Ancestral inbreeding reduces the magnitude of inbreeding depression in Drosophila melanogaster.

The influence of natural selection on the magnitude of inbreeding depression is an important issue in conservation biology and the study of evolution. It is generally expected that the magnitude of inbreeding depression in small populations will depend upon the average homozygosity of individuals, as measured by the coefficient of inbreeding (F). However, if deleterious recessive alleles are selectively purged from populations during inbreeding, then inbreeding depression may differ among populations in which individuals have the same inbreeding coefficient. In such cases, the magnitude of inbreeding depression will partly depend on the ancestral inbreeding coefficient (fa), which measures the cumulative proportion of loci that have historically been homozygous and therefore exposed to natural selection. We examined the inbreeding depression that occurred in lineages of Drosophila melanogaster maintained under pedigrees that led to the same inbreeding coefficient (F = 0.375) but different levels of ancestral inbreeding (fa = 0.250 or 0.531). Although inbreeding depression varied substantially among individual lineages, we observed a significant 40% decrease in the median level of inbreeding depression in the treatment with higher ancestral inbreeding. Our results demonstrate that high levels of ancestral inbreeding are associated with greater purging effects, which reduces the inbreeding depression that occurs in isolated populations of small size.