Causes of inbreeding depression Two 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.
Should captive rearing programs purposefully bottleneck 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 > 1/2N e Inbreeding accumulates slowly – if N e 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 significant trend 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 purposefully Bottleneck populations in order to eliminate deleterious recessives? NO!
Among population inbreeding depression Survival/reproduction 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?
Outbred individuals had increased survival & reproductive success Females: % Increase Annual rep success (lambs weaned) + 217% Annual survival + 28% Source index Source index: -1.0 = pure Bison Range 1.0 = pure immigrant 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
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?