XVI International Botanical Congess

Most studies of the evolution of dioecy in angiosperms have focussed on the transition from hermaphroditism to dioecy via gynodioecy. However, dioecy commonly occurs in families containing monoecious species, thus dioecy may frequently evolve via this pathway. We studied factors associated with the transition from monoecy to dioecy in S. latifolia. Gender expression varied considerably among 41 populations surveyed, ranging from monoecy to dioecy, with some populations containing both unisexual and hermaphrodite plants. Monoecious populations were frequently located in disturbed aquatic habitats and contained smaller plants than dioecious populations, which were restricted to large, undisturbed wetlands. A mating system analysis revealed high, yet variable, rates of selfing in monoecious populations. These results highlight the importance of ecological factors in the transition from monoecy to dioecy.