Deleterious mutations and selection for sex in spatially structured, diploid populations

By increasing local genetic drift and generating inbreeding, population spatial structure may have important effects on the evolutionary benefits of sexual reproduction. In this article, we consider a population structured according to the island model, and use two and three-locus analytical models and multilocus simulations to explore the selective forces acting on a modifier locus affecting the rate of sexual reproduction of facultatively sexual organisms, in the presence of recurrent deleterious mutations. The results show that population structure and selection combine to generate a local excess of heterozygotes at selected loci (negative FIS), for both partially recessive and partially dominant deleterious alleles. The linkage disequilibrium between deleterious alleles may be either negative or positive depending on their dominance coefficient, the degree of population structure and the rate of sex. These genetic associations combine with many other ones to generate indirect selection at the sex modifier locus, generally favoring intermediate rates of sex even when sex entails direct fitness costs. Multilocus simulations show that the equilibrium rate of sex increases moderately as the degree of population structure increases. However, population structure may also prevent the irreversible spread of asexual mutants when the cost of sex is strong.