Abstract: Sexual differences in floral traits are widespread in gynodioecious plants, yet few studies have focused on floral dimorphism at the plant level. Floral display size—defined as the number of flowers blooming simultaneously on a plant—is one of the most important plant traits influencing reproductive success in animal-pollinated angiosperms. We hypothesized that in hermaphrodite plants, a large floral display size enhances pollinator attraction and seed production but incurs a cost of low-quality offspring because of self- pollination, whereas female plants can avoid this dilemma and benefit from a large floral display size. Such sex-specific differences in female reproductive success might explain the sexual dimorphism in floral display size in gynodioecious species. We tested this hypothesis in the gynodioecious species Cyananthus delavayi by investigating the effects of floral display size on pollinator attraction, seed production, and seed quality. Compared with hermaphrodite plants, female plants had larger floral display size, which was attributable to higher intra-plant flowering synchrony. Larger flower displays increased pollinator visitation and seed number per flower in both sexes. In hermaphrodites, floral display size did not significantly affect the germination rate or outcrossing rate; however, seedling survival decreased with floral display size, and compared with selfed seeds, outcrossed seeds exhibited significantly higher survival. Our findings suggest that sex- specific differences in reproductive success may drive the sexual dimorphism in terms of floral display size in gynodioecious species, providing novel insights into the evolution of sexual dimorphism in plants.

Key words: Cyananthus delavayi, Dimorphism, Flowering synchrony, Geitonogamy, Selfing