3P

eder

sen

, 199

5E

ryth

rod

es a

riet

ina

(Rch

b. f

. & W

arm

.)A

mes

–22

.6–

10.

01

Sin

ger

& S

azim

a, 2

001b

Hab

enar

ia g

ourl

iean

a L

indl

.15

.419

.2–

133

.30

Sin

ger

& C

occu

ci, 1

997b

Hab

enar

ia h

iero

nym

i K

raen

zl.

67.1

81.0

–1

8.3

0S

inge

r &

Coc

cuci

, 199

7bH

aben

aria

mon

tevi

den

sis

Spr

eng.

49.4

61.0

–1

0.0

0S

inge

r &

Coc

cuci

, 199

7bH

aben

aria

par

vifl

ora

Lin

dl.

–0.

096

.71

–3

Sin

ger,

2001

Hab

enar

ia r

upi

cola

Bar

b. R

odr.

69.5

79.3

–1

0.0

0S

inge

r &

Coc

cuci

, 199

7bL

eoch

ilu

s sc

ript

us

(Sch

eidw

.) R

chb.

f.

–60

.086

.50

–2

Ch

ase,

198

6M

yros

mod

es c

och

lear

e G

aray

–21

.545

.01

–2

Ber

ry &

Cal

vo, 1

991

Mys

taci

diu

m v

enos

um

Lin

dl.

–0.

065

.00

–0

Lu

yt &

Joh

nso

n, 2

001

Not

ylia

nem

oros

a B

arb.

Rod

r.–

64.8

–1

–1

Sin

ger

& K

oeh

ler,

2002

Pel

exia

oes

trif

era

(Rch

b. f

. & W

arm

.)S

chlt

r.–

0.0

100.

01

–1

Sin

ger

& S

azim

a, 1

999

Ple

uro

thal

lis

och

reat

a L

indl

.–

12.5

39.4

1–

3B

orba

, Sem

ir &

Sh

eph

erd,

200

1; B

orba

&S

emir

, 200

1P

leu

roth

alli

s ra

cem

iflor

a L

indl

. ex

Hoo

k.–

17.3

–1

44.4

–A

cker

man

, 199

5

Ple

uro

thal

lis

tere

s L

indl

.–

7.0

44.3

1–

3B

orba

, Sem

ir &

Sh

eph

erd,

200

1; B

orba

&S

emir

, 200

1P

resc

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data set. No difference was detected between tropicaland temperate species; mean fruiting failure was inthe range of 50–60% of plants from both areas(F1,32 = 0.18, P = 0.67; Fig. 2). However, failuredepended on the pollination mechanism: in deceptivespecies, most plants do not set fruit (66.5%); reward-ing species have much lower failure rates (29.1%;F1,32 = 8.28, P = 0.007).

Per cent fruiting failure was significantly higher forspecies with less than ten flowers per inflorescence(68.0% ± 6.4; mean ± SE) than for those with morethan ten (42.8% ± 8.2; mean ± SE; Mann–Whitney U-test, Z = 2.353; P < 0.02; N = 46).

Moreover, median fruit set and per cent fruitingfailure were significantly negatively correlated(Spearman’s rank correlation, corrected for ties,rho = -0.655; P < 0.0001; N = 44). Thus, deceptive spe-cies with low overall fruit set and small inflorescencesize are most likely to exhibit high levels of fruitingfailure.

POLLINATOR GROUP EFFECT

Per cent median fruit set varies according to pollinatorgroup (Kruskal–Wallis H, corrected for ties = 24.82,P = 0.0004; Table 5). We tested the difference betweenthe median fruit set of orchids pollinated by moths,bees, wasps, flies, butterflies and birds (beetles andants were excluded because of small sample size) aswell as generalist orchid species. Fruit set was lowestfor species pollinated by flies and highest for those pol-linated by birds and for generalists. However, the pat-tern may be confused by the fact that all the generalistspecies with fruit set higher than 50% are temperate,nectariferous species, while of the species pollinatedby bees, many with fruit set below 10% are tropical.

GENERAL PATTERNS OF POLLINATOR LIMITATION

Our data demonstrate that fruit production in non-autogamous orchids is pollen-limited. For almost allspecies where data are available for natural and handpollination, the minimum fruit set difference of thelatter was 10% larger (mean and SE; open pollination,26.6 ± 1.7 (N = 210); cross hand-pollination, 80.0 ± 2.6(N = 98); Table 3). Median natural fruit set in temper-ate species is approximately twice that of tropical spe-cies (mean and SE: tropical 17.0 ± 2.1 (N = 91);temperate 34.6 ± 2.3 (N = 123). It is not evident whythe former should be more efficient at setting fruit. Apossible explanation (further research is required) isthat it may be a result of population size or population

Table 4. Results of three-way non-parametric ANOVAwith interactions for ranked median fruit set. Data of openpollination only, from Table 3. Distribution: temperate vs.tropical. Reward: deceit or reward. Inflorescence size: Inflo-rescence smaller than ten flowers or equal to or larger thanten flowers

Source d.f. MS F P

Distribution 1 61707.9 19.19 <0.0001Reward 1 36862.3 11.46 0.0008Inflorescence size 1 1367.1 0.43 0.51Distribution ¥ reward 1 8698.1 2.71 0.10Distribution ¥

inflorescence1 1382.1 0.43 0.51

Reward ¥ inflorescence 1 20.4 0.01 0.94Three-way interaction 1 844.7 0.20 0.61Residual 206 3215.4

Figure 2. The frequency of deceptive and rewardingplants that fail to set fruits. Data are from Table 3. Means+ SE; N = 44.

80

70

60

50

40

30

20

10

00

Temperate

Tropical

RewardDeceptive

Per

cent

fru

iting

fai

lure

Table 5. Median per cent fruit set and standard error (SE)of naturally pollinated orchids by pollinator types. Medianfruit set is significantly distinct among pollinator groups(Kruskal-Wallis H, corrected for ties = 24.82, P = 0.0004).Beetles and ants excluded because of low sample size

Pollinator N Fruit set SE

Bees 108 25.8 2.2Birds 6 53.8 11.9Butterflies 5 36.7 12.6Generalist 8 37.7 9.3Flies 29 11.9 2.7Moths 22 41.8 6.5Wasps 9 34.2 7.9



dispersion. Tropical species are frequently organizedin small groups of individuals separated by large dis-tances (Ackerman, 1986b; Tremblay, 1997c). Hyper-dispersed populations are perhaps more commonamong tropical epiphytes because they are immersedin a tangled canopy.

Data are generally lacking on population dispersionin orchids. However, an alternative explanation fordifferences among temperate and tropical orchidsmight be that they are due to taxonomic differencesand phylogenetic constraints. An approach to testingthis hypothesis would be to investigate fruit set in spe-cies of the same genus that are in both regions. If fruitproduction is phylogenetically constrained amongregions, then we would expect fruit set within a genusto be more similar than among genera.

Our results are similar to those of Neiland & Wil-cock (1998), who observed that fruit set in temperatespecies is about three times as successful as that oftropical species (38.2% vs. 13.6%). We found just a two-fold difference, which may simply be a consequence ofdifferent sample sizes: Neiland & Wilcock (1998)included 96 species while this survey has 216.

CAUSES OF POLLINATION LIMITATION

Pollinator effectivenessOrchids are visited by a number of different insects orbirds, but not all visitors are pollinators (Ackerman &Mesler, 1979; Nilsson, 1979a, b). The relative fre-quency of effective visits is not often quantified butvariation in visitor performance can be substantial.For example, the most frequent visitors to Herminiummonorchis were rarely effective pollinators and eventhe best carried pollinaria only about 70% of the time(Nilsson, 1979a).

Pollinarium removal and depositionThe frequency of effective visits, quantified as polli-narium removals and pollinations, is often very low.Although the data show substantial variation amongpopulations and species, usually just under half theflowers fail to export their pollen (Table 3).

Pollinaria removals usually exceed pollinations, soone can express efficiency as a ratio of the former tothe latter and this can be used as an index of efficiency.Mean ratio is 1.7 : 1 in Tolumnia variegata (Acker-man, Meléndez-Ackerman & Salguero-Faría, 1997),1.35 : 1 in Comparettia falcata (Salguero-Faría & Ack-erman, 1999), 2.5 : 1 in Epidendrum ciliare (Acker-man & Montalvo, 1990), and 1.96 : 1 in Ionopsisutricularioides (Montalvo & Ackerman, 1987. Therange of per cent removals to per cent fruit set is large(0.24 : 1–26.7 : 1). The most effective systems arethose of Satyrium bicorne, Vanilla barbellata(< 0.30 : 1), while the most inefficient are those of Ste-

lis argentata and Bulbophyllum ipanemense (> 24.0;Table 3; ratios not shown). Pollinarium deposition effi-ciency among species is approximately divided equallybetween efficient (< 1 : 1) and inefficient (> 1 : 1;Fig. 3). We expect pollinaria removals to depositions tobe greater than 1 unless the pollen masses can be bro-ken unto subunits. For example, in Herminiummonorchis the number of removals per depositionranged from 0.74 to 0.94 (Nilsson, 1979a). We testedthe hypothesis that removal to fruit set ratio is moreefficient in mealy pollen orchid species and found it tobe true (Mann–Whitney U-test, Z = 5.443, P = 0.001;Table 3). Orchids with mealy pollen have a mean ratioof 1.1 ± 0.19 : 1 (N = 36) whereas those with hard pol-linia have a mean of 6.97 ± 1.56 : 1 (N = 21).

Pollinator abundance and diversityFruit set within and among populations may be influ-enced by pollinator activity and diversity. In severalSwedish populations of Listera ovata it variedbetween 13 and 70% and was positively correlatedwith visitation rates (Nilsson, 1981). Visitation fre-quencies to two populations of the epiphytic Comp-arettia falcata were related to the abundance of itshummingbird pollinator (Rodríguez-Robles et al.,1992). Ackerman et al. (1997) observed the effect ofpollinator abundance and floral fragrance on fruit setin the deceptive orchid Tolumnia variegata and foundthat the main cause of fruit production among popu-lations could be attributed to pollinator abundance.Moreover, flower production often needs to be synchro-

Figure 3. Frequency distribution of pollinaria removal tofruit set ratio. Values below 1 represent species that setmore then one fruit per pollinarium removal (mainlyorchids with mealy pollen) while values larger than 1 rep-resent species that remove more than one pollinarium perfruit set.

25

20

15

10

5

0<1 2 3 4 5 6 7 8 9 10 <20 <30

Pollinaria removal per fruit set

Freq

uenc

y



nized with the pollinator emergence for effective fruitproduction (Nilsson, 1980, 1983b; Ackerman, 1981,1983; but see Zimmerman, Roubik & Ackerman,1989). Another source of variation is the compositionof the pollinator fauna. Robertson & Wyatt (1990)quantitatively studied the pollination system of Pla-tanthera ciliaris in two disjunct populations in thesouth-eastern United States. They found that boththe primary pollinator and fruit set were differentin the two populations.

Pollen quantityWhat proportions of ovules are fertilized during thedevelopment of orchid fruits? While pollinia can obvi-ously contain many thousands of pollen grains, is thequantity of pollen deposited more than enough to fer-tilize all ovules? Variation is to be expected as polli-naria contain groups of two, four, six or eight pollinia,not all of which are necessarily deposited in a singlepollination event. Moreover, orchid pollen is notalways deposited as indivisible pollinia. Pollinia maybe ‘sectile’ or ‘mealy’, that is, soft enough to be brokenapart into chunks during pollen deposition (Dressler,1981; see Pacini & Hesse, 2002; for a review of pollendispersal units in orchids).

There are indications that increasing the number ofpollinia applied to orchid stigmas causes increasedseed set in orchids. In Cattleya trianaei Linden &Rchb. f., fruit size was found to increase with the num-ber of pollinia applied to stigmas (Duncan & Curtis,1943). No difference in fruit set was observed in Epi-dendrum ciliare following hand pollination with twoor four pollinia (Ackerman & Montalvo, 1990) and inIonopsis utricularioides with one or two pollinia (Mon-talvo & Ackerman, 1987). Gregg (1991a) experimen-tally investigated the effect of variable deposition onseed development in Cleistes divaricata. As expected,seed production declined with reduced deposition,while seed fertility (per cent of seeds with embryos)was unaffected.

Pollen qualityCross- and self-pollination: Low pollination fre-quency and pollen quantity may limit seed productionin orchids, but in self-compatible plants poor qualitypollen resulting from self-pollination can also reduceseed production; this is not always reflected in fruitset. We have self- and cross-pollination data for 69 spe-cies and in 29% of these fruit set was the same. In theremainder, fruit set from outcross pollinations wasslightly higher (81%) than from self-pollinations (72%;Wilcoxon sign-rank test, tied Z = 3.425, N = 52,P = 0.0006, Table 6). In general, it appears that self-pollination does not greatly affect fruit set in orchidsin the absence of a self-incompatibility mechanism.

Self-pollination has been observed to reduce seedproduction in orchids. Most available data report thepercentage of mature seeds bearing embryos in fruitsfrom plants which have been artificially cross- andself-pollinated. Stort & Martins (1980) measuredembryo formation in 14 non-autogamous species ofCattleya in Brazil. Comparing fruit produced usingself-pollination and cross-pollination, they observedan average of 15.3% of seeds with embryos in theformer and 47.5% in the latter. Although seed set fromnatural pollination is often better than of hand-polli-nation, it rarely reaches 100% (Stort, 1973; Stort &Pavanelli, 1986; Pintaúdi, Stort & Marin-Morales,1990).

Similar results were obtained from a compilation ofthe literature involving 76 species (Table 7). Embryoformation ranged from 24% to 99% (average 64.8%)from cross-pollination and from zero to 99%(average 40.8%) from self-pollination. These differ-ences were not always apparent (e.g. Ionopsis utricu-lariodes; Montalvo & Ackerman, 1987). Variance inembryo formation within treatments is significantlylarger in fruit from cross-pollinated plants (equality ofvariance test F71,75 = 1.987, P = 0.004). This suggeststhat species that are primarily cross-pollinated areadversely affected by inbreeding whereas others areat least insensitive to self-pollination at embryoformation.

Three indices of seed production have been mea-sured for a few non-autogamous orchids in order tocompare the influence of self- and cross-pollination:weight of mature fruits, total seed mass and seed pro-duction. The weight of mature ovaries of selfed flowersof Diuris maculata was 77% of that of crossed flowers(Beardsell et al., 1986). Total seed mass of fruits fromself-pollination in Platanthera ciliaris was about 70%of that of fruits from cross-pollination (Robertson &Wyatt, 1990). Seed production of selfed flowers in Lep-orella fimbriata was 76% of that of crossed flowers(Peakall, 1989a), while in Isotria verticillata it wasapproximately the same (Mehrhoff, 1983). In general,seed production from self-pollination is less than orequal to that of cross-pollination.

