Pollen competition as an asymmetric reproductive barrier between two closely related Silene species

Reproductive barriers are important determinants of gene flow between divergent populations or species. We studied pollen competition as a post-mating reproductive barrier between Silene dioica and S. latifolia. Gene flow between these species is extensive, but early-generation hybrids are rare. In an experiment with conspecific, heterospecific and 50 : 50 mixed pollinations in the two species, pollination treatments did not significantly affect seed set and seed weight. However, molecular determination of siring success after mixed pollinations showed that fewer than expected hybrids were produced in S. latifolia (18% hybrids) but not in S. dioica (51% hybrids). This constitutes an asymmetric post-mating reproductive barrier and likely contributes to the rarity of early-generation hybrids. Our study shows that pollen competition can be an effective barrier to hybridization between closely related species that likely acts in concert with other reproductive barriers.