Determinants of mutation load in birds

Mutations contribute substantially to mortality and disease. The mutation load in a population depends on the efficacy of purifying selection in removing deleterious genetic variation. Here, we estimated the proportion of deleterious mutations segregating in population samples of 19 bird species. Exploiting the conserved avian karyotype with high variation in recombination rate and GC content, we quantified the joint effects of effective population size ( N _e ), recombination and GC-biased gene-conversion (gBGC). In agreement with the nearly-neutral theory of molecular evolution, mutation load was substantially elevated in populations with small N _e . Purging efficacy increased with recombination rate resulting in more than a two-fold difference of genetic load between large and small chromosomes. GC-biased mutations contributed about one third to the pool of deleterious mutations. Their expected accumulation in regions of high recombination was offset by purging efficacy in large, but not small populations. This study suggests that genetic risk factors for mutational meltdown in small populations are fueled by gBGC and cluster in regions of low recombination.