PREMISE: In communities with multiple co-flowering species, those species that have not evolved an avoidance mechanism may exhibit tolerance to heterospecific pollen (HP) deposition as an adaptive strategy to minimize any deleterious effects of HP transfer, but empirical evidence for the tolerance hypothesis remains scarce.

METHODS: To estimate the potential effects of heterospecific pollen deposition (HPD) on female reproductive success, we counted conspecific (CP) and HP pollen grains deposited on stigmas and assessed subsequent seed sets of both open- and hand-pollinated flowers in three co-flowering Silene species with exposed stigmas that usually received numerous HP grains in nature.

RESULTS: HP percent per flower varied from 16.6% to 43.0% among the three species. Silene chungtienensis experienced lower HP percent and the CP-HP relationship was neutral, S. gracilicaulis and S. yunnanensis experienced relatively higher HP percent and the CP-HP relationship was positive. Positive effects of CP and HP number on natural seed set were observed in all three species, but stronger negative effects of HP percent in S. chungtienensis and S. gracilicaulis. In hand-pollinated flowers of the three Silene species, there was no decrease in seed set with HP whether CP was in excess or insufficient, indicating no negative effects of HPD on seed production.

CONCLUSIONS: Consistent with the tolerance hypothesis, our results indicated that species experiencing higher HP interference are likely to be tolerant to an increase of HP percent. These species with generalist-pollinated flowers may benefit from an increase in conspecific pollen deposition, despite the associated increase in heterospecific pollen deposition.