Evolution of sexual systems

Thomas Lesaffre
Evolution of sexual systems

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, resulting 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. A central topic in the study of sexual systems is to understand how and why evolutionary transitions between different systems occur. Here, we use a combination of mathematical modelling and computer simulations to 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. For example, our most recent work elucidates how the co-evolution of sex allocation with its underlying genetic architecture readily leads to the emergence of sex chromosomes 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.


  • Lesaffre, T., Pannell, J.R., Mullon, C. A model for the gradual evolution of dioecy and heterogametic sex determination. BioRxiv (preprint). 2023.