The Role of Pollinator-Mediated Selection in the Divergence of Floral Traits between Two Closely Related Plant Species

Premise of research. Selection on floral traits by pollinators is expected to play a key role in the diversification of floral phenotypes and the maintenance of species boundaries in sympatry. However, relatively few studies have tested the generality of these assumptions in natural systems, and fewer still have examined multiple phenotypes to better understand which ones are most important to pollinators. Methodology. We created F-2 hybrids between two sympatric species of dioecious Silene to segregate groups of floral characteristics from their associated pollination syndromes. Arrays of hybrid female plants were placed among natural field populations, where we observed native pollinators (diurnal and crepuscular) in order to address which individual traits or groups of traits influenced pollinator visitation, seed set, or predation. Pollinator visits included bees, bee flies, butterflies, and moths, including the mutualist nursery pollinator/seed predator Hadena bicruris. Pivotal results. Regardless of time of day, large flowers and tall plants had higher visitation rates. However, large flowers and tall plants were also more likely to be preyed on by H. bicruris. In all treatments, pollinators showed no preferences for flower color, despite the hypothesized general importance of color for pollinator attraction. Conclusions. Floral phenotypes are the result of both floral preferences by pollinators and antagonistic interactions with floral predators in this system. These findings provide further empirical evidence that floral traits may be important for maintaining species boundaries.