Given the apparent costs of self-pollination in mostorchids, mechanisms that promote outcrossing are tobe expected. What is unusual is that in orchids theseare often structural (movement of rostellum, move-ment of column, stigmatic maturity) and not the resultof genetic self-incompatibility, dichogamy, or unisexu-ality (although examples of all of these mechanismsexist in orchids). These structural mechanisms ofteninvolve changes in size or the movement of pollinariain a way that prevents the placement on the stigma forup to several minutes of pollinator foraging, throughdifferential drying of opposing surfaces of structuresbearing the pollinia (Dressler, 1981; Borba & Semir,



Table 6. Comparison of fruit set following artificial cross- and self-pollination. Self-incompatible plants are excluded.Cross-pollination produced higher fruit set than self pollination. Significant differences in production were noted, althoughthe mean differences were not large (Wilcoxon signed rank test Z-value 3.425, P < 0.0006, N = 44, N0 = 16, N ties = 6).Values in parentheses are number of pollinations performed

Per cent fruit set

Species Self Cross References

Aerangis ellisii (Rchb. f.) Schltr. 50.0 (26) 50.0 (26) Nilsson & Rabakonandrianina,1988

Aspasia principissa Rchb. f. 60.0 (20) 61.0 (49) Zimmerman & Aide, 1989Aspidogyne argentea (Vell.) Garay 47.1 (51) 86.1 (36) Singer & Sazima, 2001bAspidogyne longicornu (Cogn.) Garay 97.3 (37) 100.0 (30) Singer & Sazima, 2001bBrassavola nodosa (L.) Lindl. 100 (20) 67.0 (60) Schemske, 1980Bulbophyllum warmingianum Cogn. 25.0 (12) 33.0 (12) Sazima, 1978Caladenia tentactulata Tate 100 (4) 100 (4) Peakall & Beattie, 1996Catasetum viridiflavum Hook. 93.3 (15) 95.8 (71) J. K. Zimmerman, unpubl. dataCleistes divaricata (L.) Ames 100 (20) 100 (7) Gregg, 1989Comparettia falcata Poepp. & Endl.

Site 1 53.8 (13) 86.4 (66) Rodriguez-Robles, Meléndez &Ackerman, 1992

Site 2 64.3 (14) 86.7 (45)Cyclopogon congestus (Vell.) Hoenhe 88.0 (49) 98.0 (47) Singer & Sazima, 1999Cynorkis uniflora Lindl. 100 (20) 90.0 (20) Nilsson, Rabakonandrianina &

Pettersson, 1992Cypripedium acaule Ait. 70.0 (10) 74.7 (75) Davis, 1986

90.0 (20) 100 (20) Primack & Hall, 1990100 (18) 100 (11) O’Connell & Johnston, 1998

Cypripedium fasciculatum Kellogg ex S. Watson 78.0 (100) 82.4 (108) Lipow, Bernhardt & Vance, 2002Cypripedium macranthos Sw. var. rebunense 50.0 (4) 25.0 (4) Sugiura et al., 2001

(Kudo) Miyabe & KudoDactylorhiza incarnata (L.) Soó 98.1 (48) 95.2 (21) M. T. Kuitunen, pers. comm.Dactylorhiza sambucina (L.) Soó 96.7 (30) 100 (29) Nilsson, 1980Encyclia cordigera (Humb., Bonpl. & Kunth)

Dressler69.9 (119) 91.0 (211) Janzen et al., 1980

Epidendrum ciliare L. 95.8 (48) 93.5 (31) Ackerman & Montalvo, 1990Epipactis consimilis Don (Druce) Soó 100 (27) 100 (27) Ivri & Dafni, 1977Galearis spectabilis (L.) Raf. 65.4 (26) 64.6 (48) J. K. Zimmerman, unpubl. dataGoodyera procera Ker-Gawl. 92 94 Wong & Sun, 1999Habenaria parviflora Lindl. 93.3 (30) 96.7 (30) Singer, 2001Ionopsis utricularioides (Sw.) Lindl. 88.7 (55) 89.7 (61) Montalvo & Ackerman, 1987Isotria medeoloides (Pursh) Raf. 82 (11) 82 (11) Vitt & Campbell, 1997Isotria verticillata Muhl. ex Willd. 32.5 (40) 36.8 (19) Mehrhoff, 1983Lepanthes rubripetala Stimson 0.0 (11) 30.0 (44) G. Pomales & R. L. Tremblay,

unpubl. dataLepanthes rupestris Stimson 0.0 (25) 31.0 (78) G. Pomales & R. L. Tremblay,

unpubl. dataLepanthes woodburyana Stimson 0.0 (50) 60.9 (92) M. Mendez & R. L. Tremblay,

unpubl. dataLiparis loeselii (L.) C. Rich. 100 (70) 94 (34) Catling, 1980Listera cordata R. Br. 100 (14) 100 (14) Meléndez-Ackerman & Ackerman,

2001Listera ovata (L.) R. Br. 100 (27) 100 (27) Nilsson, 1981Myrosmodes cochleare Garay 51.0 (105) 39.7 (63) Berry & Calvo, 1991Mystacidium venosum Lindl. 40.0 (20) 65.0 (20) Luyt & Johnson, 2001Orchis boryi Rchb. f. 97.9 (90) 98.8 (139) Gumbert & Kunze, 2001Orchis laxifora spp. Palustris Lam. 100 (20) 100 (20) A. Fritz, pers. comm.



Orchis mascula L. 100 (30) 100 (30) Nilsson, 1983bOrchis morio L. 90.0 (10) 100 (10) Nilsson, 1984Orchis spitzelii Sauter ex Koch 95.0 (20) 100 (20) Fritz, 1990Pelexia oestrifera (Rchb. f. & Warm.) Schltr. 81.0 (42) 100 (34) Singer & Sazima, 1999Platanthera bifolia (L.) Rich. 100 (27) 100 (52) Nilsson, 1983aPlatanthera blephariglottis (Willd.) Lindl. 90.9 (44) 98.6 (146) Cole & Firmage, 1984Platanthera chlorantha (Cust.) Rchb. 100 (12) 100 (12) Nilsson, 1983aPlatanthera ciliaris (L.) Lindl.

Site 1 90.9 (33) 84.8 (33) Robertson & Wyatt, 1990Site 2 71.9 (32) 79.4 (34)

Platanthera lacera (Michx.) G. Don 91 96 Gregg, 1990Platanthera leucophaea (Nutt.) Lindl. 63 100 Bowles, 1985 in Gregg 1990Pleurothallis adamantinensis Brade 4.2 80.6 Borba, Semir & Shepherd, 2001Pleurothallis fabiobarrosii Borba & Semir 5.3 78.6 Borba, Semir & Shepherd, 2001Pleurothallis johannensis Barb. Rodr. 12.4 59.0 Borba, Semir & Shepherd, 2001Pleurothallis ochreata Lindl. 12.4 39.4 Borba, Semir & Shepherd, 2001Pleurothallis teres Lindl. 12.6 44.3 Borba, Semir & Shepherd, 2001Pogonia japonica Rchb. f. 80.0 (20) 75.0 (20) Matsui, Ushimaru & Fujita, 2001Prescottia densiflora Lindl. 69.2 (26) 100.0 (20) Singer & Sazima, 2001aPrescottia plantaginea Lindl. 48.0 (102) 48.1 (52) Singer & Sazima, 2001aPrescottia stachyodes Lindl. 93.7 (223) 95.9 (218) Singer & Sazima, 2001aProsthechea cochleata (L.) W. Higgins

Site 1 88 (16) 88 (16) Ortiz-Barney & Ackerman, 1999Site 2 82 (16) 82 (16)

Sacoila lanceolata (Aubl.) Garay var.lanceolata

100 (6) 100 (6) Catling, 1987

Sarcoglottis fasciculate (Vell.) Schltr. 98.0 (43) 95.0 (42) Singer & Sazima, 1999Sauroglossum elatum Lindl. 94.9 (158) 98.1 (162) Singer, 2002Schomburgkia tibicinia Bateman 71.2 (59) 67.4 (46) Rico-Gray & Thien, 1987Spiranthes lacera (Raf.) Raf. var. lacera 100 (15) 100 (17) Catling, 1982Spiranthes lucida (H. H. Eaton) Ames 100 (12) 100 (12) Catling, 1982Spiranthes ochroleuca (Rydb.) Rydb. 100 (20) 100 (17) Catling, 1982Spiranthes romanzoffiana Cham. 64 (7) 100 (12) Catling, 1982Spiranthes spiralis (L.) Chevall. 75.0 35.0 Willems & Lahtinen, 1997Spiranthes vernalis Engelm. & Gray 100 (30) 100 (30) Catling, 1982Stelis argentata Lindl. 1.9 (54) 29.3 (185) Christensen, 1992Tetramicra canaliculata (Aubl.) Urb. 10-14 80 J. D. Ackerman, unpubl. dataTipularia discolor (Pursh) Nutt. 91.3 (133) 69.1 (156) Whigham & McWethy, 1980Mean and SE 71.6 (4.0) 80.8 (2.9)

Per cent fruit set

Species Self Cross References

1999a; Johnson & Edwards, 2000). This provides aquestion for future research: is it the very existence ofpollinaria and column that has promoted the relianceof orchids on structural mechanisms that promote out-crossing?

Self-incompatibility: While most orchids appear to beself-compatible (Table 6), self-incompatibility hasbeen reported in orchids of diverse lineages (Table 8).It is common in Epidendroideae, although with such a

small sample size and the limited literature on thesubject it is premature to reach a conclusion. In tem-perate orchids, self-incompatibility has been reportedin Galearis spectabilis (Dieringer, 1982) although apopulation in Maryland has been found to be self-compatible (J. K. Zimmerman, unpubl. data). To date,no one has attempted to describe the self-incompati-bility mechanism in any of these orchids.

Fruit set is often extremely low in self-incompatibleorchids. In Tolumnia variegata it is often less than 2%

Table 6. Continued



Table 7. Per cent embryos formed (EF) following pollination (self vs. outcross). Results are those of artificial pollination(results from natural pollination in parentheses). Seeds from cross-pollinated plants have significantly more embryos(Wilcoxon signed rank test tied Z-value 6.50, P < 0.0001, N = 72, N0 = 3, N ties = 8). The variance in embryo formation issignificantly larger in self-pollinated plants (s2 = 905.0) than in cross-pollinated ones(s2 = 455.5; equality of variance testF76,75 = 1.987, P = 0.004). Autogamous species have higher (mean and SE; 68.6 ± 11.3) selfing fruit set than non-autogamousspecies (38.0 ± 3.5; Mann–Whitney U with ties, Z-value 2.49, P = 0.001, N = 76). No differences in fruit set between cross-pollinated autogamous and non-autogamous species (Mann–Whitney U with ties, Z-value 1.484, P = 0.14; mean and SE;autogamous 77.3 ± 8.0; non-autogamous, 63.8 ± 2.6)

Species Autogamy

EF (%)

ReferenceSelf Cross

Bulbophyllum weddellii (Lindl.) Rchb. f. N 68.3 40.0 Borba, Sheppard & Semir, 1999Bulbophyllum ipanemense Hoehne N 44.4 51.8 Borba, Sheppard & Semir, 1999Bulbophyllum involutum Borba, Semir &

F. BarrosN 52.7 48.1 Borba, Sheppard & Semir, 1999

Caladenia tentactulata Tate N 74.0 73.7 Peakall & Beattie, 1996Catasetum viridiflavum Hook. N 67.8 98.3 J. K. Zimmerman, unpubl. dataCattleya amethystoglossa Linden &

Rchb. f. ex WarnerN 5.9 70.3 Stort & Martins, 1980

Cattleya aurantiaca (Bateman) P. N. Don Y 79.4 62.8 Stort & Martins, 1980Cattleya bicolor Lindl. N 32.8 35.5 Stort & Martins, 1980Cattleya dormaniana (Rchb. f.) Rchb. f. N 25.2 42.5 Stort & Martins, 1980Cattleya elongata Barb. Rodr. N 36.3 57.6 Stort & Martins, 1980Cattleya forbesii Lindl. N 6.4 57.0 Stort & Martins, 1980Cattleya gaskelliana Rchb. f. N 7.7 24.5 Stort & Martins, 1980Cattleya guttata Lindl. N 25.2 54.6 Stort & Martins, 1980Cattleya harrisoniana Bateman ex Lindl. N 2.3 36.3 Stort & Martins, 1980Cattleya labiata Lindl. N 9.3 30.1 Stort & Martins, 1980Cattleya leopoldii Verschaff. ex Lem. N 20.8 48.6 Stort & Martins, 1980Cattleya loddigesii Lindl. N 11.7 76.1 Stort & Martins, 1980Cattleya measuresiana (Willd.) Blumensch. N 23.2 65.9 Stort & Martins, 1980Cattleya schofeldiana Rchb. f. N 7.7 28.2 Stort & Martins, 1980Cattleya warneri Moore N 0.0 36.7 Stort & Martins, 1980Cleistes divaracata (L.) Ames N 64.0 89.0 Gregg, 1989Comparettia uspida Poepp. & Endl. N 97.2 95.8 Salguero-Faría & Ackerman,

1999Cynorchis uniflora Lindl. N 36.0 61.0 Nilsson et al., 1992Dactylorhiza sambucina (L.) Ames N 43.0 75.0 Nilsson, 1980Disa atricapilla (Lindl.) Bolus N 82.2 94.7 Steiner, Whitehead & Johnson,

1994Disa ferruginea (Thunb.) Sw. N 36.4 86.4 Johnson, 1994Diuris maculata R. Br. N 94.0 82.0

Beardsell et al., 1986Epidendrum ciliare L. N 79.3 74.5 Ackerman & Montalvo, 1990Epidendrum nocturnum Jacq. Y 20.9 48.7 Stort & dos Santos Pavanelli,

1986Epidendrum rigidum Jacq. Y 51.5 -- Iannotti, Stort & Morales, 1987Epidendrum tridens Poepp. & Endl. Y 44.2 71.5 Stort & dos Santos Pavanelli,

1986Goodyera oblongifolia Raf. N 40.0 60.0 Kallunki, 1981

N 52.7 83.8 Ackerman, 1975Goodyera pubescens (Willd.) R. Br. N 77.0 64.5 Kallunki, 1981Goodyera repens (L.) R. Br. var. ophioides

FernaldN 36.0 63.5 Kallunki, 1981

Goodyera tesselata Lodd. N 88.0 79.0 Kallunki, 1981Ionopsis utricularioides (Sw.) Lindl. N 99.0 99.0 Montalvo & Ackerman, 1987



Laelia caulescens Lindl. N 33.5 66.6 Stort & de Lima Galdino, 1984Laelia cinnabarina Bateman N 59.9 70.5 Stort & de Lima Galdino, 1984Laelia crispa Rchb. f. N 8.7 42.1 Stort & de Lima Galdino, 1984Laelia crispilabia A. Rich. ex R. Warner N 16.6 54.0 Stort & de Lima Galdino, 1984Laelia flava Lindl. N 12.7 39.3 Stort & de Lima Galdino, 1984Laelia grandis Lindl. & Paxton N 9.4 41.5 Stort & de Lima Galdino, 1984Laelia longipes Rchb. f. N 0.2 29.3 Stort & de Lima Galdino, 1984Laelia millerii Blum N 0.0 26.6 Stort & de Lima Galdino, 1984Laelia mixta Hoehne N 14.4 48.1 Stort & de Lima Galdino, 1984Laelia ostermayerii Hoehne N 0.7 52.5 Stort & de Lima Galdino, 1984Laelia perrinii Rchb. f. N 42.2 69.7 Stort & de Lima Galdino, 1984Laelia pumila Rchb. f. N 33.0 52.6 Stort & de Lima Galdino, 1984Laelia tenebrosa Rolfe N 12.9 46.5 Stort & de Lima Galdino, 1984Laelia xanthyna Lindl. ex Hook. N 8.5 38.1 Stort & de Lima Galdino, 1984Leporella fimbriata (Lindl.) A. S. George N 40.3 50 Peakall, 1989aListera cordata R. Br. N 88.5 94.2 Meléndez-Ackerman & Ackerman,

