Schedule

Brief meeting tomorrow morning to discuss paper?

Rich Fredrickson

Sally AitkenUniversity of British ColumbiaChapter 21: Climate Change

Rich Fredrickson Gordon Luikart

Mexican wolves Bighorn sheep

Final Exam Period Schedule

Addis, BrettAusband, DavidBrekke, TomCross, ToddErickson-Davis, MorganFannin, BarbHoneycutt, KenRoffler, GretchenTempa, Tshering

TODAY

Turn-in Paper

Chapter 16: Units of Conservation (continued)ESA & Distinct population segments (DPS)

Chapter 17: HybridsHybrid fitnessNatural hybridizationAnthropogenic hybridizationHybrids & the ESAGenetic rescue

Signed by President Nixon on 28 December 1973

“The ultimate goal of the Endangered Species Act is the conservation of the ecosystem on which all species depend for survival”.

U.S. Congress, 1978

Under the ESA, a “species” is -

• A species

• A subspecies

• A distinct population segment (DPS) of a vertebrate (not an invertebrate or plant).

Joint (USFWS & NMFS) DPS policy in 1996:

1.Discreteness of the population segment in relation to the remainder of the species to which it belongs;

2. The significance of the population segment to the species to which it belongs; and

Questions???

A double-edged sword

hybrida: the Latin word for a piglet resulting from the union of a wild boar and a tame sow.

Derived from the Greek hubris, meaning “arrogance or insolence against the gods”.

Hybridization and ConservationComplex and controversial

Even the definition is controversial

Harmful effects of hybridization  Sterile hybrids

Reduced reproductive success because of loss of gametes.

Introgression

Extinction through genetic mixing.

Hybridization: interbreeding of individuals from genetically distinct populations, regardless of the taxonomic status of the populations.

Introgression: gene flow between populations whose individuals hybridize.

Hybrid Fitness(1) Hybrid vigor

Hybrids have greater fitness than either parent

(2) Intermediate fitness

(3) Outbreeding depression

Hybrids have lower fitness than either parent

Hybrid Vigor (heterosis)

Inbreeding depression in reverse

Causes

(1) Sheltering of deleterious recessive alleles

(2) Overdominance (heterozygous advantage)

Outbreeding depression: a reduction in fitness in hybrid individuals relative to the parental types.

  Intrinsic: results from genetic incompatibility between hybridizing taxa. 

Chromosomal (rearrangements that disrupt meiosis).

  Genic (interactions within or between loci)

 Extrinsic: results from reduced adaptation to environmental conditions.

PNAS 99:12955-12958. (2002)

COX = cyt c oxidase mtDNA

CYC = nuclear gene

Genic: interactions within or between loci

Intrinsicor

Extrinsic?

Capra ibex

• Introduced ibex from Austria to reestablish extinct population in Czechoslovakia

• Later introduced ibex from Turkey and the Sinai

- Rutted earlier in the fall and gave birth in February resulting in high juvenile mortality

Intrinsicor

Extrinsic?

parental parental

hybrids

Whitetail Mule deer

X

Lingle, S. 1992. Escape gaits of white-tailed deer, mule deer and their hybrids gaits observed and patterns of limb coordination. Behaviour 122:153-181.

VideoThank you Susan Lingle!

gallop

stot

stumble

whitetail

mule deer

F1 hybrid

2001. Trends in Ecology & Evolution 16:613-622.

Hybrid taxon: an independently evolving, historically stable population or group of populations possessing a unique combination of heritable characteristics derived from two or more discrete parental taxa.

Virgin River roundtail chub (Gila seminuda) are listed as endangered under the ESA. It is a hybrid taxon that appears to have originated from hybridization between G. elegans and G. robusta in the Pleistocene long before human influence in the Colorado River system.

Type 2: Natural Introgression

Intraspecific

DPS

Interspecific

Molecular leakage

Darwin’s finches, etc.

Humans

Some gene flow between species with small Ne is essential for long-term viability of species.

Nature Rev Genetics 12:603-614 (2011)

An estimated 3% of the human genome resulted from hybridization between modern Europeans and Neandertals some 50,000 years ago

May 2010

YES!

Type 3: Hybrid zone

Yellow- and red-shafted flickers Colaptes auratus

Anthropogenic hybridization

Habitat modifications

Translocation, introductions, etc.

high Turbidity low

Bull trout = BL (L= homozygous)

Brook trout = BR (R = homozygous)

South Fork of Lolo Creek

Yellowstone cutthroat trout

(YCT)

O. c. bouvieri

Rainbow trout

(RT)

O. mykiss

(Illustrations by Joseph R. Tomelleri)

Westslope cutthroat trout

(WCT)

Oncorhynchus clarkii lewisi

aaa

15

N

RTWCT

YCT

WCT and YCT are extremely genetically divergent. They are fixed for different alleles at 10 out of 52 allozyme loci.

