schedule brief meeting tomorrow morning to discuss paper?
TRANSCRIPT
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.
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