2001Liparis loeselii (L.) ex Lindl. Y 95.0 95.0 Catling, 1980Listera ovata (L.) R. Br. N 89.3 97.7 Nilsson, 1981Oeceoclades maculata (Lindl.) Lindl. Y 92.0 88.0 González-Díaz & Ackerman, 1988Orchis mascula L. N 59.8 75.1 Nilsson, 1983aOrchis morio L. N 10.1 35.2 Nilsson, 1984Orchis spitzelii Saut. ex Koch N 54.0 86.9 Fritz, 1990Platanthera bifolia (L.) Rich. N 43.8 84.2 Nilsson, 1983bPlatanthera chlorantha (Cust.) Rchb. N 23.9 73.5 Nilsson, 1983bPlatanthera ciliaris (L.) Lindl. N 66.0 76.0 Gregg, 1990Platanthera lacera (Michx.) G. Don N 67.0 47.0 Gregg, 1990Pleurothallis adamantinensis Brade N 28.0 92.0 Borba, Semir & Shepherd, 2001*Pleurothallis fabiobarrosii Borba & Semir N 30.0 94.0 Borba, Semir & Shepherd, 2001*Pleurothallis johannensis Barb. Rodr. N 20.0 91.0 Borba, Semir & Shepherd, 2001*Pleurothallis ochreata Lindl. N 28.0 95.0 Borba, Semir & Shepherd, 2001*Pleurothallis teres Lindl. N 5.0 90.0 Borba, Semir & Shepherd, 2001*Sacoila lanceolata (Aubl.) Garay var.

lanceolataN 80.0 75.0 Catling, 1987

Sacoila lanceolata (Aubl.) Garay var.paludicola (Luer) Sauleda, Wunderlin

Y 97.5 97.5 Catling, 1987

& B. F. HansenSatyrium bicorne Thunb. N 14.8 66.1 Ellis & Johnson, 1999Satyrium coriifolium Sw. N 29.3 65.6 Ellis & Johnson, 1999Satyrium erectum Sw. N 14.5 57.9 Ellis & Johnson, 1999Sophronitis purpurata (Lindl. & Paxton)

C. Berg & M. W. Chase N 11.6 43.8 Stort & de Lima Galdino, 1984Vanilla claviculata (W. Wright) Sw. N 63.0 88.0 L. R. Nielsen & J. D. Ackerman,

unpubl. dataXylobium squalens Lindl. N 74.7 82.3 Pintaúdi, Stort & Marin-Morales,

1990Mean and SE 40.8 (3.8) 64.6 (2.5)

Species Autogamy

EF (%)

ReferenceSelf Cross

Table 7. Continued

*Values are approximate, from interpretation of figure 2 in mentioned reference



(Ackerman & Montero Oliver, 1985; Calvo, 1993; Ack-erman et al., 1997) and Ackerman (1989) recorded avalue of 4% for Psychilis krugii while Christensen(1992) reported 2.2% for Stelis argentata. J. K. Zim-merman (unpubl. data) observed a level of 1.8% in aPanamanian population of Oncidium stipitatum andwe observed 1.6% fruit set in a Puerto Rican popula-tion of O. altissimum. In the tropical terrestrial Mal-axis massonii, fruit set was 1.4% and 3.4% in twopopulations (Aragón & Ackerman, 2001). Dieringer(1982) recorded an average fruit set of 5% in two Ohiopopulations of Galearis spectabilis (the self-compati-ble Maryland population, meanwhile, exhibited fruitset of 53%; J. K. Zimmerman, unpubl. data). Averagefruit set over 2 years in a Maryland population ofLiparis lilifolia was 1.6% (Whigham & O’Neil, 1991).

One suggestion as to why fruit set is higher in somespecies is that they do not possess elaborate mecha-nisms, evident in many other orchids (Dressler, 1981),which prevent self-pollination. Low fruit set in self-

incompatible species may be the result of much self-and geitonogamous pollination, resulting in flower orfruit abortion. Alternatively, mechanisms for prevent-ing self-pollination may include sequential flowering(Psychilis spp. - Ackerman, 1989; Malaxis massonii -Aragón & Ackerman, 2001), dichogamy/protandry(Goodyera oblongifolia - Ackerman, 1975; Spiranthes- Catling, 1983a; Prescottia stachyodes - Singer &Sazima, 2001a; Erythrodes arietina - Singer & Saz-ima, 2001b; Sauroglossum elatum - Singer, 2002;Mesadenella cuspidate (Lindl.) Garay - Singer, 2002;Notylia nemorosa Barb. Rodr. - Singer & Koehler,2002) and temporal variation in pollinarium size (Bul-bophyllum weddellii (Lindl.) Rchb. f., B. ipanemenseHoehne and B. involutum - Borba & Semir, 1999a).

Autogamy: This is not infrequent among orchid spe-cies. Van der Pijl & Dodson (1966) estimated thefamily-wide occurrence of autogamy to be about 3%,but recent estimates from local floras suggest it may

Table 8. Self-incompatible species

Species References

Acampe pachyglossa var. renschiana (Rchb. f.) Senghas Agnew, 1986Acampe praemorsa (Roxb.) Blatt. & McCann Agnew, 1986Angraecum cultiforme Summerh. Agnew, 1986Cattleya warneri Moore Stort & Martin, 1980Dendrobium (44 spp.) Johansen, 1990Doritis pulcherrima Lindl. Agnew, 1986Epidendrum cinnabarium Salzm. ex Lindl. East, 1940Epipactis atrorubens (Hoffm. ex Bernh.) Besser East, 1940Galearis spectabilis (L.) Raf. Dieringer, 1982Malaxis massonii (Ridl.) Kuntze Aragón & Ackerman, 2001Lepanthes woodburyana Stimson G. Pomales, M. Méndez & R. L. Tremblay, unpubl. dataLiparis lilifolia (L.) Lindl. Whigham & O’Neil, 1991Liparis makinoana Schltr. Oh et al, 2001Oeoniella polystachys (Thouars) Schltr. Agnew, 1986Oncidium ascendens Lindl. Parra-Tabla et al., 2000Oncidium cavendishianum Bateman East, 1940Oncidium cimiciferum (Rchb. f.) Beer East, 1940Oncidium crispum Lodd. East, 1940Oncidium divaricatum Lindl. East, 1940Oncidium lemonianum Lindl. East, 1940Oncidium sphacelatum Lindl. East, 1940Oncidium unicorne Lindl. East, 1940Phalaenopsis schilleriana Rchb. f. Agnew, 1986Psychilis krugii (Bello) Sauleda Ackerman, 1989Psychilis monensis Sauleda S. Aragón, unpubl. dataSobennikoffia humbertiana H. Perrier Agnew, 1986Stelis argentata Lindl. Christensen, 1992Tolumnia variegata (Sw.) Braem Ackerman & Montero Oliver, 1985Trichocentrum microchilum (Bateman ex Lindl.) East, 1940

M. W. Chase & N. H. Williams



be higher. Catling (1990) suggested that autogamouspollination may occur in between 5% and 20% of thefamily. Ackerman (1985) estimated that 15% of theNorth American orchid flora was autogamous com-pared to 10% for Barro Colorado Island, Panama and25% for Puerto Rico.

There is evidence that autogamy increases with lat-itude and in insular areas. In eastern Canada, forexample, 17% of orchids are autogamous, while valuesfor Europe range from 27 to 50% (Kirchner, 1922a;Hagerup, 1952; Catling, 1983b, 1990). Self-pollinationis more frequent in austere colder habitats (Catling,1990 and references therein) where pollinator activitymight be unpredictable.

Autogamy in orchids is often suggested by high lev-els of fruit set. Among orchids in which mechanisms ofself-pollination are known (Table 9), average fruit setis high, 77.0 ± 5.0% (between 14% and 100%, N = 21),much higher than it is in allogamous orchids (Table 3).Clearly, one result of autogamy is that pollination lim-itation is reduced (depending on the degree to whichthe pollen of a single flower can fertilize all potentialovules) or absent and the evolution of self-fertilization

under these conditions follows the ReproductiveAssurance Hypothesis of Hagerup (1952) and Jain(1976).

While fruit production is assured under self-fertili-zation, there are trade-offs involved. Inbreedingdepression, the loss in offspring fitness due to theexpression of deleterious recessive alleles and othercauses (Charlesworth & Charlesworth, 1979, 1987;Dudash, 1990), can offset increases in seed production.However, because offspring from self-pollination pos-sess two copies of the parental genome compared toone in outcrossed offspring, the critical level ofinbreeding depression determining the evolution ofautogamy is assumed to be 50% (Charlesworth &Charlesworth, 1979; Lande & Schemske, 1985; but seeHolsinger, 1988). On this basis Gill (1989, 1996) hassuggested that high inbreeding depression mayexplain the failure of Cypripedium acaule to developautogamy from its current ‘inefficient’ mating system(natural fruit set is often < 2% in this species, Table 3).Data on embryo formation from cross- and self-pollination in non-autogamous species do not suggesta level of inbreeding depression greater than 50%

Table 9. Natural fruit set (per cent fruits from flowers) in autogamous orchids

Species % FS Location Reference

Aplectrum hyemale (Mulh. ex Willd.) Torr. 64 Illinois, USA Hogan, 1983Bletia stenophylla Schltr. 88 Venezuela R.N. Calvo, unpubl. dataCaularthron bilamellatum (Rchb. f.) R. E. Schult. 86 Panama J.K. Zimmerman, unpubl. dataCephalanthera austinae (A. Gray) A. Heller 14 California, USA Kipping, 1971C. odontorhiza (Willd.) Poir. 84 Maryland, USA J.K. Zimmerman, unpubl. dataC trifida Chatel. 93 Ontario, Canada Catling, 1983bDisa glandulosa Harv. ex Lindl. 78 South Africa Johnson, Steiner & Kurzweil,

1994Disa vaginata Burch. ex. Lindl. 74 South Africa Johnson, Steiner & Kurzweil,

1994Goodyera procera Ker-Gawl. 94 Hong Kong Wong & Sun, 1999Isotria medeoloides (Pursh) Raf. 67 Main, USA Vitt & Campbell, 1997Isotria verticillata Muhl. ex Willd. 57 Southeastern USA Mehrhoff, 1983Liparis loeselii (L.) Rich. 71 Germany Kirchner, 1922bOeceoclades maculata (Lindl.) Lindl. 36 Florida, USA Calvo, 1990b

52 Puerto Rico González-Díaz & Ackerman,1988

Ophrys apifera Huds. 100 England Darwin, 1877Phragmipedium lindenii (Lindl.) Dressler & N. H.

Williams>95 Ecuador L. McCook, pers. comm.

Platanthera clavellata (Michx.) Luer 50 USA Gregg, 1990Platanthera hyperborea (L.) Lindl. 99-100 Michigan, USA Catling, 1983bSacoila lanceolata (Aubl.) Garay var. paludicola

(Luer) Sauleda, Wunderlin & B. F. Hansen100 Florida, USA Catling, 1987

Thelymitra carnea R. Br. 100 Australia Fitzgerald in Darwin, 1877Thelymitra circunsepta Fitzg. 96 Australia Sydes & Calder, 1993Mean (SE) 78.1 (4.8)



(Tables 6, 7). The level of inbreeding depression in theself-compatible Puerto Rican Prosthechea cochleatashowed very little difference in fitness among self andcross progeny (Ortiz-Barney & Ackerman, 1999); inthis species a selfing mutant could easily swamp theoutcrossing phenotype. However, more thorough stud-ies of inbreeding depression focusing on the completeorchid life span are required to address this issue.

Comparison of seed formation in autogamousorchids with that of outcrossing species suggests theelimination of an inbreeding fitness bottleneck inancestral outcrossing stock. The autogamous Brazil-ian Cattleya aurantiaca (Bateman) Don studied byStort & Martins (1980) did better under self- (79.5%)than cross-pollination (62.8%). In Oeceoclades macu-lata and Sacoila lanceolata (Catling, 1987; González-Díaz & Ackerman, 1988) embryo formation wassimilar in fruits from cross- and self-pollination(Table 7). In addition, seed mass of fruits inO. maculata was unaffected by self- and cross-pollination (González-Díaz & Ackerman, 1988).

Facultative autogamy may occur in a number of spe-cies (for review see Catling, 1990) and may be anappropriate strategy when frequency of pollination ishabitually low. An occasional crossing event may besufficient to infuse enough genetic variability todiminish any effects of inbreeding depression from fre-quent self-pollination. However, supporting evidenceis not yet at hand.

Pollen diversity: Increasing pollen diversity on thestigma can positively affect the quality and quantity ofthe progeny (Marshall & Ellstrand, 1986; Montalvo,1992). For orchids, the effects of multiple parentageare incompletely known yet it may be important forsome species, particularly those with mealy or sectilepollen. The incidence of multipaternal pollination inorchids is, similarly, usually not known, although it isnot likely to be frequent, as many orchid flowers arenot visited at all and when they are, they wither afterpollen deposition (Arditti, 1976; Proctor & Harder,1995). Nevertheless, multipaternal pollinations arepossible because pollinators may carry more than onepollinarium (Nilsson et al., 1987; Luyt & Johnson,2001). When evidence for such pollination has beensought under field conditions, it was either notdetected (Tremblay, 1994; Peakall, 1989b; Nilsson,Rabakonandrianina & Pettersson, 1992; Salguero-Faría & Ackerman, 1999) or found to be rare (Folsom,1994). What is the effect of pollen diversity on seedquality? Tremblay (1994) hand-pollinated Cypripe-dium calceolus, where treatments included pollinationby outcrossing with a single father or a mixture of tendifferent fathers. Multipaternal pollination resultedin a higher germination rate although embryo sizeswere generally smaller.

POLLINATION LIMITATION SUMMARY

In nearly all non-autogamous species examined, fruitset may be increased by hand pollination, which sug-gests that the observed variation in fruit set is mainlydependent upon the level of pollinator activity. This inturn may vary among populations and among years.Thus orchids are generally pollination limited andseverely so. Pollen quantity, quality and frequency ofselfing may also affect fruit and seed set, but are gen-erally not correlated with the degree of pollinatoractivity.

RESOURCE CONSTRAINTS IN ORCHIDS

SHORT-TERM EFFECTS

What role does the availability of resources play inorchid reproduction? The usual evidence for resourcelimitation of fruit and seed set is that additional pol-lination fails to increase fruit production, whereas anaugmentation of resources succeeds in doing so. Anexperimental study evaluating the effect of nutrientapplication (N, P and K) to Platanthera bifolia andDactylorhiza incarnata showed mixed results (Mattila& Kuitunen, 2000). They found that for P. bifolia,plant size was the factor in determining fruit set of fer-tilized plants. Small plants had higher fruit set whengiven nutrients, whereas large plants showed no dif-ferences (Mattila & Kuitunen, 2000). Nutrient fertili-zation of D. incarnata did not effect fruit initiation andproduction, or leaf area in the following year (Mattila& Kuitunen, 2000).

Another experiment evaluated the effect of addi-tional water resources on fruit production in Platan-thera bifolia (Mattila, 2000). While it resulted inhigher fruit production, no increase in leaf area wasnoted on plants the following year (Mattila, 2000).Fisher (1992) examined the consequences of the asso-ciation of the self-pollinating Caulathron bilamella-tum with an ant which inhabits the pseudobulb. Therefuse of the ant community presumably fertilizes theplant. Such uptake has been demonstrated in Myrme-cophila tibicinis (Rico-Gray et al., 1989). Fisher’sremoval of the ants and their refuse reduced the num-ber of fruits produced, which suggests that reproduc-tion is resource limited. Zimmerman & Whigham(1992) showed that severance of old corms in Tipu-laria discolor and experimental defoliation (herbivoryeffect) had little effect on fruit production. Aragón &Ackerman (2001) had similar results in their defolia-tion experiments with Malaxis massonii as did Pri-mack, Miao & Becker (1994) in their work onCypripedium. Not surprisingly, the response level wasdependent on the severity of the costs. Multi-yeardefoliation of T. discolor resulted in failure to produceflowers (Whigham, 1990).



