HIERARCHICAL ANALYSIS OF INBREEDING DEPRESSION IN PEROMYSCUS POLIONOTUS

Abstract

Abstract.-The severity of inbreeding depression appears to vary among taxa, but few ecological or other patterns have been identified that predict accurately which taxa are most sensitive to inbreeding. To examine the causes of heterogeneity in inbreeding depression, the effects of inbreeding on reproduction, survival, and growth were measured in three replicate experimental stocks for each of three subspecies of Peromyscus polionotus mice. Inbreeding of the dam reduced the probability of breeding, the probability of producing a second litter, and litter size. Inbreeding of the litter caused depression of litter size, juvenile viability, and mass at weaning, and caused an increase in the withinlitter variance in mass. In spite of differences between the subspecies in natural population sizes, genetic variation, and mean rates of reproduction and survival, all variation observed between experimental populations in their responses to inbreeding could be attributed to random founder effects. The genetic load of deleterious alleles in each replicate was unequally partitioned among its founder pairs, and different founders contributed to the load affecting different fitness components. Thus, inbreeding depression for anyone fitness component, in our experimental environment, must be due to relatively few deleterious alleles with major effects. Genetic loads so comprised would be expected to diverge among natural populations due to both random drift and selective removal of recessive deleterious alleles during population bottlenecks. The near universality of inbreeding depression would be maintained, however, if different alleles contribute to inbreeding depression of different fitness components and in different environments.

Conservation genetics of population bottlenecks: the role of chance, selection, and history

“…This is apparent when estimating the effects of inbreeding on male and female fitness (e.g., female fecundity and male fertility; Fry et al 1998). However, it is not uncommon to find that inbreeding may have drastic effects on particular fitness measures while no significant effects on others (Lacy et al 1996). As a general rule, there seems to be little correlation on the effects of inbreeding among different fitness trait components (Kuke Bijlsma, personal communication).…”

“…Under the detrimental paradigm of inbreeding depression (upper pathway), arrows presuppose a consistent an unambiguous relationship between population size, genetic diversity, fitness and extinction risks fitness. Classic studies by Pray and Goodnight (1995) and Wade et al (1996) on Tribolium castaneum, Lacy et al (1996) on Peromyscus polionotus, and Holtsford (1996) on Clarkia have shown population-specific effects of bottlenecks on inbreeding depression. That is, while some populations showed signs of decreased genetic diversity and inbreeding depression, others revealed no declines or even increases in genetic variation and fitness following bottlenecks.…”

Fine decomposition of the inbreeding and the coancestry coefficients by using the tabular method

“…Note that d includes both the Mendelian sampling variability and the ignorance about the knowledge of the parents. Lacy et al (1996) and Lacy (1997) described how to decompose an inbreeding coefficient in several parts attributable to each population founder. The method presented here makes use of the same procedure, allowing a complete decomposition of the inbreeding coefficients or the coancestries, including the effects of the non-founder animals.…”

“…Finally, Lacy et al (1996) suggested a different partition for both inbreeding and coancestries as partial components due to each founder. The sum, across all founders, of the partial inbreeding coefficients for an individual is equal to the overall inbreeding for that individual.…”

“…We will compare the results against the nodal decomposition (Wright 1922) and the founders based decomposition (Lacy et al 1996;Lacy 1997). In this paper, two small pedigrees will be used to illustrate the procedure, including the pedigree of the bull Comet.…”