PARENT SESSION
Poster Session #46: Seed Production, Recruitment, and Pollination.
Friday, August 10, 2001. Presentation from 10:30 AM to 12:00 PM. Exhibition Hall

Plant density and reproductive success in two wind pollinated species of Thalictrum (Ranunculaceae).

Steven, Janet¹, Waller, Donald¹, ¹

ABSTRACT- Flowering plants exhibit a wide diversity of breeding systems and pollination syndromes. To account for this diversity, we must understand how evolutionary transitions between systems occur and the population-level selection pressures that drive them. We are examining shifts from hermaphroditism to dioecy and insect to wind pollination in the genus Thalictrum. We used a molecular phylogeny to identify sister taxa with these contrasting breeding systems and are now using field data to characterize the relative efficiency of the two modes of pollination. The effectiveness of both pollination modes depends on the distance between plants, but wind pollinated plants appear more likely to suffer reductions in seed set with increasing distance between mates. In dioecious and wind pollinated T. fendleri, distance to nearest male neighbor did not affect seed set, but seed set increased with local male density. Similarly, distance to nearest male neighbor did not affect seed set in a population of T. dioicum. However, when pollen density was decreased by experimentally clipping male flowers before anthesis in parts of another population, seed set increased with increasing distance to nearest male neighbor. We are currently investigating biparental inbreeding depression as a cause for this pattern. We are also extending this approach to a closely related hermaphroditic species and examining patterns of resource allocation to further analyze the selective forces driving transitions to wind pollination and dioecy.

KEY WORDS: Thalictrum dioicum, Thalictrum fendleri, wind pollination, dioecy