Fire has been thought to stimulate flowering in ter-restrial orchids, although Barnett (1984) found thatamong 22 species at one site in Australia most wereadversely affected and only a few responded positively.Primack et al. (1994) noted that fire also had a nega-tive effect on Cypripedium acaule that became addi-tive when coupled with simulated herbivory. D. E. Gill(pers. comm.) noted a positive effect on the same spe-cies, but an indirect one: fire opened up the forest can-opy, stimulating flowering of food plants for thepollinators of C. acaule, while the orchid benefitedfrom increased pollinator populations and visitationrates.

Disease can affect resource availability and repro-ductive success in many plants (Burdon, 1987), but itrarely has been studied in natural populations oforchids. Meléndez & Ackerman (1993) studied a com-mon rust disease in a population of the epiphytic Tol-umnia variegata and found that it had little impact oneither short-term vegetative growth or reproductivesuccess.

In some studies in which fruit production wassignificantly increased by hand pollination, within-season effects of resource availability have beendocumented. These include reduced flower productionor vegetative growth, decreased fruit size withincreasing fruit number, position-dependent fruitsize, and abortion of initiated fruits (Montalvo & Ack-erman, 1987; Ackerman, 1989; Zimmerman & Aide,1989; Calvo, 1993). A series of experiments evaluat-ing the effect of a reduction in photosynthesis demon-strated that it affected neither fruit production norleaf area in Dactylorhiza maculata, which suggeststhat orchids either are either heavily dependent onmycorrhizae or utilize stored resources for fruit devel-opment, capsule weight and subsequent growth (Val-lius & Salonen, 2000; Vallius, 2001). In addition tothese short-term effects that are widely known amongorchidists, long-term costs to elevated fruit set alsooccur.

LONG-TERM EFFECTS

Ample evidence for within-season pollinator limita-tion in orchids exists, yet lifetime fruit production maybe resource-limited rather than pollinator-limited(Montalvo & Ackerman, 1987; Zimmerman & Aide,1989). In a study in which hand pollination resulted ina 13-fold increase in fruit set over natural pollinationin Encyclia cordigera, Janzen et al. (1980) argued thatthe results of single-season experiments could be mis-leading because the experimental increase in fruit setmight have detrimental effects on future growth andreproduction (i.e. it may result in an increased cost ofreproduction). Determining the long-term effect ofincreasing fruit set was tested on the epiphyte, Ionop-

sis utricularioides, by Montalvo & Ackerman (1987).Natural fruit set in this species was 6.1%; in hand-pollinated plants it was between 51% and 66%, abouta 10-fold increase. However, this increase carried acost. There was a significant negative linear correla-tion between fruit set in 1982 and the per cent changein leaf area between 1982 and 1983, and plants with>25% fruit set also had a significantly lower probabil-ity of flowering in 1983 than those with lower fruit set(67% failed to flower vs. 19%, respectively). Further-more, mineral fertilization in a shaded greenhouseresulted in higher growth rates than in the field.These data indicate that the availability of nutrientswas a limiting factor for growth and reproduction, andthat there was a trade-off in resource allocation tothese two functions. Significant negative effects ofartificially increased fruit set on growth have beenreported in four other tropical epiphytes and two tem-perate species (Ackerman, 1989; Snow & Whigham,1989; Zimmerman & Aide, 1989; Ackerman & Mon-talvo, 1990; Primack & Hall, 1990; Meléndez-Ackerman, Ackerman & Rodríguez-Robles, 2000).However, no effects of fruit set on either growth orreproduction were detected in multiyear studies ofCyclopogon cranichoides (Calvo, 1990a) and Cleistesdivaricata (Gregg, 1989).

RESOURCE CONSTRAINTS AND OVULE DEVELOPMENT

One corollary of the resource limitation hypothesis(Stephenson & Bertin, 1983) is that the relative cost offlowers is small compared to that of fruits, such thatlarge floral displays are relatively inexpensive. Forexample, Bookman (1984) showed that the biomassand nutrient content of abscised flowers and fruits ofAsclepias speciosa Torr. was < 3.2% that of maturepods, suggesting that the cost of aborting the struc-tures was relatively small. Similar adaptations to pol-lination and resource limitation may have occurred inorchids.

As noted previously, hyperovulate ovaries (and theenormous number of seeds produced per fruit) is ahallmark of orchid reproduction. Although firstdescribed in the 1940s (Duncan & Curtis, 1943; seeWirth & Withner, 1959 for citations), it is still notwidely recognized that ovule development in orchids isusually completed after pollination. As a result, theperiod between pollination and fertilization in orchidscan be quite long, ranging from a week to almost ayear (Wirth & Withner, 1959). In orchids with singleanthers, ovaries develop to a stage where three pla-cental ridges, consisting of undifferentiated tissue, areevident. Cypripedoid orchids have two anthers anddifferentiation proceeds to where megaspore mothercells (which will divide by mitosis to form megagame-tophytes) are recognizable. In both types of orchids



further development of ovules is triggered by auxinreleased from pollen that has been deposited on thestigma (Wirth & Withner, 1959).

Can the incomplete development of orchid ovariesbe interpreted as part of a reproductive strategyrelated to pollen-limited fruit reproduction? If flowersfrequently remain unpollinated, then the arresteddevelopment of the ovaries may represent a strategyfor limiting the cost of flower production (Dressler,1981) in favour of pollinator attraction. FollowingBookman’s (1984) example, Zimmerman (unpub-lished) collected data on the relative mass of flowersand fruits from a Maryland population of Tipulariadiscolor. Plants were sampled at anthesis, at the timethe ovaries of pollinated flowers had expanded (andincluded only the placental ridges in the hollowovary), and when fruits were mature. Ovaries atanthesis weighed about 0.5 mg (dry weight after >72 h at 60 ∞C). The average mass of expanded ovarieswas 6.25 mg (12.5 times that of ovaries at anthesis)and that of mature fruits was 8.65 mg (over 17 timesthat of an unexpanded ovary). Thus, the mass ofmature fruits is only about 1.4 times that of matureflowers and if T. discolor were to regularly abortunpollinated, fully developed flowers, it would be rel-atively costly in terms of biomass.

A test of this idea would come from an orchid inwhich fruit set of flowers was assured. Wirth & With-ner (1959) noted one exception in orchids in whichovary development proceeds, prior to pollination, tolevels beyond those observed in other orchids. Thisoccurs in a triandrous form of the normally monan-drous Prosthechea cochleata. This form is autogamousand differentiation of the placental ridges precedescontact of the pollen and stigmatic fluids (Duncan &Curtis, 1943). One consequence of autogamy is thatpollen-limitation is alleviated and fruit set is assured.In autogamous orchids, therefore, one would expect toobserve a relatively greater degree of ovule develop-ment in advance of pollination.

EVALUATING SHORT AND LONG-TERM COSTS

OF FRUIT SET

Calvo & Horvitz (1990) investigated the relative rolesof pollination level and cost of reproduction on overallfitness using a transition matrix demographic modelof orchid life-history. The model incorporated life-his-tory traits characteristic of orchids, such as skeweddistribution in fruit production, the ability to regressin size in subsequent years, and high fecundity havinga cost. Through simulation, the model predicted thatmaximum fitness would be attained at intermediatelevels of pollination in most cases. The results stressedthe importance of the trade-off between the additionalfecundity gained through increased fruit production

and its effects on subsequent growth. For a given mag-nitude of cost, there was a threshold return in fecun-dity beyond which higher fruit production wasfavourable.

Tolumnia variegata provides a good example of thedisparity between an increase in fruit set and the costof the increase. In Calvo’s (1993) study population,hand-pollination of field plants resulted in 72% fruitset vs. 0% in naturally pollinated plants. In the follow-ing year the former were, on average, about 30%smaller than the latter. This model showed that thecost was too insignificant to override the potential ben-efit of higher fruit set.

Re-examination of the evidence for Ionopsis utricu-larioides (Montalvo & Ackerman, 1987) and Aspasiaprincipissa (Zimmerman & Aide, 1989) in terms oftheir demographic model, led Calvo & Horvitz (1990)to suggest that these species may indeed be pollinator-limited, mainly because the fecundity gained throughhigher fruit production appeared to be relativelyhigher than the negative effects of costs. Meléndez-Ackerman et al. (2000) took a different approach fromCalvo & Horvitz (1990). From experimental data onComparettia falcata, they calculated a relative fitnessindex to compare pollen augmented plants and natu-rally pollinated plants and found that lifetime fitnessin this species can be resource limited. Nutrient avail-ability affects flowering in a number of orchids andconsequently resources do affect reproductive success(Dijk, Willems & Van Andel, 1997). Nevertheless, theavailable evidence seems to indicate that, althoughresource limitation may impose an upper limit to fruitproduction in orchids, many species could sustain sig-nificantly higher levels of pollination throughout theirlifetimes.

Many orchids face low fruit production as a result oflow pollination, but they are able to produce manyfruits if pollinated. If pollination is uncertain at theindividual level, then producing many fruits in a given‘good’ year may be beneficial to the plant even if itentails a cost, because the probability of being polli-nated in the subsequent year may be very low, regard-less of whether or not the plant sets fruit in thecurrent year (Zimmerman & Aide, 1989; Calvo & Hor-vitz, 1990). For example, the lady’s slipper, Cypripe-dium acaule, produces a single flower per year andconsistently exhibits strikingly low fruit set in closedcanopy forests where the orchids are usually found(below 10%; Davis, 1986; Gill, 1989). Fruiting doeshave a significant cost, but only after four consecutiveyears, which may occur with a probability of 0.0001(Primack & Hall, 1990). On the rare occasion that theforest canopy opens up after a fire, pollinator activityabruptly increases as does fruit set (Gill, 1996). Thissit-and-wait strategy probably pays off because thelong intervals of low activity allow plants to replenish



their resources before another fruiting event takesplace.

ECOLOGICAL EFFECTS ON REPRODUCTIVE SUCCESS

Variation in the probability of a given flower settingfruit is influenced by a number of factors. Phenology,inflorescence, habitat, plant density, population size,and temporal variation (Kindlmann & Balounová,2001) may affect reproductive success as well as thecomposition of the surrounding plant community.

PHENOLOGY

The degree of synchronization between flower anthe-sis and pollinator availability may affect visitationrates. In several temperate orchid species, individu-als that flower early in the spring are pollinated at ahigher rate than those that flower later, as inCalypso bulbosa (Ackerman, 1981), Orchis morio(Nilsson, 1984), Orchis mascula (Nilsson, 1983b) andPlatanthera blephariglottis in Michigan (cf. Cole &Firmage, 1984). The same phenomenon was observedin the tropical epiphyte Tolumnia variegata (Sabat &Ackerman, 1996). However, constant rates of pollina-tion throughout the season were reported for thesame species of Platanthera in Maine (Smith &Snow, 1976), and for the tropical epiphyte, Psychiliskrugii (Ackerman, 1989), which flowers all year. InDactylorhiza sambucina the highest rates of visita-tion and pollinia removal occurred at the beginningof the flowering season (Nilsson, 1980), but thisobservation was not discussed in terms of fruitingsuccess.

Zimmerman et al. (1989) reported asynchronousphenologies of Catasetum viridiflavum and itseuglossine bee pollinator, Eulaema cingulata (Fabri-cius) in Panama. Peak flowering occurred up to sixmonths later than the peak of pollinator activity, withfruit set of early flowering plants being about threetimes higher than later flowering ones. However, earlyinitiated fruits were shed at a higher rate, even whenthey appeared to be fully developed. This shedding offruits was presumably related to the long time fruitshave to be retained until seed dispersal, which occursduring the next dry season. Zimmerman et al. (1989)discussed the possibility of conflicting selective pres-sures on the flowering time of C. viridiflavum. In sev-eral cases, the phenology of orchids and theirpollinators has been recorded (Smith & Snow, 1976;Nilsson, 1978b, 1983a; Ackerman, 1983; Inoue, 1985;Nilsson et al., 1985), but the effects of individual vari-ation in flowering phenology on fruit set was not fullydiscussed.

INFLORESCENCE EFFECTS

Differential fruit set can occur, as a result of the posi-tion of the flower on the inflorescence (Nilsson, 1980,1983b, 1984; Cole & Firmage, 1984; Berry & Calvo,1991; Vallius, 2000). In all reported cases, fruit set washighest toward the bottom of the inflorescence anddecreased steadily toward the top. In Orchis morio,O. mascula, and Dactylorhiza sambucina, which arenectarless orchids with acropetal floral development,the reduction in fruit set along the inflorescence wasdramatic. Fruit set of flowers at the top was between50 and 90% lower than that of those at the bottom(Nilsson, 1978a, 1980, 1983b, 1984; Vallius, 2000). Asimilar pattern was observed in the nectar producingOreorchis patens in Japan (Sugiura, Okajima &Maeta, 1997). These patterns may result from pollina-tor foraging behaviour. Pollinators tend to moveupwards along inflorescences in order to forage fornectar (Ackerman, 1975; Best & Bierzychudek, 1982);they may depart after unsuccessfully probing a fewflowers, thus resulting in reduced pollination of flow-ers located towards the top (Nilsson, 1980, 1983b).

It is possible that the nectar reward is insufficient toencourage multiple flower visits or that the top flowersare smaller in size, thus affecting reproductive success(Sugiura et al., 1997). Alternatively, in reward species,pollinators may lose interest as rewards in the popu-lation diminish below a certain threshold as happenswith pollinators of other plant populations (Heinrich,1975). In the case of deceptive species, the pool ofnaive pollinators may become exhausted so that flow-ers emerging in the latter half of the season wouldreceive few visitations (see ‘Phenology’ above). Fruit-ing would therefore be more common on the lower andolder parts of the inflorescence.

A unique situation has been described for the highaltitude Andean orchid, Myrosmodes cochleare (Berry& Calvo, 1991). As in other orchids, fruit set decreasesmarkedly toward the top of the inflorescence; however,in this case flowers open in a basipetal sequence (i.e.top flowers open first) and flower size increasessteadily from the top to the bottom of the inflores-cence. Controlled pollination revealed that the abilityto set fruit decreases from the bottom to the top, andappears to be associated with the change in flowersize. Because pollen viability is high along the inflo-rescence, Berry & Calvo (1991) argued thatM. cochleare is functionally andromonoecious.

Position-related fruit set is common in other fami-lies, but it has been generally explained in terms ofcompetition for resources among developing ovaries(Lee, 1988). Basal, early initiated fruit may pre-emptresources that could otherwise be allocated to otherfruit (Stephenson, 1981; Bawa & Webb, 1984). Thus, inresource-limited species, position-related fruit set is a



consequence of differential flowering or fruit abortion.Orchids, however, are largely pollination-limited andthe pattern seems to be the result of differential pol-lination along the inflorescence. Consequently, thecommonness and influence of resource allocationneeds to be studied and could explain position-dependent fruit set, especially in terrestrial, many-flowered species.

HABITAT AND COMMUNITY EFFECTS

Another potential source of variation in attractivenessto pollinators (and thus in fruit set) among individualsof a population is the microhabitat they occupy. In gen-eral, plants that attract their pollinators by means ofvisual cues may be less visited if they grow amongother plants or in shady spots (e.g. Orchis galilaea;Bino, Dafni & Meeuse, 1982). Reduced fruit set hasbeen attributed to ‘closed habitat’ in Platanthera cili-aris (Smith & Snow, 1976), and P. blephariglottis (Cole& Firmage, 1984), where plants were somewhat hid-den by the surrounding vegetation. In Cephalantheralongifolia, fruit set of plants located in shady micro-habitats was 40–50% lower than that of those insunny areas in two consecutive years (Dafni & Ivri,1981b). Similarly, M. Morales & J. D. Ackerman(unpubl. data) found that in a population of the epi-phytic Tolumnia variegata, plants in the sun had 20%fruit set whereas those in the shade had only 3%.Inoue (1985) studied two moth-pollinated species ofPlatanthera; in one there was a detectable reductionin fruit set related to plant cover, but in the other norelationship was detected.

