Pollinator Selectivity and Its Implications for the Evolution of Dioecy and Sexual Dimorphism

Pollinator selectivity is thought to influence the evolution of separate sexes in plants because of its potential to limit plant reproductive success. Selective visitation could also constrain or promote the phenotypic divergence of the sexes. In this study, I explored the causes and consequences of selectivity by generalist pollinators of a gynodioecious wild strawberry (Fragaria virginiana) and thus provide insight into potential pollinator-mediated selection for dioecy and sexual dimorphism. I found that flowers of F. virginiana show pronounced sexual dimorphism in petal length, stamen length, nectar and pollen production, and that this results in dramatic and consistent levels of sex-differential visitation by ants, bees, and flies. I performed manipulations of hermaphrodite flowers to understand the basis of selectivity and found that much of bee and fly preference for hermaphrodite flowers derived from their strong preference for longer petals, but also from a more subtle preference for pollen-filled anthers. These studies also revealed that other traits contribute to the observed discrimination against females. A stronger relationship existed between bee visitation and pollen receipt in females than between bee visitation and pollen removal from hermaphrodites. An analysis of natural variation in petal and stamen length confirmed the central role of petal length and also showed a lack of an effect of vestigial stamen length in pollination success of females. It also revealed a significant effect of stamen length, but not of petal length, on pollen removal. The data suggest that pollinator selectivity may affect the evolution of floral sexual dimorphism, both by exerting selection that could lead to the maintenance of stamens in females and by exerting selection to increase petal length in females.