Floral Biology of Gender Monomorphism and Dimorphism in Wurmbea dioica (Colchicaceae) in Western Australia

We compared the floral biology of female, male, and cosexual plants in sympatric populations of Wurmbea dioica (Colchicaceae) from Western Australia to assess evidence that changes in pollination biology accompanied a transition between monomorphic to dimorphic sexual systems in this species. In W. dioica, sex‐specific differences in floral design and display, the quantity and quality of rewards, and reproductive phenology represent alternative strategies for pollinator attraction and the avoidance of self‐pollination. Unisexual plants had smaller flowers containing less pollen than flowers of cosexuals; males and cosexuals produced nectar with higher sugar content than did females. These patterns were associated with differences in the composition and visitation rates of pollinators. Nectar‐foraging flies visited all three sexes, but pollen‐collecting bees visited only cosexuals. Fly‐pollinated females achieved greater seed fertility relative to their bee‐pollinated cosexual relatives, and males and cosexuals received longer and more frequent visits than did females. Although there were no differences in flower number among sexes, females maintained larger displays throughout flowering. In contrast, males and cosexuals staggered both floral anthesis and anther dehiscence as mechanisms for promoting enhanced pollen dispersal. We propose that sex‐based differences in floral design and display in W. dioica have mediated a shift in the effectiveness of nectar‐foraging insects as pollinators and that gender dimorphism enforces outcrossing under pollination conditions that would otherwise cause selfing. Differences in pollination biology probably also contribute to the maintenance of combined versus separate sexes in sympatric populations of W. dioica in Western Australia.