Dynamics of Deleterious Mutations and Purifying Selection in Small Population Isolates.

The genomic consequences of prolonged population decline and isolation are increasingly recognized, but quantitative assessments of mutation loads have been limited by low population-level replication in individual studies. Moreover, how inbreeding and purifying selection shape the genomic landscape of deleterious variation remains poorly understood. We evaluated the abundance and frequency of putative deleterious mutations, characterized the landscape of deleterious variation, and measured the efficacy of purifying selection in 17 wild nine-spined stickleback (Pungitius pungitius) populations covering varying levels of inbreeding (FROH = 0.015 to 0.912) and histories of isolation. We found significantly more deleterious homozygous mutations and a greater frequency of mildly deleterious variants in long-term small, isolated, and inbred populations than in larger outbred populations. Deleterious homozygotes were enriched in runs of homozygosity regions across all study populations, but the extent of enrichment was more pronounced in larger outbred populations than in small inbred populations. Historical effective population sizes serve as an indicator of the strength of purifying selection for mildly deleterious alleles but not for strongly deleterious alleles. The results demonstrate that the accumulation and purging of deleterious variants can occur simultaneously and that a large fraction of segregating strongly deleterious variants are recessive lethals. These findings, which are based on analyses of highly replicated samples of populations, suggest that the level of inbreeding is a good predictor of realized loads of deleterious mutations and that the genomic consequences of prolonged isolation in small populations are predictable.