Conditions Favoring Stable Mixed Mating Systems with Jointly Evolving Inbreeding Depression

Abstract

Theory suggests that mixed mating systems can be evolutionarily stable if fitness declines monotonically with consecutive generations of self-fertilization. However, most models which assume inbreeding depression due to recessive deleterious mutations predict that inbreeding depression will be purged upon selfing, and thus will not show a monotonic decline in fitness. Fitness changes are tracked over consecutive generations of self-fertilization in a model which permits purging of load to occur. The results suggest that very mildly deleterious mutations (S = 0·01) approximate a monotonic decline in fitness within selfing lineages. This is because they are purged more slowly than the increase in homozygosity with each generation of continued selfing. The fitness functions expected at several selfing rates for mutations of small, moderate and severe effect are then calculated, assuming that purging within selfing lineages is negligible, and using this function, we predict the stable selfing rate. In several cases, the predicted stable selfing rate matches that used to calculate the fitness function, indicating that mixed mating is a stable outcome of the joint evolution of the mating system and inbreeding depression. Mixed mating is most likely when inbreeding depression is caused by partly recessive (h = 0·2-0·35) very mildly deleterious mutations with per genome mutation rates between 1·0 and 2·0. Empirical studies of mutation suggest that these parameters are not uncommon. We discuss studies of inbreeding depression which relate the selfing rate to the changes in fitness with selfing.