Evolution of a Flower Dimorphism: How Effective is Pollen Dispersal by "Male" Flowers?

Abstract

Many plants produce more flowers than will set fruit. In some species, a subset of flowers are actually female—sterile and morphologically distinct; these flowers are presumed to enhance fitness primarily through pollen donation. Individual plants of the hummingbird—pollinated shrub Besleria triflora produce both long—styled and short—styled flower morphs, characteristic of these "andromonoecious" sexual systems. Short—styled flowers set fruit rarely when hand—pollinated and are therefore functionally staminate. To test the hypothesis that selection for increased male function maintains flower dimorphism, I measured relative pollen dispersal per unit investment by staminate vs. perfect (long—styled) flowers. In the field, anthers of perfect flowers dispersed pollen for a significantly longer time than did those of staminate flowers. In laboratory experiments, hummingbirds transferred more pollen per visit from perfect flowers than from staminate flowers, in part due to differences in flower—morph pollen production. As a result, perfect flowers dispersed substantially more pollen than staminate flowers over an average flower lifetime. The two flower morphs did not differ in the amount of biomass invested through the time of corolla abscission, suggesting that staminate flowers do not conserve resources substantially. These results fail to support the "male—function" hypothesis for the evolution of andromonoecy. Staminate flowers contribute relatively little to plant reproductive success directly through male function, suggesting they are maintained for reproductive functions other than pollen donation.