Sometimes fruiting success of an individual maysimply depend on whether it is located where its pol-linators are present. Ackerman et al. (1997) found thatpopulation variation in fruit set of Tolumnia variegatawas associated with the abundance of their Centrisbee pollinators. Nilsson (1983c) noted that fruit set inCephalanthera rubra, which deceives its solitary malebee pollinator, Chelostoma spp., was 30% in areas thatwere patrolled by the bees, and only 7% in unpatrolledareas (Nilsson, 1983c). Finally, deceptive orchids mayhave higher fruit set if the population overlaps with arewarding species and they share one or more pollina-tors. Cypripedium acaule benefits from bumblebeeactivity on interspersed Vaccinium (Gill, 1996). Dafni& Ivri (1981a) observed that fruit set of the nectarlessOrchis israelitica, which resembles flowers of the nec-tariferous Bellevalia flexuosa Boiss. (Liliaceae), wasbelow 5% where Bellevalia was absent and higherthan 35% with it present. Similarly, fruit set in Orchiscaspia was 2–6 times higher where nectar producingspecies of other families were present (Dafni, 1983).However, fruit production of Epidendrum radicansPav. ex Lindl. was apparently unaffected by the pres-

ence of Asclepias curassavica L. and Lantana camaraL., two nectar producing species with similar floraldisplays that share pollinators with it (Bierzychudek,1981b), although other nectar resources were nottaken into account.

In the rewardless Orchis boryi, reproductive successcan depend on the flower colour of the plants in thesurrounding community (Gumbert & Kunze, 2001).Bees that preferentially foraged on flowers similar incolour to that of the orchid were more likely to visit theorchid as well. Visits to O. boryi were also influencedby the density of both the orchid and the surroundingrewarding plants.

Reproductive success can vary dramatically betweenadjacent sites where habitats differ. Sugiura et al.(1997) observed that at an exposed seashore site inJapan, the unpollinated flowers of Cypripedium mac-ranthos lasted an average 5.1 days and fruit set was8.3%. At a nearby sheltered area, longevity was twiceas long yet fruit set was only 1.2%. The authors suggestthat the lower fruit set was a direct consequence ofanthropogenic intervention (tourism, site two only:1500 visitors/day) interfering with the pollinators.

DENSITY EFFECTS

Clumped distributions may result in higher pollina-tion rates if pollinators respond to the larger floral dis-play. However, not many studies have addressed thispossibility. In Calopogon tuberosus, clumps of interme-diate size (2–8 plants in a 1-m radius) were found tohave higher fruit set than either solitary plants orclumps of nine or more plants (Firmage & Cole, 1988).More pollinators may be attracted to the largerclumps, but because C. tuberosus has a deceptive pol-lination system, they might be abandoned after a fewunrewarding visits, resulting in lower overall fruit set(Firmage & Cole, 1988). No effect of clump size orplant densities was found in Brassavola nodosa(Schemske, 1980), Leporella fimbriata (Peakall,1989a) or Malaxis massonii (Aragón & Ackerman,2001). Plant densities also had no effect on reproduc-tive success in Listera cordata except where plantswhere highly dispersed (Meléndez-Ackerman & Ack-erman, 2001). Alternatively, Schemske (1980) sug-gested that pollinators of Brassavola nodosa mayfocus on the inflorescence as the unit of attractioninstead of clusters of inflorescences. This is corrobo-rated by some studies (Montalvo & Ackerman, 1987;Aragón & Ackerman, 2001), but not by others(Meléndez-Ackerman & Ackerman, 2001). In one caseSabat & Ackerman (1996) found that frequency of vis-itation to a population of Tolumnia variegata wasmore closely related to the number of flowers on a hosttree than to the number of flowers on a host branch ororchid inflorescence.



POPULATION SIZE EFFECTS

Population size can have an effect on reproductive suc-cess. For orchids, the distribution of population sizes isskewed, with most populations holding only a fewindividuals and few populations are large (Whigham& O’Neill, 1988; Tremblay, 1997c). Fritz & Nilsson(1994) observed the effect of pollinaria removal andfruit set in populations of differing size in three spe-cies of deceptive orchids in Sweden (Orchis spitzelii,N = 10; Orchis palustris Jacq., N = 8, and Anacamptispyramidalis, N = 10). The number of pollinaria remov-als and fruit set increased with population size, butthe proportion of pollinaria removed per plantdecreased. The effect on fruit set was inconsistentamong the three species: a reduction was observed inthe butterfly-pollinated A. pyramidalis but no correla-tion was observed in the other two species. In general,larger populations had: (1) more pollinaria removedand higher fruit set; (2) decreased pollinaria removalsper individual; (3) increased variance in male (but notfemale) reproductive success; (4) an increased ratio ofpollinaria removal to fruit set; and (5) increase in vari-ation of pollinator types.

In addition to Fritz & Nilsson’s (1994) study,Donaldson et al. (2002) examined the effect of habitatfragmentation on pollinator diversity and reproduc-tive success and showed that these factors can influ-ence reproductive success by demonstrating thatisolated small populations can fail to set fruit. Murren(2002) investigated the effect of fragmentation onreproductive success in Catasetum viridiflavum in tenislands created by the Panama Canal and five nearbymainland sites. Reproductive success, as measured asfruit set, was significantly lower on islands as com-pared to mainland populations in two of three years.

We have supplementary data on the number of pol-linations or fruits set and variance in male and femalereproductive success. Ackerman (1981) found thenumber of fruits set in the deceptive Calypso bulbosavar. occidentalis was strongly related to populationsize [linear regression, N = 7, r2 = 0.84, P = 0.004 (ouranalysis)] as did Peakall (1989a) for the number of pol-linations in populations of Leporella fimbriata [N = 16,r2 = 0.46, P = 0.004 (our analysis)]. However, Peakall(1990) did not find such a relationship for Drakaeaglyptodon [N = 9, P = 0.3 (our analysis)]. Fritz & Nils-son (1994) showed that only variance in female repro-ductive success was unrelated to population sizewhereas Tremblay (unpubl. data) found that variancein both male and female reproductive success in threespecies of Caribbean Lepanthes was independent ofthe size of populations (all P > 0.20).

Although we often find a positive relationshipbetween population size and the number of pollina-tions or fruits set, per cent fruit set shows no such

relationship. Per cent fruit set was independent ofpopulation size in the rewardless Calypso bulbosa var.occidentalis [Ackerman (1981) linear regression,r2 = 0.22, N = 7, P = 0.61 (our calculations)], althoughthere was a tendency for reduced per cent fruit set invery small or very large populations. Alexandersson &Ågren’s (2000) similar study of the same species noteda negative relationship between pollen removal andpopulation size, although the pattern was onlyobserved in one of two years. Moreover, fruit set waspopulation size independent. Similarly, in populationsof the sexually deceptive Australian terrestrials, Dra-kaea glyptodon (N = 9), Caladenia tentactulata (N = 5)and Leporella fimbriata (N = 16), the percentage offlowers pollinated was independent of population size[linear regressions: all P ≥ 0.3 (our calculations)] butthere was a trend for the largest populations to havelower rates of pollination (Peakall, 1989a, 1990;Peakall & Beattie, 1996). Per cent fruit set of SouthAfrican populations of sexually deceptive Disa atrica-pilla (N = 6, P = 0.39) and D. bivalvata (N = 4,P = 0.28) were again unrelated to population size(Steiner, Whitehead & Johnson, 1994). Furthermore,pollinator behaviour may be affected by populationsize variation which may itself be density dependent(Gillman & Dodd, 2000). Thus, in general, reproduc-tive success as measured by per cent fruit set or flow-ers pollinated in populations of deceptive species inAfrica, Australia, North America and the Caribbean,appears to be unrelated to most population sizesencountered.

TEMPORAL VARIATION IN REPRODUCTIVE EFFORT

Fruit set in some populations of both tropical and tem-perate orchids is consistent from year to year. Popula-tions of Orchis mascula showed 4.8–7.1% fruit set overa period of 6 years (Nilsson, 1983b), and V. C. Quesnel(pers. comm.) reported that fruit set was consistentlylow in a Trinidad population of Cyrtopodium parviflo-rum Lindl. (1978, 10%, 1979, 1%, 1980, 3% and 1981,8%). Fruit set was also consistent in Epidendrum cil-iare and was even unaltered after a hurricane severelyaltered the habitat (Ackerman & Moya, 1996).

Consistency may commonly exist, but not all popu-lations of a species behave in a similar fashion. Tol-umnia variegata fruit set differed ten-fold amongPuerto Rican populations, but was consistent amongyears (Ackerman et al., 1997). Usually, Cypripediumacaule also has a fairly constant fruit set (Gill, 1989;Primack & Hall, 1990) and a low one too, but unlikeE. ciliare, a large increase in fruit production occurredwhen the environment was radically modified. Afteran outbreak of gypsy moth caused trees above theorchids to defoliate, the increased light brought a dra-matic increase in flower production in orchids and



blueberries. The latter were an important nectarsource for bees, pollinators of both the blueberries andthe orchids. The bee populations increased, and orchidfruit set leaped to 20% (Gill, 1996). Such environmen-tal perturbations may be responsible for the variablefruit set reported in other orchid populations such asEpipactis palustris (5.01–19.80% in 3 years; Nilsson,1983b) and Orchis morio (4.3–30.2% over a 5-yearperiod; Nilsson, 1984).

EVOLUTIONARY PROCESSES AND CONSEQUENCES

Studies of patterns of evolutionary divergence inorchids have flourished over the last decade, withnumerous papers addressing evolutionary relation-ships among species, genera, tribes and subfamilies ofthe Orchidaceae (Benzing, 1987; Chase & Palmer,1997; Barkman, 2001; Barkman & Simpson, 2002;Mant et al., 2002). These studies have generated muchdiscussion, spawning hypotheses of evolutionary pro-cesses to explain the patterns of relationships. How-ever, our understanding of the processes by whichorchids evolve is still in its infancy. In this section wediscuss how variation originates in orchids, describethe evolutionary processes associated with theseplants and review the evidence for each.

SOURCES OF VARIATION

Evolution can only occur if there is variation amongindividuals. The most commonly acknowledged causeof variation is point mutation. Using phylogeneticdata, Barraclough & Savolainen (2001) estimated neu-tral molecular evolution in flowering plants and dem-onstrated that a positive relationship between speciesnumber and rate of molecular change exists. What isthe cause of molecular rate change? Barraclough &Savolainen (2001) suggested that three possible com-ponents could influence and cause higher evolutionaryrates: short generation time, small population size andhigh mutation rates. Mutation rates may vary amongdifferent plant lineages for a number of reasons,including mismatch repair efficiency and mutagenexposure (Merrell, 1981; Gaut et al., 1996; Barra-clough & Savolainen, 2001).

In addition to DNA mutation, variation is alsoaffected by meiotic recombination (Adelson, 2001). Inorchids, every fruit produces many seeds; the largestcontain over a million (Arditti & Ghani, 2000). Thus,the opportunity for meiotic recombination is high,where each new combination constitutes an experi-ment in variation.

Another process influencing variation in orchids ispolyploidy and aneuploidy, both of which are notuncommon. (Tanaka & Kamemoto, 1984). From a hor-

ticultural point of view, polyploidy often results inmore robust plants with flowers that are wider,thicker, more erect, sturdy or compact. However,Tanaka & Kamemoto (1984) note that whereas polyp-loidy leads to distinct differences from parental plants,aneuploidy rarely leads to differences from the paren-tal plant except for deviations from the basic chromo-some number.

Hybridization generates considerable variation andis thought to be a major reason for plant diversifica-tion (Stebbins, 1959; Campbell Waser & Meléndez-Ackerman, 1997). In fact, coupled with polyploidy, itmay lead to sympatric speciation of allopolyploidicpopulations (Hedrén, 1996). Natural hybrids occuramong orchids and new examples are reported everyyear. However, convincing genetic evidence is rarelypresented for putative hybrids. Hybrid swarms andhybrid origins of species appear to be commonplace inEurope (Scacchi, De Angelis & Lanzara, 1990) but arerarely reported elsewhere. The high frequency inEurope may be overstated (Hedrén, 1996) whereas thelow occurrence elsewhere may be understated. In atleast one neotropical genus, Tolumnia, a number ofprocesses seem to be operating that generate variationand these do include both hybridization and poly-ploidy (Withner, 1976; Braem, 1988).

Whatever the origin of variation within and amongorchid populations, the consequences for orchid evolu-tion depend on the same processes. Rather thanreview and debate the relative importance of varioussources of variation, we focus on the conditions andprocesses that shape that variation.

NATURAL SELECTION AND GENETIC DRIFT

Two processes can lead to evolution: natural selectionand genetic drift. Natural selection is a process thatrequires (a) variation in character states among indi-viduals (variation); (b) a relationship between traitvariants and the ability of the individuals to leave off-spring (fitness differences); and (c) that the trait beheritable between parents and offspring (inheritance).

Genetic drift requires (a) and (c), but (b) is absent(Endler, 1986). Population sizes necessary for naturalselection and genetic drift to occur are likely to be dif-ferent. Natural selection is more likely in populationswith large effective sizes, unless the selection coeffi-cient is high, while genetic drift is more likely in pop-ulations with small effective sizes. The presence ofnatural selection does not exclude genetic drift andvice versa. It is important to appreciate that in smallpopulations, small selection coefficients are effectivelyneutral: the smaller the population, the larger theselection coefficient must be to overcome genetic drift.Consequently, for natural selection to be dominantand observable in small populations, the selection



coefficients need to be high (Merrell, 1981; Tremblay,1997a).

Are fruit set and pollinaria removals goodindicators of fitness?Attempts to understand the reproductive ecology oforchids and flowering plants in general have focusedon fruit and seed production. Arguments about polli-nator vs. resource limitation, pollinator efficiency andfitness have been made under the assumption that iffruit set is higher, then fitness would be higher. Evo-lutionary arguments about pollinator limitation andresource limitation are applicable only to the extent towhich fruit production affects overall fitness and isconsequently subjected to selection. The only study todate that addresses this question in orchids is Acker-man, Sabat & Zimmerman (1996), who found thathigher fruit production in Tolumnia variegataresulted in an increase in seedling establishment.Thus, in one of 20 000 or so species of orchids, fruit setis a good indicator of reproductive success and, in par-ticular, female fitness.

Occasionally the number of pollinaria removals isused as a measure of male fitness; again, however, wehave found only one study that has tested this notion.Nilsson et al. (1992) used microtags on pollinaria ofAerangis ellisii and found that there was a very strongrelationship between the number of pollinariaremoved and success as a pollen donor.

Fitness is a measure of the relative reproductivesuccess of individuals (fruit set or pollinaria removals)as compared to the mean of the population. Conse-quently, it is the ability of an individual to leave off-spring as compared to others, making the number offruits produced or pollinia donated important. On theother hand, per cent fruit set is a measure likely to bemore relevant for comparing growth among popula-tions.

EVIDENCE OF NATURAL SELECTION

There is no doubt that selection is responsible formany (probably most) of the floral adaptations oforchids. However, there are few examples that showhow natural selection occurs in orchids and thestrength and direction of the selection. We describesome of the evidence in this section.

Influence of inflorescence display size onreproductive successInflorescence size (number of flowers per inflores-cence) or display size (number of flowers open) variesin most orchids. If reproductive success is associatedwith size or display, then the number of flowers pro-duced may be affected by selection. Plants with largerinflorescences had a higher probability of setting fruit

in Brassavola nodosa (Schemske, 1980), Lepantheswendlandii (Calvo, 1990b), Calopogon tuberosus (Fir-mage & Cole, 1988), Ionopsis utricularioides (Mon-talvo & Ackerman, 1987) and Aspasia principissa(Zimmerman & Aide, 1989). This was not the case withdisplay size in the self-incompatible Psychilis krugii(Ackerman, 1989), Cypripedium fasciculatum (Lipow,Bernhardt & Vance, 2002) or inflorescence size in Epi-dendrum exasperatum (Calvo, 1990b). However, pro-longed sequential production of flowers in Psychilismonensis improves probability of both male andfemale success (S. Aragón, unpubl. data).

