XVI International Botanical Congess

A molecular phylogeny of Thalictrum species was reconstructed using the nuclear ribosomal DNA ITS region and the chloroplast DNA rpl16 intron, and four outgroups. There was high bootstrap support for most clades. Thalictrum species vary in their breeding system and in their pollination mechanisms within a breeding system. Ecological correlates of dioecy such as woodiness, fleshy fruit, island habitat and wet or dry habitats, either do not occur in the genus or are not associated with dioecy. The evolution of dioecy was examined using the methods of character optimization in MacClade. Two separate origins of dioecy were detected in North America, and no reversal back to hermaphroditism was observed. In at least one clade wind-pollination preceded the evolution of dioecy, suggesting that wind-pollination may represent a causal mechanism in the evolution of dioecy in this group.