CONDITIONS FAVORING STABLE MIXED MATING SYSTEMS WITH JOINTLY EVOLVINGINBREEDING DEPRESSION

Theory suggests that mixed mating systems can be evolutionarily stable if fitness declines monotonically with consecutive generations of sel f-fertilization. However, most models which assume inbreeding depressi on due to recessive deleterious mutations predict that inbreeding depr ession will be purged upon selfing, and thus will not show a monotonic decline in fitness. Fitness changes are tracked over consecutive gene rations 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 l ineages. This is because they are purged more slowly than the increase in homozygosity with each generation of continued selfing. The fitnes s 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 pre dict the stable selfing rate. In several cases, the predicted stable s elfing rate matches that used to calculate the fitness function, indic ating that mixed mating is a stable outcome of the joint evolution of the mating system and inbreeding depression. Mixed mating is most like ly 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 depressi on which relate the selfing rate to the changes in fitness with selfin g.