Sex allocation theory predicts how plants should allocate resources between male and female functions. Shifts in reproductive success achieved through male vs. female function in hermaphrodites could favor increasing allocation to either gender, eventually promoting the evolution of dioecy. It is generally assumed that insect-pollinated species experience diminishing gains in male reproductive success as allocation to anthers and pollen increases because local pollinators become saturated. In contrast, wind-pollinated plants seem likely to face more continuous increases in male reproductive success with increased investment in pollen. If this is occuring, wind-pollinated species should allocate more to male function than insect-pollinated hermaphrodites. We investigated patterns of resouce allocation and gender in Thalictrum, a genus in which wind pollination has apparently favored a transition to dioecy. The variable breeding systems and pollination syndromes in Thalictrum provide a valuable opportunity to examine these evolutionary transitions while controlling for lineage and general reproductive traits. In the hermaphroditic species, wind-pollinated Thalictrum alpinum indeed allocates proportionally more biomass to male function than insect-pollinated Thalictrum sparsiflorum. In Thalictrum fendleri, which is both wind-pollinated and dioecious, male and female plants have similar reproductive effort except in one population where low seed set limited female effort. Thus, the evolution of wind pollination in Thalictrum appears to favor greater resource allocation to male function which in some instances may favor dioecy.

Key words: biomass, dioecy, hermaphroditism, insect pollination, resource allocation, sex allocation theory, Thalictrum, wind pollination