Pollen Competition as a Target for Sexual Selection in Plants

Abstract: According to the theory of sexual selection, traits can evolve because they confer a mating advantage in competition with members from the same sex. In animals, sexual selection is considered as an important evolutionary force. In this thesis I have investigated the potential for sexual selection in plants. I have concentrated on the phase of reproduction that occurs after pollinators have deposited pollen on the stigma. At this stage, pollen competition can take place between individual pollen donors with the goal to fertilise a high proportion of the ovules. In greenhouse studies of Viola tricolor, pollen tube growth rate of donors was important for siring ability in competitive situations. Other pollen traits, such as pollen germination ability, pollen grain size and the ability to inhibit pollen from other donors was of less significance. Pollen tube growth rate was further heritable, which indicates that this trait can respond to selection. Genotype by environment interactions influenced pollen tube growth rate and sporophytic traits in V. tricolor. A theoretical model where both life phases were affected by genotype by environment interactions, showed that a balance between diversifying selection and pollen flow can maintain variation in pollen competitive ability despite strong selection. Pollen tube growth rate may not only be of importance for male reproductive success, but could also function as a cue for female choice. In V. tricolor, pollen donors with a high pollen tube growth rate showed a superior sporophytic quality, and sired offspring with enhanced fitness. Female traits that increase the probability to be fertilised by highly competitive pollen (e.g. a long pistil) may thus be selected. A theoretical analysis of both male and female interests during pollen competition indicated that pollen tube growth rate and pistil length can co-evolve in response to each other, though the response of both traits is not always positively correlated.