Divergence of mating systems in Primula oreodoxa: Insights from transplant experiments and mating portfolios

Transitioning from outcrossing to self-fertilization is a widespread reproductive strategy in plants, especially in environments where pollination is limited. Despite its prevalence, this transition has rarely been examined using transplant experiments, and previous studies have overlooked the contribution of the male parent in elucidating mating diversity. In this study, six transplanted populations were generated to investigate the relationship of the pollination environment with plant mating patterns and fecundity in Primula oreodoxa, a species that exhibits both distyly (predominantly outcrossing) and homostyly (predominantly selfing), based on data from 3582 individuals and 11 SSR markers. Homostylous plants had fruit and seed sets comparable to those of distylous plants at lower elevations but exhibited a clear reproductive advantage at higher elevations, particularly compared with the S morph. As elevation increased, the populational selfing rates increased, and the genetic diversity among the progeny was reduced. Furthermore, the visitation frequency of long-tongued pollinators was negatively and positively correlated with the selfing rate and number of mates, respectively, in the L and S morphs. In contrast, short-tongued pollinator visitation showed opposite correlations with the selfing rate and number of mates in homostylous morphs. In most populations, individuals functioned consistently as both female and male, and mating occurred randomly, suggesting a breakdown of the distyly polymorphism. Overall, our results provide experimental validation of the reproductive advantages of homostyly at high elevations by revealing that pollinator visitation shapes the selfing rate and mating diversity within populations, potentially driving the divergence of mating systems along environmental gradients.