Inbreeding Depression “You might be a redneck if you think the theory of relativity has something...
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Transcript of Inbreeding Depression “You might be a redneck if you think the theory of relativity has something...
Inbreeding Depression
“You might be a redneck if you think the theory of relativity has something to do with inbreeding”
Causes of inbreeding depressionTwo main hypotheses:
(1) Increased homozygosity for deleterious recessive alleles
Deleterious recessive alleles are thought to be responsible for the majority of inbreeding depression
(2) Decreased heterozygosity at loci with heterozygous advantage
Even moderate amounts of inbreeding (say mean F=0.10) within populations can greatly increase the frequency of homozygotes for rare alleles.
p = 0.9, q = 0.1
How do we measure inbreeding depression?
Lethal Equivalents (LEs)
A lethal equivalent is a set of deleterious alleles that would cause death if homozygous.
Thus, one lethal equivalent may either be a single allele that is lethal when homozygous, two alleles each with a probability of 0.5 of causing death when homozygous, or 10 alleles each with a probability of 0.10 of causing death when homozygous.
What is the probability that E is not aa or bb?
One LE:
a and b are lethal alleles
The probability of E being homozygous aa is (1/2)4 = 1/16; similarly, there is a 1/16 probability that individual E will be homozygous bb. Thus, the probability of E not being homozygous for a recessive allele at either of these two loci is (15/16) (15/16) = 0.879.
Thus, one LE per diploid genome will result in approximately a 12% reduction (1 - 0.879) in survival of individuals with an F of 0.25.
0.50.40.30.20.10.0
1.0
0.8
0.6
0.4
0.2
0.0
Inbreeding Coefficient
Rel
ativ
e S
urvi
val
Mean reduction in relative juvenile survival to six months of age in 40 captive mammal populations (Ralls et al. 1988).
brother X sister
-33%LE = 3.14
Environmental effects on inbreeding depression: Measures of inbreeding depression in captivity are likely to be underestimates.
Inbreeding depression in Mus in semi-natural enclosures
Males
FemalesF= 0F = 0.25
4X lab estimates
8X
Inbred steelhead (F = 0.25) had much lower marine survival than non-inbed steelhead (F = 0.0)
No consistent pattern of inbreeding depression in captivity
15
Soay sheep overwinter mortality: Parasitism increases inbreeding depression
Coltman et al., 1999 Evolution
Control TreatmentOpen – lived
Shaded - died
Sta
ndar
diz
ed M
ult
i-lo
cus
Het
eroz
ygos
ity
Control Open – lived
Shaded - died
Sta
ndar
diz
ed M
ult
i-lo
cus
Het
eroz
ygos
ity
Treatment
Sta
ndar
diz
ed M
ult
i-lo
cus
Het
eroz
ygos
ity
Treatment
The cumulative effects of inbreeding depression on different traits can cause large reductions in the fitness of inbred individuals
Purging of deleterious recessive alleles
Should captive rearing programs purposefullybottleneck populations in order to eliminate deleterious recessives?
Purging could be effective when:
• Inbreeding depression is caused mostly by deleterious recessive alleles
• the effect of most deleterious alleles is large... s > 1/2Ne
• Inbreeding accumulates slowly – if Ne is small (inbreeding accumulates quickly) drift is likely to overwhelm selection against deleterious recessives
• Purged deleterious alleles are not reintroduced
“ancestral inbreeding shows a highly significanttrend toward reducing inbreeding depression,but the effect within any one species, in general, is weak”… PURGING WAS INEFFECTIVE!!
Inbreeding depression was slightly weaker for individuals with inbred ancestors in 15/17 species
Purging of deleterious recessive alleles
Should captive rearing programs purposefullyBottleneck populations in order to eliminate deleterious recessives?
NO!
Among population inbreeding depression
Su
rviv
al/r
epro
du
ctio
n
F
B = 0
If the genetic load is fixed/nearly fixed, or if F is high but has very low variance, the relationship between fitness and F will be artificially weak
Among population inbreeding depression
In such cases, ‘population level’ inbreeding depression can be detected by comparing the fitness of resident offspring and those from matings between individuals from separate populations…
Genetic rescue experiments
What if we fail to detect inbreeding depression by correlating fitness to F?
An
nual
rep
rodu
ctiv
e su
cces
s
0.00 0.25 0.50 0.75 1.00Expected fraction of immigrant alleles (S)
An
nual
rep
rodu
ctiv
e su
cces
s
0.00 0.25 0.50 0.75 1.00Expected fraction of immigrant alleles (S)
Outbred individuals had increased survival & reproductive success
Females: % Increase
Annual rep success (lambs weaned) + 217%
Annual survival + 28%
Males:
Annual rep success (lambs to weaning) + 257%
Annual survival + 27%
(estimates controlled for age & year/environment)
Hogg et al. 2006 Proc. Royal Soc. B
46 birdsMillion of birds 2000 birds
The strength of inbreeding depression varies:
• Temporally– Variation in exposure to stressors
• Among populations– Systematic environmental variation– Chance differences in genetic load
• Among individuals– the distribution of deleterious recessives is often not
even across individuals (Douglas fir example, founder specific inbreeding coefficients)