The effect of inflorescence size on fruit productioncan be time dependent. Rodríguez-Robles et al. (1992)found a positive relationship in Comparettia falcata inone year but not in another. Furthermore, response tovariation in inflorescence size may differ among polli-nators. Inoue (1986a) showed in a study of the moth-pollinated Platanthera metabifolia that species of thefamily Sphingidae were clearly the most effective pol-linators and responded positively to inflorescence size,whereas those of the Noctuidae were poor pollinatorsand did not favour the larger inflorescences. Maad(2000) found display size to be highly significant anddirectional in P. bifolia in three consecutive years.Thus, flower production in most cases is positivelyrelated to fitness so that one would expect selection forincreased flower production in the absence of anyother constraints.

Floral display, whether measured by inflorescenceor flower size, may differentially affect male andfemale reproductive success. A role for male-malecompetition in the evolution of display size assumesthat pollen is not limiting to reproduction (Stephenson& Bertin, 1983; Willson & Burley, 1983). Therefore, inpollen-limited taxa, one might expect that display sizewould influence male and female reproductive successequally. In those orchids that have been studied, thisappears to be true. In each of these studies, malereproductive success was estimated from pollinariaremovals and female reproductive success was mea-sured as fruit set (usually) or pollination, as deter-mined by the presence of pollinia on the stigma. In thetropical orchids Aspasia principissa (Zimmerman &Aide, 1989), Psychilis krugii (Ackerman, 1989), andComparettia falcata (Rodríguez-Robles, Meléndez &Ackerman, 1992) relative male and female reproduc-tive success does not vary with inflorescence size andis therefore parallel with respect to display size. How-ever, these relationships can be site dependent.Schemske (1980) reported a disproportionate increasein both male and female reproductive success withrespect to inflorescence size in Brassavola nodosa, buttrends for male and female function were nonethelessparallel. Murren & Ellison (1996) found that largerdisplays in B. nodosa at another locality had dispro-



portionately greater male than female reproductivesuccess. Similar results were observed for Psychiliskrugii (Ackerman, 1989).

The relationship between display size and male andfemale reproductive success is equally complex fortemperate orchids. Piper & Waite (1988) concludedthat secondary floral characteristics were male spe-cialized in Epipactis helleborine because polliniaremovals were greater than depositions. However, aspointed out by Snow & Whigham (1989), trends inestimates of male and female reproductive successacross the range of inflorescences were largely paral-lel, indicating no differential effect of this aspect ofdisplay. They also cited unpublished data from Tipu-laria discolor supporting the same conclusion. Fir-mage & Cole (1988) suggested that inflorescence sizeof Platanthera blephariglottis influenced male andfemale reproductive success in a parallel fashion oversmall inflorescences (1–4 flowers) but that femalereproductive success levelled off with increasing inflo-rescence size while male reproductive success contin-ued to increase. However, samples of largeinflorescences were few and it is not clear that thistrend was real.

The relationship between inflorescence size andmale and female reproductive success in temperateand tropical orchids appears to suggest parallelresponse for most species with few exceptions. More-over, we agree with Nilsson (1992) that clarification ofthis relationship may be dependent on whether flow-ers develop synchronously or sequentially. Furtherresearch is required.

Some constraints on floral displayInflorescence size may be influenced by multiple fac-tors over ecological and evolutionary time. Total flowerproduction is dependent on plant size in many plantspecies (Harper, 1977) including orchids (Montalvo &Ackerman, 1987; Zimmerman & Aide, 1989; Calvo,1990a). We have shown that larger inflorescences oforchids generally have greater male and female repro-ductive success. However, plant size distribution inmany populations is strongly hierarchical, with themajority of the individuals being small (Weiner & Sol-brig, 1984; Gregg, 1991b; Leeson, Haynes & Wells,1991). Consequently, most plants may have relativelysmall inflorescences (e.g. Schemske, 1980; Firmage &Cole, 1988). Even if larger inflorescences are benefi-cial, an increase in the mean inflorescence size of apopulation may be limited by energetic and allometricconstraints.

Another factor that may impose limits on inflores-cence size is the frequency of geitonogamous pollina-tion (Ackerman, 1989), which may be greater in largerinflorescences and result in higher rates of flower andfruit abortion due to self-incompatibility or inbreeding

(e.g. Wyatt, 1982; Hessing, 1988; Klinkhamer & deJong, 1993). There are numerous mechanisms whichenhance the probability of cross-pollination in orchidsand manipulation of display size is one of them. Someorchids have small floral displays yet produce manyflowers over long periods (e.g. Lepanthes and Psychilisspp.). Displays at any given moment may be small, buttotal flower production in these plants is positivelyrelated to reproductive success (Lepanthes spp., R. L.Tremblay, unpubl. data; Psychilis monensis, Aragón &Ackerman, 2004).

Although reproductive success in orchids is stronglypollen limited, resource availability plays a major rolein affecting display size. We could suggest that thereshould be selection for more flowers, but the action ofthat selection may be for efficient procurement andstorage of resources for reproductive events. There isscope here, too, for further investigation.

Effect of flower size on reproductive success: Thenumber of flowers produced is one component of dis-play and flower size is another. Larger flowers may bea more powerful signal to pollinators than smallerones and may be affected by selection in a similarmanner to flower production. We found five studiesthat compared flower size with reproductive success.Murren & Ellison (1996) showed that larger flowershad higher male reproductive success in Brassavolanodosa. However, male and female reproductive suc-cesses were independent of flower size in Tolumniavariegata (Sabat & Ackerman, 1996), Myrmecophilatibicinis (Malo, Leirana-Alcocer & Parra-Tabla, 2001)and Cypripedium acaule (O’Connell & Johnston,1998).

Due to the near ubiquity of pollen-limited orchidreproduction, one would expect no differential effect offlower size on male and female reproductive success.However, more experimental studies should be con-ducted, perhaps utilizing modification of display sizeas in Malo et al. (2001) to show whether or not this istrue. If flower size indeed influences male and femalereproductive success equally, then selection to ensurepollinator visits should be higher than in specieswhere male and female reproductive success isunequal. As a result of pollinator limitation selectionwould favour the showiest variants in a population oforchids.

Spur characteristics and reproductive success: Spurlength, width of spur mouth and nectar reward shouldbe closely correlated with reproductive success. Dar-win recognized that effective pollination is dependenton the coupling of the morphology of the flower withthat of the vector. One of the most commonly givenexamples of adaptation is the extraordinary spurlength of Angreacum sesquipedale Thou. and thematching proboscis length of the hawkmoth pollinator



Xanthopan morgani (Walker) ssp. praedicta. Variationin spur length and spur-mouth width has been impli-cated in orchid evolution and reproductive isolation(e.g. Nilsson, 1980, 1983a, b, 1985, 1988). While thereis little debate that the spur length of orchids is adap-tive, recognition of such features is not indicative ofthe processes involved.

Most of the evidence relating natural spur lengthvariation to reproductive success is circumstantial. Itseffects have been evaluated in few species. Luyt &Johnson (2001) failed to detect selection for length inMystacidium venosum, while Inoue (1986b) found thatin natural populations of Platanthera mandarinorumssp. hachijoensis, variation in length was independentof reproductive success. However, he tested the effectof spur-shortening and a simulation of spur elongationand found that it caused a reduction in male andfemale reproductive success. Inoue interpreted theresults to suggest that extreme variants are selectedagainst and that stabilizing selection is likely.

Nilsson (1988) also examined the importance of spurlength variation on male and female reproductive suc-cess. For Platanthera bifolia and P. chlorantha heshowed that both male and female fitness increasewith spur length and that it should be under direc-tional selection.

The most comprehensive approach to studyingselection in orchids was performed by Maad (2000).She observed that long spurs of Platanthera bifoliahad a female advantage but the selection differentialwas only observed in one of three years. Selection canbe variable not only across seasons, but within a sea-son as well. Maad (2000) observed that a combinationof the time of flowering and the length of the spuraffected reproductive success. Plants with short spurswere favoured through female function early in theseason while those with long spurs were favouredlater in the season.

Floral colour and reproductive success: Most pollina-tors are sensitive to colour. Consequently, colour vari-ation among orchids in a population should beperceived by pollinators and the most attractive colourmorph should be preferentially selected. Such varia-tion exists and can be substantial, particularly indeceptive orchids (e.g. Dactylorhiza sambucina - Nils-son, 1980; Orchis caspia - Dafni, 1983; Thelymitra epi-pactoides - Cropper & Calder, 1990). Sometimes thevariation is continuous (Psychilis monensis - Aragón& Ackerman, 2004) while at other times there are dis-tinct colour morphs.

Have we found selection for flower colour? Few stud-ies have analysed this problem and only two includebehavioural data of pollinators. Smithson (2001) andGigord, MacNair & Smithson (2001) found in Dacty-lorhiza sambucina that there is a reproductive advan-

tage to the rare corolla colour morph of thenonrewarding orchid. Smithson (2001) corroboratedher field studies with behavioural experiments in thelaboratory. Negative frequency-dependent selectionseems to occur for flower colour in her system.Koivisto, Vallius & Salonen (2002) examined thebehaviour of pollinators on two colour morphs ofD. maculata and noted that bumblebees preferred thedarker coloured morphs while other visitors had nopreferences. In contrast, R. L. Tremblay (unpubl. data)examined colour variation in Lepanthes rupestris pet-als and found no evidence of differential selection innatural populations.

J. D. Ackerman & W. Carromero (unpubl. data) mea-sured reproductive success in two colour morphs(magenta and white) of Bletia patula in the DominicanRepublic and discovered that there was no significantrelationship between flower colour and male or femalereproductive success. Nevertheless, there was a ten-dency for pollinaria removals to be higher in the whitemorph (P = 0.056). Aragón & Ackerman (2004) exper-imentally manipulated flower colour variation in pop-ulations of Psychilis monensis and detected negativefrequency-dependent selection in two of three trials.Thus, flower colour in orchids can be under selectionbut not necessarily all the time.

Community structure of flower colour can also affectorchid reproductive success. Pollinators of Orchisboryi react to the diversity of flowers in the immediatevicinity of the orchid; the similarity of these (model)plants can have a positive effect on the orchid’s repro-ductive success. Orchis boryi had a higher reproduc-tive success if pollinators foraged on flowers of similarcolour (Gumbert & Kunze, 2001).

Floral rewards and reproductive success: Nectar, fra-grances, oils, pseudopollen, resins and waxes (van derPijl & Dodson, 1966; Dondon et al., 2002) and even pol-len (Vogel, 1981 reported in Pridgeon et al., 1999;Gregg, 1991c) have been identified as pollinatorrewards in orchids. Variation in these rewards isexpected to directly influence pollinator behaviour andconsequently affect plant reproductive success eitherthrough frequency of visits or quality of pollination.Nevertheless, among all pollinator attractants, weperhaps know the least about variation in rewardsand its consequences.

Nectar is the most common pollinator rewardoffered by orchids and is the only one in which vari-ability has been associated with plant reproductivesuccess. Variation in the quantity of nectar existswithin inflorescences of Platanthera blephariglottisbut it is not consistently associated with the probabil-ity of pollination (Cole & Firmage, 1984). Interplantvariation in nectar production was extensively studiedin Comparettia falcata by Rodríguez-Robles et al.



(1992). They found no relationship between naturalvariation in nectar availability and pollinator visita-tion. However, when Ackerman, Rodríguez-Robles &Meléndez (1994) experimentally removed nectar,reproductive success declined. On the other hand,when nectar was added, there was no effect, not evenan increase in geitonogamous pollinations (Salguero-Faría & Ackerman, 1999). These results strongly sug-gest that stabilizing selection may favour the meagrenectar reward offered by C. falcata.

Perhaps the most intriguing story concerning nectarproduction in orchids is actually the lack of it: approx-imately one third of all species produce no pollinatorreward (Ackerman, 1986a). Visitors are deceived byvarious means; most commonly the deception is basedon a general resemblance to food plants without anyspecific mimicry. Naive pollinators visit a few flowersbefore learning to avoid them (Heinrich, 1975).

When orchids are so pollinator-limited, how couldsuch a pollination system have evolved (Gill, 1989)?Under pollen-limitation there should be strong selec-tion to increase pollinator visits. What better waycould there be than to provide a more enticing reward?Any plant carrying a mutation that enhances itsreward would likely be more successful than one with-out it. This may have happened in the genus Disa.Johnson, Linder & Steiner (1998) showed that nectarreward systems in the genus are derived and evolvedindependently three times. These results are quiteintriguing because they contradict the hypothesis thatdeception systems are more specialized than reward-based systems and are derived from them (Ackerman,1986a; Dafni, 1987).

If we do indeed find that deceptive systems evolvedfrom rewarding ones in some lineages, then whatwould have led to the loss of a reward? Selection mayhave favoured a loss if having a reward resulted in (1)high levels of geitonogamy (selfing) or (2), resourcedepletion through high costs of nectar or fruit produc-tion. Johnson & Nilsson (1999), Smithson & Gigord(2001) and Smithson (2002) tested the geitonogamyhypothesis by adding nectar to rewardless orchids(Orchis spp., Barlia and Anacamptis). They foundthat pollinator visitation increased dramatically asexpected, but that there was little or no effect on thenumber of geitonogamous pollinations or total fruitset except in Anacamptis. In the first two taxastudied, delayed caudicle bending likely preventedself-pollinations. As for resource depletion, we havenothing but suggestive data: nectar production maybe expensive (Southwick, 1984; Koopowitz & March-ant, 1998; Luyt & Johnson, 2002), and as alreadyreviewed, there is ample evidence of a physiologicalcost to reproduction.

Smithson & Gigord (2001) identified a third possiblemechanism that would lead to evolution of a deception

system. In their nectar-addition experiment, theyfound that nectarless plants had ten times more pol-linaria removals as reward plants. If a population hada nectarless mutant, then it would have a much higherprobability of fathering seed than a nectar-producingplant. Such mutants would rapidly spread through apopulation even when the effective population size issmall. Smithson & Gigord (2001) argued that a malefunction advantage is the more likely explanation forthe evolution of deception systems.

Selection may not be the only avenue for the evolu-tion or maintenance of deception. Nectar productionmay have been lost through genetic drift if populationor display sizes were insufficient to attract or main-tain pollinator interest regardless of reward quality orquantity. Coincidentally, when population sizes aresmall, both drift and competition for pollinator atten-tion are more likely to occur. Tremblay & Ackerman(2001) showed that conditions for drift do occur inorchids, and the widespread pollen-limitation inorchids implies that competition for pollinators issevere. We still need experimental data on reproduc-tive success across an array of population sizes to cor-roborate this hypothesis.

Thus, we have phylogenetic evidence that nectarproducing orchids may have evolved from deceptivespecies (Johnson et al., 1998) which is also supportedby theoretical considerations that deception should bean unstable system (Gill, 1989). On the other hand,there are data which indicate that deceptive speciesmay have evolved from reward species via costs toreproduction or reward production (Ackerman & Mon-talvo, 1990), or through male function advantage(Smithson & Gigord, 2001). Perhaps there is no singleor dominant process by which deceptive pollinationhas evolved or is maintained. However, the wide-spread nature of the pollination system in orchidsargues against this possibility.

