Inbreeding Depression

“You might be a redneck if you think the theory of relativity has something to do with inbreeding”

Inbreeding depression: reduction in fitness of individuals whose parents are related

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.

B = number of LEs per gamete

0

1S

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bab

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F

0 1

0 LEs

1 LE

0.5 LEs

How strong is inbreeding depression?

From 40 captive mammal populations

3.14 LE

0.50.40.30.20.10.0

1.0

0.8

0.6

0.4

0.2

0.0

Inbreeding Coefficient

Rel

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e S

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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

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Control Open – lived

Shaded - died

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Treatment

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Treatment

Mean LE = 12 across entire life history

The cumulative effects of inbreeding depression on different traits can cause large reductions in the fitness of inbred individuals

Reduction in volume, survival probability of trees produced by selfing relative to outcrossing

Inbreeding depression for embryonic survival in Douglas fir.

Purging of deleterious recessive alleles

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

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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

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0.00 0.25 0.50 0.75 1.00Expected fraction of immigrant alleles (S)

An

nual

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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)

Inbreeding Depression and Conservation

• Under what conditions will inbreeding depression have strong effects on population growth?