Pollen performance and sex-ratio evolution in a dioecious plant

There has been a proliferation of studies, in a variety of taxa, that have detected sex-linked or cytoplasmic genes that enhance their own transmission via sex-ratio distortion. One of the most important parameters influencing the dynamics of these elements is the magnitude of their transmission advantage. In many systems, the mechanism of sex-ratio distortion is to abort X- or Y-bearing gametes. With this mechanism, the transmission advantage associated with sex-ratio distortion is diminished when the production of male gametes limits offspring production or when competition among the gametes of different males is intense. In this study, we analyzed the outcome of pollen competition between males that produced different sex ratios in the dioecious plant, Silene alba, and estimated how the sex-ratio bias influenced the transmission properties of the sex chromosomes. We varied the intensity of pollen competition by controlling the quantity of pollen used in crosses and used a combination of single-male pollinations and pollen mixtures to evaluate the effects of multiple paternity. Paternity in pollen mixtures was estimated using allozymes. Sex-ratio bias was directly influenced by the quantity of pollen, but the magnitude of this effect was small. The relative performance of pollen from different males varied substantially, especially when there was multiple paternity. Specifically, males with biased sex ratios sired far fewer offspring of either sex in pollen mixtures. In crosses involving single males, however, these "sex-ratio" males produced the same number of offspring as other males, so the female bias caused a significant transmission advantage for X-linked genes. X-linked genes could enhance their transmission via sex-ratio distortion in Silene populations, but the magnitude of this transmission advantage will depend on the ecological circumstances that influence the opportunity for multiple paternity.