Odour and reproductive success: As in other charac-teristics associated with pollinator attraction, floralfragrances are usually considered to be adaptive. Thecharacterization of floral fragrances took immenseleaps in the 1960s and early 1970s, particularly fororchids pollinated by male euglossine bees (Dodsonet al., 1969; Williams & Dodson, 1972). Field andexperimental studies have linked floral fragrances inboth temperate and tropical genera to specific pollina-tors or groups of pollinators (Dressler, 1968; Hills, Wil-liams & Dodson, 1972; Borg-Karlson, Bergström &Kullenberg, 1987; Bergström et al., 1992; Alcock,2000; Plepys, Ibarra & Löfstedt, 2002). Because theemphasis was on reproductive isolating mechanisms,characterization tended to be typological. It is cer-tainly undeniable that interspecific variation in fra-grances exists and that this may be attributed to



differences in pollinators. Nonetheless, there is con-siderable intraspecific variation in floral fragranceproduction and composition as well.

The first population-level sampling of orchid floralfragrances that we know of showed that there wasconsiderable variation among plants in fragrances ofeuglossine bee-pollinated Cycnoches species (Gregg,1983). Within- and among-population variation wasalso described in the European Platanthera bifoliaand P. chlorantha (Tollsten & Bergström, 1989, 1993).In contrast, Patt, Rhoades & Corkill (1988) foundmuch quantitative but little qualitative variation inpopulations of North American P. stricta. In a detailedstudy of fragrance variation, Moya & Ackerman (1993)showed that fragrances of Epidendrum ciliare are pro-gressively more variable among flowers of an inflores-cence, among inflorescences of a plant, among plantsof a population, and finally among populations. Inanother study, some plants of the widespread Anti-llean epiphyte, Tolumnia variegata, were found to befragrant whereas others were not (Ackerman, Melén-dez-Ackerman & Salguero-Faría, 1997).

What are the causes of intraspecific variations infloral fragrances? Some variation may be attributableto hybridization (Tollsten & Bergström, 1993),whereas genetic drift may be important in other cases.Drift may come about due to founder events, smalleffective population size of isolated populations (Toll-sten & Bergström, 1993) or loss of function, as hasapparently happened for other characteristics(Steiner, 1998). More recently it has been suggestedthat variation in fragrance production and composi-tion may be driven by natural selection, particularlyin species that employ deceptive means of attractingpollinators. In the pseudocopulated Ophrys sphegodes,part of the fragrance spectrum is invariant whereasother parts are not. The invariant part mimics thefemale sex pheromones of the pollinators while thenon-biologically active part of the fragrance is quitevariable. It is this variable portion of the fragrancethat impairs avoidance learning on the part of theduped male bees (Ayasse et al., 2000).

Negative frequency-dependent selection has beenproposed to maintain high levels of variation in char-acteristics associated with pollinator attraction fordeception-pollinated species (Ackerman & Galarza-Pérez, 1991). Although negative frequency-dependentselection on flower colour has been shown, we have notbeen able to demonstrate it for fragrance. An attemptwas made to test this hypothesis by manipulating thefrequencies of fragrant and odourless plants of Tolum-nia variegata but without success. As it turns out, thefragrant plants are highly variable in fragrance com-position (A. Cuevas, unpubl. data), a factor which mayhave affected the outcome of the experiment. We areunaware of any other attempt to associate reproduc-

tive success with odour variation in natural popula-tions of orchids.

Because pollinators associate rewards with fra-grances quite readily (Kunze & Gumbert, 2001), weexpect that some deceptive species would have anodour identical to rewarding species, be odourless orhave a highly variable fragrance (Moya & Ackerman,1993; Kunze & Gumbert, 2001). With the partialexception of the Ophrys model (Ayasse et al., 2000), weare unaware of any deceptive species with a fragrancethat mimics that of a rewarding model (Nilsson, 1980;Kunze & Gumbert, 2001). Thus far, deceptive speciestend to be either odourless or quite variable.

Current evidence for natural selection on floralodours is mostly indirect, coming from studies whichshow that different fragrances are associated with dif-ferent pollinators. Others have demonstrated theadaptiveness of particular compounds (Schiestl &Ayasse, 2001). In cases where variation is minimal, weexpect that the pattern would have been the result ofstabilizing selection. On the other hand, drift, nega-tive frequency-dependent selection or hybridizationmay be responsible for high levels of fragrance varia-tion in some species.

Inflorescence height and reproductive success: Howhigh flowers are from the ground or substrate couldaffect reproductive success if pollinators have foragingheight preferences. Handel & Peakall (1993) showedthat thynnine wasps preferred flowers of the sexuallydeceptive Chiloglottis reflexa Druce that were lower tothe ground (15 cm) compared to flowers higher up inthe vegetation (55 and 105 cm). Maad (2000) found sig-nificant selection differentials for stalk length in Pla-tanthera bifolia in all three years of her study. However,the selection differentials were more consistentthrough the female function. O’Connell & Johnston(1998) found that stalk length (flower height) was sig-nificantly correlated with male and female reproduc-tive success in Cypripedium acaule. In the Caribbeangenus Psychilis, inflorescences have an elongate scapeup to 2 m in some species. Aragón & Ackerman (2004)detected a significant positive effect of height aboveground and length of inflorescence on reproductive suc-cess for P. monensis but similar data failed to show anysuch relationship for the related P. krugii (J. D. Ack-erman & R. L. Tremblay, unpubl. data).

Multivariate selectionSelection can affect multiple characters simulta-neously and can occur either as a result of charactersthat are linked genetically or because of allometriccorrelations. We found two studies that attempted tomeasure multivariate selection in orchids and bothlooked at phenological, vegetative and floral charac-teristics. O’Connell & Johnston (1998) found negative



correlational selection on plant height with floweringtime in one of two populations of Cypripedium acaulewhereas Maad (2000) found no evidence of correla-tional selection in Platanthera bifolia. In the absenceof experimental manipulation of trait distributions,future studies should consider using regression anal-ysis as a powerful tool (Lande & Arnold, 1983) forobtaining estimates of the strength of selectionacting both directly (univariate) and indirectly(multivariate).

EVIDENCE OF GENETIC DRIFT

SHIFTING BALANCE THEORY OF EVOLUTION

While natural selection is the change in trait distribu-tion across generations caused by selection of advan-tageous characteristics, genetic drift is the changeresulting from random events. There are presentlytwo main hypotheses for evolution through geneticdrift: Wright’s shifting balance theory (Wright, 1968,1969, 1977, 1978) and genetic drift including only thetwo first steps of Wright’s theory (Coyne et al., 1997).Wright’s shifting balance theory describes a mecha-nism that involves genetic drift, followed by mass nat-ural selection and migration among subpopulations. Itimagines an adaptive landscape with peaks and val-leys of relative fitness. For example, peaks in fitnessmay correspond to characteristics of flowers that pro-mote the efficient deposition and receipt of pollen fromcertain sized pollinators. Valleys are combinations offloral traits that do not match available pollinators.

How then do members of a species pass from onepeak to another, thereby achieving reproductive isola-tion and resulting in speciation? Wright (1978) pro-posed three stages to his theory. First, relativelyisolated subpopulations or demes become geneticallydifferentiated through genetic drift, the stochasticchange in gene frequencies caused by small effectivepopulation size (Ne). Put simply, Ne is the number ofindividuals in a population actually leaving offspringin the next generation. However, its meaning andmeasurement are much more complicated than this(Wright, 1939; Kimura & Crow, 1963; Waples, 1989;1991; Wade, 1991; Barrowclough & Rockwell, 1993;Nunney, 1993, 2000; Orive, 1993). Demes ‘drift’ ran-domly into new genetic combinations and this processis more likely with decreasing Ne. When foundingevents involve very few individuals colonizing a newlocale, extreme changes can occur, particularly if thisresults in the alteration of genetic variance-covariancematrices controlling the relationships between indi-vidual genes (Carson, 1990). In this way, a deme maypass or ‘skip’ fitness valleys, placing them on an oppo-site slope of the adaptive landscape.

The second stage is mass selection, perhaps themost straightforward aspect of the theory (Crow,

Engels & Denniston, 1990; Coyne et al., 1997). Once anew variant has become established, one that achievessome modicum of pollination efficiency from a new pol-linator, selection will act to improve this efficiency.Additive genetic variance allowing selection to occur isassumed to be maintained by polymorphic mutationand recombination (Kimura, 1965; Lande, 1975) andmay be augmented by the scrambling of genetic vari-ance-covariance matrices during a founding event(Carson, 1990; Roughgarden, 1996). Owing to overallpollinator limitation, less fit individuals may repro-duce infrequently if at all, speeding the fixation of pol-lination efficient genotypes. As a concrete example,Inoue (1986b) noted that the effect of spur length onestimates of male and female reproductive success inPlatanthera mandarinorum was greater at lower pol-lination frequencies than at higher ones. Thus, selec-tion proceeds most rapidly under pollinator limitation,again possibly distinguishing orchids from many otherplants if variance in reproductive success is higher ormore frequent in orchids.

The establishment of new pollination strategies ofclosely related species with different pollinators maybe speeded up if only a few fruits are needed to set afounder population. The fact that species of Catase-tum can be distinguished by a few fragrance compo-nents (Hills et al., 1972) may be the result of the quickestablishment of new genotypes. Thus, low fruit pro-duction in orchids, coupled with the high number ofseeds produced per pod, may have provided a mecha-nism for speciation and diversification in pollinationsystems.

The third stage involves the movement of these‘more’ fit genes back to the original populations. Intime, the different populations take on the new com-bination of phenotypes from the original founding pop-ulation (Wright, 1969; 1977; Lande, 1979; Coyne et al.,1997). However, this process has never been docu-mented in natural populations (Coyne et al., 1997).Furthermore, it would not result in cladogenesis andconsequently cannot fully explain the great diversityof orchids.

Commonality of small effective population size (Ne)We have argued and shown elsewhere (Zimmerman &Aide, 1989; Calvo, 1990b; Ackerman & Galarza-Pérez,1991; Tremblay, 1996, 1997a; Tremblay & Ackerman,2001) that reproductive patterns in orchids may pro-mote small Ne. Within a season, few flowering individ-uals produce fruit and contribute to the seed pool,particularly in populations of tropical orchids(Table 3). Fruiting failure in a reproductive season ishigh (Fig. 4). This may hold true even over longer sam-pling periods because fruit production can suppressflowering in subsequent years. Estimates of Ne for thethree species of the tropical epiphytic Lepanthes



revealed it to be below 20% for all species and popu-lations; most were below 10% of the standing adultpopulation (Tremblay & Ackerman, 2001). Only 18%of flowering individuals in a population of Aspasiaprincipissa produced fruit during the three years of asurvey by Zimmerman & Aide (1989). Furthermore,the variances in female and male reproductive successand in total lifespan are high and can further reduceNe. If the variance in recruitment is as skewed as theprevious variables, then Ne may be lowered even more.While numerous factors contribute to Ne, we believethat these life history characteristics separate orchidsfrom many other plant groups and are all directlylinked to the extreme degree to which reproduction ispollinator-limited. It will be necessary to investigatein detail the commonality of small Ne in orchids beforewe can critically evaluate the frequency of this processin orchid evolution.

Gene flow in orchidsGene flow is an essential component of evolutionaryprocesses and critical to our argument that geneticdrift is an important component of diversification inorchids. The amount of gene flow among local popula-tions will determine whether or not individual popu-lations (demes) can evolve independently and begenetically distinct and plausibly lead to cladogenesis.Theoretically, a gene flow rate greater than two indi-viduals per generation is sufficient to prevent localadaptation/evolution. Consequently, local selectionand drift will be overcome by migration (Merrell,1981). Meanwhile, a flow rate less than one individualper generation will likely result in population differ-

entiation through natural selection or genetic drift(Merrell, 1981; Roughgarden, 1996). If the product ofNm (the effective number of migrants per generation;

, N = effective population size, m =

migration rate) lies between one and two, there will beconsiderable variation in gene frequencies among pop-ulations (Merrell, 1981). The determining factor relat-ing gene flow and natural selection is the effectivepopulation size; the smaller it is, the more gene flow isrequired to prevent local differentiation (Roughgar-den, 1996). Thus, many small groups of individualswith high gene flow would evolve as a unit if the sumof the populations has a large Ne. Consequently, nat-ural selection will likely dominate as the main evolu-tionary process. Therefore, we need to know theamount of gene flow among populations or groups ofindividuals in order to identify the evolutionary pro-cesses involved.

Gene flow and dispersal can be measured throughdirect and indirect methods (Bohomak, 1999). Directobservation of gene flow can be viewed by the use ofmark and recapture studies (for mobile organisms orstained pollen) or tracking marker alleles (paternityanalysis) over a short number of generations. Indirectmethods involve obtaining allele frequencies and pro-vide an estimate of the average long-term effect of dif-ferentiation by genetic drift. The alleles are assumedto be neutral so that differentiation based on thesemarkers would be a consequence of genetic driftrather than natural selection. Bohomak (1999) con-cluded that simple population genetic statisticsare robust for inferring gene flow among groups ofindividuals.

Few orchid studies have attempted to directlyobserve gene flow and thus far only staining ormicrotagging of pollen have been used (Peakall,1989b; Nilsson et al., 1992; Folsom, 1994; Tremblay,1994; Salguero-Faría & Ackerman, 1999). All suchstudies examined gene flow only within populations.

Current estimates of gene flow have been madeusing indirect methods. They indicate that there ismuch variation among species (Table 10). Data areavailable for 70 species and subspecies of orchids. Nogenetic differentiation among populations wasobserved in two of the species. Twenty-nine specieswith high levels of estimated gene flow (Nm > 2), 20 ofthe species have intermediate to low levels of geneflow (1 < Nm < 2) and 22 species have low levels(Nm < 1; Fig. 5). There is much variation within agenus, but there appears to be some consistency. Forexample, Orchis species typically have high estimatesof gene flow among populations (Scacchi et al., 1990;Corrias et al., 1991; Rossi et al., 1992) whereas Lepan-thes spp. have much lower ones (Tremblay & Acker-man, 2001). The present indirect approach to

FNm

st =+

14 1

Figure 4. Distribution of fruiting failure in natural popu-lations of orchids. Frequency of plants (N = 36) that fail toset fruits. Data from Table 3.

Per cent fruiting failure

90.0 –100.0

80.0 – 90.0

70.0 – 80.0

60.0 – 70.0

50.0 – 60.0

40.0 – 50.0

30.0 – 40.0

20.0 – 30.0

10.0 – 20.0

0.0–10.0

Freq

uenc

y

8

6

4

2

0



Table 10. Estimates of gene flow in orchids. Nm(S) = gene flow estimates (number of migrants per generation) based onSlatkin’s private allele model; Nm(W) = gene flow estimates based on Wright’s statistics; GST coefficient of genic differen-tiation among populations

Species References Nm(S) Nm(W) GST

Caladenia tentaculata Tate Peakall & Beattie, 1996 7.101 0.0346

Calypso bulbosa (L.) Oakes Alexandersson & Ågren, 2000 3.20 0.072Cephalanthera damasonium (Mill.)

DruceScacchi, De Angelis & Corbo, 1991 --5 --5

Cephalanthera longibracteata Blume Chung, Nason & Chung, 2004 0.762 0.247Cephalanthera longifolia (L.) Fritsch Scacchi, De Angelis & Corbo, 1991 2.151 0.104Cephalanthera rubra (L.) Rich. Scacchi, De Angelis & Corbo, 1991 0.761 0.247

Brzosko & Wroblewska, 2003 14.21 0.017

estimating Nm is actually the sum and interaction ofthe historical and present gene flow (Slatkin & Bar-ton, 1989).

