Evolutionary Transitions Between Hermaphroditism and Dioecy in Animals and Plants

Abstract

We review transitions between hermaphroditism and dioecy in animals and (mainly flowering) plants. Although hermaphroditism and dioecy represent two end states in a sex-allocation continuum, both vary in major ways among clades. However, drawing on sex-allocation theory and distinguishing between self-fertilization and outcrossing, we recognize five broad paths to dioecy and two broad paths to hermaphroditism. Which path is taken likely depends on the starting state (especially in terms of the mating system), as well as the ecological setting or genetic context of the transition. Androdioecy may have been more important in some transitions to dioecy than current theory would suggest, and gynodioecy may often be an evolutionary end point in itself rather than a step between hermaphroditism and dioecy. Transitions between environmental and genetic sex determination may also play an important role in sexual-system evolution. Further theory is required to address these possibilities. Detailed empirical work is also greatly needed, especially in animal clades that vary in their sexual system.