Individual variation, gametophytic competition and style length: Does size affect paternity?

Post-pollination competition among pollen grains for the fertilization of ovules can potentially filter out inferior male gametophytes from the reproductive process. It has been assumed that, in relatively long styles, competition among genetically diverse pollen grains will be more intense than in relatively short styles. However, very few empirical studies have examined the influence of style length on differences among pollen donors in siring success. We developed a model of pollen tube growth based on measures of in vivo pollen performance and style length of an annual herb (Clarkia unguiculata) to examine the relationship between style length and the percentage of seeds sired by two competing donors with differing pollen performance parameters. We found that there is no increase in the siring success of a superior donor as style length increases under a range of conditions. Moreover, there was a positive relationship between style length and style filtering ability only in styles shorter than those found in wild populations of C. unguiculata, suggesting that variation in style length has little influence on filtering ability in natural populations of this species. We conclude that both male and female reproductive success can be influenced by style length, but under a restricted set of conditions.