Range Maps. RT ranges north and south beyond map area.

Hybrid swarm Forest Lake, Montana

Genotypes at 8 diagnostic allozyme loci and mtDNA from Forest Lake, Montana.

W = homozygous WCT

WY = heterozygous

Y = homozygous YCT

Hybrid swarm: all individuals are hybrids by varying numbers of generations of backcrossing with parental types and mating among hybrids.

1614121086420

3

2

1

0

WCT genes

Fre

quen

cy

Pure YCT Pure WCT

15 individuals

Hybrid index (2 x 8 diagnostic loci)

Examined hybridization in 42 putative WCT samples from Flathead River.

WCT (n=17)

Flathead Lake

North Fork

Middle Fork

Hybridization widespread

Hybridized (n=25)

WCT (n=17)

Flathead Lake

North Fork

Middle Fork

Hybridizationspreading

Hybridized (n=17)

Hybridized post-1985

(n=8)

1,333 sample sites

>20,000 individuals <1% admixture

WCT

Langford Creek

Estimated individual admixture and number of progeny produced over a five year period with 16 microsatellite loci.

Sheltering of deleterious recessive alleles in first-generation hybrids can increase effective rate of gene flow and cause loss of local adaptations.

First-generation hybrids

Female fitness

WCT females produced ~14X progeny than RT

~50% reduction in fitness

Male fitness

Why is hybridization spreading so rapidly if the hybrids have such reduced fitness?

~50% reduction in fitness

… parental taxa will trend toward extinction as introgression proceeds in spite of even a heavy fitness penalty for the hybrids.

Epifanio and Philipp (2001)

“Genomic Ratchet”

• All progeny of hybrid will be hybrids.

• Frequency of hybrids within a population may increase even when most of the hybrid progeny do not survive.

Consider a population of grey duck hybridizing with mallards. Assume that we start with 90% grey and 10% mallards that mate at random, and the fitness of the hybrids is reduced by t.

That is, the fitness of the grey ducks is 1 and the fitness of the hybrids is 1 – t.

(Fitness of the mallards is irrelevant since they are so rare; 1% after panmixia.)

Hybrid fitness = 0.75 (t = 0.25)

1086420

1.00

0.75

0.50

0.25

0.00

Generations

Pro

port

ion

Hyb

rids

t = 0

t = 0.25

t = 0.50

Genomic ratchet

The proportion of hybrids increases rapidly even if they have greatly reduced fitness (25%).

t = reduction in fitness of hybrids

New Zealand grey duck Anas superciliosa

Hybridize with introduced mallard ducks. Few (none?) pure populations remain (Murray Williams, NZ VUW).

New Zealand grey duck is an example of “genomic extinction” (the irretrievable loss by hybridization of genome-wide combination of genotypes that have evolved over long periods of evolutionary time).

What has been lost?

Not the genes; they still are present in the admixed hybrid swarm.

The genome has been lost through admixture with mallard genes.

What is the effect of this?

The grey duck genome (genotype) has been lost.

The grey duck and mallard duck differ at most loci in the genome:

Grey duck X Mallard duck

AABBCC . . . aabbcc . . .

AaBaCc . . .

Consider a random mating hybrid swarm with 50:50 admixture of grey and mallard duck alleles. What is the frequency of the “pure” grey duck genotype?

One locus:

AA 25% grey duck

Aa 50%

aa 25%

Grey duckNo. loci genotype Frequency

1 AA 0.250 2 AABB 0.063 3 AABBCC 0.016 4 AAB . . CDD 0.004 5 AA . . . . EE 0.001 . 10 AA . . . . JJ 9.5 x 10-7

Fitness

Alleles that enhance fitness may reduce fitness in the novel genetic background producedby hybridization (Bateson-Dobzhansky-MullerIncompatibilities).

Loss of the grey duck genotype is expected to bring about the loss of adaptations due to multiple locus interactions.

Hybrids and the US ESA 

• An early series of interpretations concluded that hybrids should NOT receive protection under the ESA. 

• This ‘Hybrid Policy’ was withdrawn in December 1990 because ‘New scientific information concerning genetic introgression has convinced us that the rigid standards set out in those previous opinions should be revisited’.

• A proposed policy on ‘intercrosses’ was published in 1996. This Intercross Policy was scheduled to be finalized in 1997, but still has not been finalized.

♂

♀