Variation in gene flow estimates can be extensive,even within the same species. In Cephalantherarubra, Scacchi et al. (1991) found high genetic popu-lation substructure in Italy (FST = 0.245), whereasBrzosko & Wroblewska (2003) found very little struc-ture in South Korea (FST = 0.017). Similar large varia-tions have been observed in Epipactis helleborine inEurope and North America (Hollingsworth & Dickson,1997; Scacchi, Lanzara & De Angelis, 1987; Squirrellet al., 2001) and Cypripedium calceolus in NorthAmerica and Poland (Case, 1993, 1994; Brzosko,Wróblewska & Ratliewicz, 2002; Brzosko, Ratkiewicz& Wroblewska, 2003)

Are there phylogenetic associations with gene flow?The Orchis and Lepanthes data are suggestive butmuch more extensive sampling is needed for both tem-perate and tropical species. Curiously, Lepanthes andOrchis have very different population genetic param-

eters yet both are species-rich genera and are likely ina state of evolutionary flux.

EVOLUTIONARY DIVERSIFICATION MODELS

The commonness of pollinator limitation in orchids isevident from the literature and we suggest it to be ofprimordial importance in explaining evolutionary pro-cesses in the family. The low levels of fruit set and theskewed reproductive success and lifespan among indi-viduals will result in small Ne when gene flow amongpopulations is limited.

The presence of short and long-term resource con-straints is not likely to have effected the importance ofpollinator limitation as a component of evolution inthe Orchidaceae because resource constraints havemostly been observed between reproductive bouts andnot through fruit abortion. For example, if fruit abor-tion occurred because of resource constraints, thiswould result in a reduction in the variance in repro-ductive success among individuals and an increase inNe.

Interestingly, the process of natural selection inorchids has rarely been detected in the field eventhough it might seem obvious that it must have beeninvolved in the evolutionary process. Moreover, asmentioned earlier, we are still ignorant of the strengthof natural selection, its frequency and directions. Theevidence we use to explain the great morphologicaldiversity that we assign to natural selection is eitherpattern-based or circumstantial. We believe that thegreat diversity of vegetative and floral charactersexhibited by the Orchidaceae is best explained by aprocess that involves more than just natural selection.In this section we will discuss our view of the processof evolution in orchids after differentiating naturalselection and Wright’s shifting balance theory andexplain why these processes alone cannot account forevolution in this family.

Figure 5. Frequency distribution of gene flow estimates(Nm(W)) among populations of orchids. Data from Table 10.

0

2

4

6

8

10

12

14

16

18

20

22

Freq

uenc

y

0 2 4 6 8 10 12 14 16Nm(W)



Cymbidium goeringii Rchb. f. Chung & Chung, 1999 8.48 2.30 0.098Cypripedium acaule Ait. Case, 1994, 2002 1.271 0.164Cypripedium arietinum R. Br. Case, 2002 --5 --5

Cypripedium calceolus L. Case, 1993, 1994 1.631 0.196Brzosko, Ratkiewicz & 17.6 0.014

Wroblewska, 2002Cypripedium candidum Muhl.

ex Willd.Case, 1994, 2002 3.371 0.069

Cypripedium fasciculatumKellogg ex S. Watson

Aagaard, Harrod & Shea 1999 6.00 0.04

Cypripedium kentuckienseC. F. Reed

Case et al., 1998; Case, 2002 1.121 0.182

Cypripedium parviflorum Salisb. var.pubescens (Willd.) O. W. Knight

Case et al., 1998; Case, 2002 1.281 0.163

Southern populations Wallace & Case, 2000 0.94 0.209Northern populations 1.57 0.137

var. makasin (Farw.) Sheviak var. 1.00 0.199parviflorum species level 1.43 0.149

0.83 0.232Cypripedium reginae Walter Case, 1994, 2002 0.471 0.349Dactylorhiza romana (Sebastiani) Soó Bullini et al., 2001 3.321 0.07Dactylorhiza sambucina (L.) Soó Bullini et al., 2001 1.311 0.16Epidendrum conopseum R. Br. Bush, Kutz & Anderton, 1999 1.433 0.149Epipactis helleborine (L.) Crantz Scacchi, Lanzara & De Angelis,

1987; Squirrell et al., 20017.31 0.033

European populations 1.001

0.241,40.2000.5064

North American 2.531 0.0904

Hollingsworth & Dickson, 1997 0.791 0.240Epipactis youngiana Richards & Porter Harris & Abbott, 1997 2.431 0.093Eulophia sinensis Miq. Sun & Wong, 2001 --

0.1331,30.00.6533

Gooyera procera Ker-Gawl. Wong & Sun, 1999 0.2211

0.3971,30.5230.3863

Gymnadenia conopsea (L.) R. Br. Scacchi & De Angelis, 1989 0.2801 0.471Gustafsson, 2000 3.917 0.06

Gymnadenia conopsea (L.) R. Br.conopsea

Soliva & Widmer, 1999 2.96 0.078

Gymnadenia conopsea (L.) R. Br.subsp densiflora (Wahl) E.G.Camus & A. Camus

Soliva & Widmer, 1999 0.39 0.391

Gymnadenia odoratissima (L.) Rich Gustafsson & Sjögren-Gulve, 2002 1.066 0.19Lepanthes caritensis Tremblay &

AckermanW. Carromero, R. L. Tremblay & J. D.

Ackerman, unpubl. data0.23 1.30 0.167

Lepanthes eltoroensis Stimson Tremblay & Ackerman, 2001 1.54 0.89 0.220Lepanthes rubripetala Stimson Tremblay & Ackerman, 2001 0.48 0.62 0.270Lepanthes rupestris Stimson Tremblay & Ackerman, 2001 1.15 1.84 0.170Lepanthes sanguinea Hook. W. Carromero, R. L. Tremblay & J. D.

Ackerman, unpubl. data1.74 1.45 0.144

Lepanthes woodburyana Stimson W. Carromero, R. L. Tremblay & J. D. Ackerman, unpubl. data

0.61 7.5 0.032

Leporella fimbriata Peakall & James, 1989 5.431 0.044


Table 10. Continued



Nigritella rhellicani Teppner &Klein

Hedrén, Klein & Teppner, 2000 1.381 0.153

Orchis laxiflora Lam. Scacchi, De Angelis & Lanzara, 1990 2.851 0.08Arduino et al., 1996 1.971 0.116

Orchis longicornu Poir. Corrias et al., 1991 4.20 12.252 0.02Orchis mascula (L.) L. Scacchi, De Angelis & Lanzara, 1990 2.761 0.083Orchis morio L. Scacchi, De Angelis & Lanzara, 1990 3.661 0.064

Rossi et al., 1992 4.751 0.05Orchis palustris Jacq. Arduino et al., 1996 0.311 0.448Orchis papilionacea L. Scacchi, De Angelis & Lanzara, 1990 6.331 0.038Orchis pauciflora Ten. Scacchi, De Angelis & Lanzara, 1990 6.001 0.040Orchis provincialis Balb. Scacchi, De Angelis & Lanzara, 1990 10.621 0.023Orchis purpurea Huds. Scacchi, De Angelis & Lanzara, 1990 5.701 0.042Orchis tridentata Scop. Scacchi, De Angelis & Lanzara, 1990 6.161 0.039Paphiopedilum micranthum T. Tang

& F. T. WangLi, Luo & Ge, 2002 0.061 0.7977

Platanthera leucopaea (Nutt.) Lindl. Wallace, 2002 0.081

0.7110.7540.263

Pleurothallis adamantinensis Brade Borba, Semir & Sheppard, 2001 4.851 0.049Pleurothallis fabiobarrosii Borba

& SemirBorba et al., 2001 2.841 0.081

Pleurothallis johannensis Barb. Rodr. Borba et al., 2001 5.191 0.046Pleurothallis ochreata Lindl. Borba et al., 2001 4.061 0.058Pleurothallis teres Lindl. Borba et al., 2001 0.971 0.205Pterostylis aff. alata M. A. Clem. Sharma et al., 2001 0.811 0.235

Sharma, Jones & French, 2003 4.381 0.054Pterosylis angusta A. S. George Sharma et al., 2001 1.301 0.161Pterosylis aspera D. L. Jones & M. A.

Clem.Sharma et al., 2001 1.011 0.198

Pterostylis gibbosa R. Br. Sharma, Clements & Jones, 2000 1.42 0.15Pterostylis hamiltonii Nicholls Sharma et al., 2001 0.861 0.225Pterosylis rogersii E. Coleman Sharma et al., 2001 1.101 0.186Pterostylis scabra Lindl. Sharma et al., 2001 0.831 0.232Spiranthes diluvialis Sheviak Arft & Ranker, 1998 5.44 0.044Spiranthes hongkongensis S. H. Hu

& BarrettoSun, 1996 --5 --5

Spiranthes sinensis (Pers.) Ames Sun, 1996 0.53 1.19 0.174Tipularia discolor (Pursh) Nutt. Smith, Hunter & Hunter, 2002 0.357 0.415Tolumnia variegata (Sw.) Braem Ackerman & Ward, 1999 8.00 2.50 0.09Vanilla barbellata Rchb. f. Nielsen & Siegismund, 2000 1.78 0.123Vanilla claviculata (W. Wright) Sw. Nielsen & Siegismund, 2000 1.33 0.158Zeuxine gracilis Blume Sun & Wong, 2001 0.5001

0.21410.3330.5393

Zeuxine strateumatica Schltr. Sun & Wong, 2001 0.0213 0.9243


Table 10. Continued

1Nm calculated by the present authors from GST or FST using formula on p. 320 of Hartl & Clark (1989).2Recalculated using previous formula, original Nm value 3.70.3Calculated from RAPD markers.4Calculated from cpDNA.5No genetic differentiation found among populations.6Calculated according to Weir and Cockerham’s statistics.7Estimated using RAPD and AMOVA.



DARWINIAN AND WALLACIAN GRADUALISM

Both Wallace (1858) and Darwin (1859) promoted theview that change in character states among genera-tions is gradual and driven by natural selection.Armed with an understanding of gene expression, pop-ulation genetics, and techniques to measure selection,gradualism has come under fire from many quarters,including orchid biologists.

Gradual change may occur when effective popula-tions are large or gene flow is substantial amongsmaller populations. Under these circumstances, mostgenetic variation would occur within populationsrather than among them. We have some evidence,mostly indirect, that such conditions do occur inorchids. Pollen and seed dispersal is the usual meansfor gene flow. Pollinators can move pollinaria long dis-tances, presumably from population to population(Janzen, 1971; Williams & Dodson, 1972; Ackerman &Montalvo, 1985). The dust-like seeds are ideally suitedfor wind dispersal (Carey, 1998; Murren & Ellison,1998) and may be carried over great distances (Arditti& Ghani, 2000). Nevertheless, fine-scale genetic struc-turing (Chung, Nason & Chung, 2004), mathematicalmodels and wind tunnel experiments suggest limitedgene flow (Murren & Ellison, 1998). The rapid rangeexpansion of Oeceoclades maculata (Stern, 1988)throughout the neotropics clearly demonstrates thepotential orchids have for dispersal. In addition, somepopulation genetic data are consistent with the highgene flow hypothesis. For outbreeding species, varia-tion among populations can be quite low, especially fortemperate terrestrial orchids (Table 10). If evolution-ary change occurs in such populations, then it wouldlikely be a gradual process.

Nevertheless, we have also presented substantialevidence that evolutionary change for many orchids isnot likely to be gradual. Effective population sizes maybe small because of a combination of factors: fewadults per population, pollinator limitation, skewedreproductive success among individuals, and perhapsskewed progeny recruitment. In such populations,gene flow is restricted, and a high proportion of thegenetic variation occurs among populations. Gradualevolutionary change is not consistent with those con-ditions, so we advocate an alternative process that weoutline below.

THE DRIFT-SELECTION MODEL OF ORCHID EVOLUTION

A simpler scenario than the shifting balance theory isthe founding event–genetic drift scenario without thethird stage of Wright’s process. In this alternativeapproach to evolution, genetic drift would be acommon occurrence in local populations followed bynatural selection, ultimately causing sufficient differ-

entiation and the beginning of cladogenesis. Subse-quently, this new taxon would colonize new sites.Assumptions of this model include commonality ofsmall populations (low effective population sizes), lowor infrequent gene flow among populations, heritablevariation, and high or consistent selection coefficientsamong generations in local populations.

Our present vision of orchid evolution involves bothgenetic drift and natural selection working simulta-neously. For most orchids, population sizes are smalland are effectively further reduced by skewness inboth reproductive success and lifespan of individuals.Such conditions would likely result in frequent char-acter changes among generations. Most of thesechanges would be neutral or negatively selected withthe odd character having some positive selection(directional, disruptive, stabilizing selection or somenon-parametric distribution). If true, then we shouldobserve large heritable character variation amongindividuals, populations and generations.

The original evolutionary model for drift-selection isDodson’s (1962) ‘leap-frog’ model of orchid speciationin Stanhopea. This genus is pollinated by relativelysmall euglossine bees of the genus Euglossa and rela-tively large ones of the genera Eulaema and Eufriesea(Kimsey, 1982). Dodson proposed that speciation hadoccurred repeatedly by the switching of species fromsmall to large pollinators and vice versa. This couldpossibly occur through random changes in floral fra-grances caused by genetic drift, followed by strongselection on morphological characteristics to accom-modate a different pollinator. This model was recentlygiven support by a study which demonstrated that themolecular phylogeny of Stanhopea is not congruentwith phylogenies based on floral fragrances (Williams& Whitten, 1999). A similar model of evolutionarydiversification may apply to the Australian genusChiloglottis and its Thynninae wasp pollinators. Mantet al. (2002) showed that the pattern of pollinatorchange is conservative in the sexually deceptive orchidand that the phylogenies based on the DNA sequencesof the orchids and pollinators are highly congruent(but not perfectly so). They emphasized that theobserved pattern is not evidence of coevolutionbecause relative branch length differences betweenplant and insect groups were not similar. They alsosuggested that pollinator shifts are dependent on theavailability of pollinators within the geographicalarea, the phenology of flowering and wasp emergence,and the changes in floral odour chemistry that mimicfemale wasp pheromones. Thus, random changes infloral fragrances could attract a different species ofwasp and lead to cladogenesis.

It is unlikely that we will observe in the field aseries of evolutionary events reflecting the totality ofthe drift-selection model. However, it is possible to



assemble sufficient field observations and experi-ments to reconstruct the process. The model may besupported by demonstrating that: (1) gene flow amongpopulations is restricted in time or space; (2) effectivepopulation sizes are usually (or occasionally) small,and (3) selection is episodically intense and differentamong populations.

CONCLUSIONS

In this review, our central question has been: Whataspect(s) of orchid reproduction explain(s) the intri-cate pollination mechanisms of this family? We haveargued that pollen-limitation of both male and femalereproductive success, in combination with the uniquestructure of the orchid flower (the column), largelyexplains the extravagant pollination mechanisms.Low reproductive success, leading to a small propor-tion of reproducing individuals and low Ne, makesgenetic drift a more likely source of evolutionarychange in orchids that, combined with episodic selec-tion, explains much of the diversification of the family.Paradoxically, it is the low reproductive success oforchids, not the opposite, that may account for boththeir unique pollination mechanisms and extremediversity. The Orchidaceae is a remarkable group ofplants and the fundamental processes by which thefamily has diversified are far more fascinating thanDarwin ever suspected.

ACKNOWLEDGEMENTS

This review was begun in the early 1990s by JKZ andRNC, then abandoned as careers changed. They thankRLT and JDA for reviving the project and achievinglevels of understanding never contemplated. JKZwishes to thank the support of the Smithsonian Envi-ronment Research Center and Dennis Whigham dur-ing the incipient phase of development. We greatlyappreciate critical comments from W. M. Whitten, N.W. Williams and two anonymous reviewers. We alsothank all our colleagues who have contributed unpub-lished data in the many areas of orchid biology.

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