Transcript Document

Schedule
Brief meeting tomorrow morning to discuss paper?
Rich Fredrickson
Sally Aitken
University of British Columbia
Chapter 21: Climate Change
Rich Fredrickson
Gordon Luikart
Mexican wolves
Bighorn sheep
Final Exam Period Schedule
Addis, Brett
Ausband, David
Brekke, Tom
Cross, Todd
Erickson-Davis, Morgan
Fannin, Barb
Honeycutt, Ken
Roffler, Gretchen
Tempa, Tshering
TODAY
Turn-in Paper
Chapter 16: Units of Conservation (continued)
ESA & Distinct population segments (DPS)
Chapter 17: Hybrids
Hybrid fitness
Natural hybridization
Anthropogenic hybridization
Hybrids & the ESA
Genetic 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 Conservation
Complex 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
Intrinsic
or
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
Intrinsic
or
Extrinsic?
hybrids
parental
parental
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.
gallop
whitetail
stot
mule deer
stumble
F1 hybrid
Video
Thank you Susan Lingle!
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.
Some gene flow between species with
small Ne is essential for long-term
viability of species.
Humans
Nature Rev Genetics 12:603-614 (2011)
May 2010
YES!
An estimated 3% of the
human genome resulted
from hybridization between
modern Europeans and
Neandertals some 50,000
years ago
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
Westslope cutthroat trout
(WCT)
Oncorhynchus clarkii lewisi
Yellowstone cutthroat trout
(YCT)
O. c. bouvieri
Rainbow trout
(RT)
O. mykiss
(Illustrations by Joseph R. Tomelleri)
Range Maps. RT ranges
north and south beyond
map area.
15
RT
WCT
N
YCT
WCT and YCT are
extremely genetically
divergent. They are fixed
for different alleles at 10
out of 52 allozyme loci.
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.
Hybrid index (2 x 8 diagnostic loci)
Frequency
3
15 individuals
2
1
0
0
2
4
6
8
10
12
14
16
WCT genes
Pure YCT
Pure WCT
Examined hybridization in 42 putative
WCT samples from Flathead River.
North Fork
Hybridization
widespread
WCT (n=17)
Hybridized (n=25)
Middle Fork
Flathead Lake
North Fork
Hybridization
spreading
WCT (n=17)
Hybridized (n=17)
Hybridized post-1985
(n=8)
Middle Fork
Flathead Lake
<1%
admixture
WCT
1,333 sample sites
>20,000 individuals
Estimated individual
admixture and number of
progeny produced over a
five year period with 16
microsatellite loci.
Langford Creek
First-generation hybrids
Sheltering of deleterious recessive alleles in
first-generation hybrids can increase effective rate
of gene flow and cause loss of local adaptations.
Female fitness
~50% reduction in fitness
WCT females produced ~14X progeny than RT
Male fitness
~50% reduction in fitness
Why is hybridization spreading so rapidly if
the hybrids have such reduced fitness?
“Genomic Ratchet”
… parental taxa will trend toward extinction as
introgression proceeds in spite of even a heavy fitness
penalty for the hybrids.
Epifanio and Philipp (2001)
• 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)
Genomic ratchet
1.00
Proportion Hybrids
t=0
0.75
t = reduction
in fitness of
hybrids
t = 0.25
0.50
t = 0.50
0.25
0.00
0
2
4
6
8
10
Generations
The proportion of hybrids increases rapidly even
if they have greatly reduced fitness (25%).
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
AABBCC . . .
Mallard duck
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%
Aa
50%
aa
25%
grey duck
No. loci
1
2
3
4
5
.
10
Grey duck
genotype
AA
AABB
AABBCC
AAB . . CDD
AA . . . . EE
AA . . . . JJ
Frequency
0.250
0.063
0.016
0.004
0.001
9.5 x 10-7
Fitness
Alleles that enhance fitness may reduce fitness in
the novel genetic background produced
by hybridization (Bateson-Dobzhansky-Muller
Incompatibilities).
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.
♂
♀