Format: ORAL

John R. Pannell

Department of Ecology and Evolution University of Lausanne 1015 Lausanne Switzerland

Transitions from hermaphroditism to dioecy have occurred numerous times in the diversification of angiosperms. While the role of inbreeding avoidance in driving these transitions has received substantial attention in both theoretical and empirical studies, we know much less about how transitions might evolve selection for sexual specialisation, despite the fact that sexually dimorphic specialization often accompanies the evolution of dioecy. In this presentation, I present new theoretical results that show how dioecy may evolve from hermaphroditism when there are advantages to sexual specialisation such as those associated with pollen or seed dispersal. The theory also predicts an association between the shape of gain curves, which relate fitness to sex allocation, and the evolution of XY versus ZW sex determination. I illustrate the process modelled with empirical results from a long-term experimental evolution study in which we observed, first, the breakdown of dioecy via loss of a Y chromosome and, subsequently, the early stages of a transition back towards separate sexes via selection on variation at sex-allocation loci unrelated to the original sex chromosomes. The study thus draws a theoretical link between sex-allocation and sex-chromosome evolution and draws on the remarkably rapid evolution of sex allocation under experimental conditions that shows how such a link may be manifest.