| The association of wind pollination and dioecy occurs in many plant lineages, including the genus Thalictrum. Here, I explore the role of wind pollination in promoting the evolution of dioecy in four species of Thalictrum that vary in pollination syndrome and breeding system. None of the study species employ reproductive strategies that rely entirely on insect pollinators, suggesting that poor pollinator service in an insect-pollinated ancestor may have promoted the evolution of wind pollination. Thalictrum species have a labile floral form and lack nectar as a pollinator reward, characters that may predispose species to evolve wind pollination.; Sex allocation theory predicts that unisexual morphs can invade a population and lead to the evolution of dioecy when the relationship between investment in a sex function and reproductive success through that function is positive and accelerating. The evolution of wind pollination may favor greater investment in male function, which consequently favors the evolution of dioecy. The wind-pollinated species we studied had a higher male biomass cost per flower and greater allocation to male function overall than insect-pollinated and self-compatible T. sparsiflorum. In addition, the wind-pollinated species require this higher level of male investment to achieve pollination success. Seed set in the two dioecious, wind-pollinated species (T. fendleri and T. dioicum) decreased when plant density was low, while seed set in T. sparsiflorum was density-independent and consistently high. These results suggest that the evolution of wind pollination does in fact result in selection pressure for greater male allocation, which may have consequently favored the evolution of dioecy in Thalictrum.; Inbreeding depression in a hermaphroditic population may also promote the evolution of dioecy as an outcrossing mechanism. The increase in pollen production accompanying the evolution of wind pollination may have increased the selfing rate and consequent levels of inbreeding depression. Selfed T. sparsiflorum seeds showed a decreased sensitivity to germination cues, longer germination times, and fewer leaves after 15 weeks of growth than outcrossed seeds. Thus, both inbreeding depression and changes in resource allocation following the evolution of wind pollination may have worked in conjunction to promote the evolution of dioecy in Thalictrum. |
