Mating pattern of a distylous primrose in a natural population: unilateral outcrossing and asymmetric selfing between sexual morphs

Abstract

Heterostyly is a genetic polymorphism of anther and stigma heights, which are reciprocally placed among morphs. The maintenance of heterostyly has often been ascribed to disassortative mating, but alternative explanations are also possible, which can be examined by analyzing mating patterns. However, parental analysis with genetic markers has only been applied to a limited number of taxa, despite the phylogenetic diversity of heterostylous species. Here, we examined the mating patterns of a distylous primrose, Primula kisoana (Primulaceae), in its natural habitat, using 11 microsatellite markers. We also examined other fitness components. We found no differences in seed production, vegetative growth, or survival between morphs. However, the siring success of thrum fathers was much higher than that of pin fathers; outcrossing occurred unilaterally between thrum fathers and pin mothers, and selfing was more frequent in thrum plants than in pin plants. These findings are consistent with the explanation that distyly in this species is maintained either by overdominance due to previously reported recessive lethal effects of the thrum allele or by effectively dioecious mating if inbreeding depression is strong. We built a mathematical model of the overdominance hypothesis to confirm that it is logically sound. These results warrant further investigations into the possible roles of overdominance and dioecy in the maintenance of heterostyly.