In anisogamous organisms, how individuals allocate their resources to the production of male and female gametes (sex allocation) can vary extensively both between species and between individuals within species. This results in a wide diversity of sexual systems ranging from hermaphroditism, where individuals produce male and female gametes, to separate sexes, where individuals specialise in one sex. We study how and why evolutionary transitions between different sexual systems occur using a combination of mathematical modelling and computer simulations. For example, we investigate how these transitions are influenced by the co-evolution of sex allocation with other traits, and how evolution shapes the genetic basis of sexual systems during these transitions. Our most recent work elucidates how the co-evolution of sex allocation with its underlying genetic architecture readily leads to the emergence of heterogametic sex determination (XY/ZW systems) during the transition from hermaphroditism to separate sexes.
The pictures at the top of the page show an example of sex allocation variation in the andromonoecious flowering plant Pulsatilla alpina, which produces hermaphroditic flowers (with both pistils and stamens, left-hand panel) and pure male flowers (stamens only, right-hand panel). This species is perennial, and individuals generally produce a single flower per flowering season. Sex allocation is size-dependent: individuals behave as males when they are small and start producing hermaphroditic flowers once they grow larger, presumably because they then have more resources at their disposable. Credit for these beautiful pictures goes to Kai-Hsiu Chen.