| Silene species were used to explore the evolution of reproductive isolation and the influence of pollen competition on inbreeding depression. These topics are united by the role of pollen-tube dynamics in both processes. If reproductive isolating barriers exist, closely related species may remain distinct despite regions of sympatry. This process can involve reinforcement, whereby prezygotic reproductive isolating barriers are stronger in sympatry than allopatry. I tested for reinforcement between two closely related species by performing hybrid crosses with multiple allopatric and one sympatric population. Results indicated that seed set following crosses between Silene latifolia mothers from sympatry and S. diclinis fathers was impeded by the inability of S. diclinis pollen to fertilize ovules in the long-styled flowers of sympatric S. latifolia females. However, reinforcement was rejected as the mechanism responsible, as S. latifolia from large-flowered populations in allopatry also exhibited reduced seed set when pollinated with S. diclinis pollen. Although the reciprocal cross did not exhibit this prezygotic barrier, seed germination from such crosses was lower when the sire was from the sympatric population, indicating postzygotic isolation likely also contributes to the lack of natural hybrids. Silene vulgaris was used to test the hypothesis that competition among pollen results in more fit offspring. Pollen competition may be finely manipulated in this species by placement of pollen at the tip or base of the receptive styles. Moreover, haploid pollen genomes are expressed during pollen-tube growth, allowing the expression of deleterious recessive alleles. Hence, pollen competition might decrease inbreeding depression. To test these hypotheses, I manipulated pollen competition via tip vs. base pollination using self vs. outcross pollen, utilizing a novel crossing scheme to assess inbreeding depression. I found no effect of pollen competition on traits associated with offspring vigor, demanding reevaluation of these hypotheses. |
