Dioecy in a wind-pollinated herb explained by disruptive selection on sex allocation

The evolution of dioecy from hermaphroditism is widely thought to be a response to disruptive selection favouring males and females, driven by advantages of inbreeding avoidance, sexual specialisation, or both. However, it has hitherto been difficult to uncouple the importance of these two advantages of dioecy. Here, we estimate the selfing rate and fitness gain curves with and without inbreeding depression in wind-pollinated Mercurialis annua, based on variation in sex allocation among hermaphrodites that evolved from dioecy following the experimental removal of males. Because males had been removed, we could rule out any important contribution to disruptive selection due to advantages of sexual specialisation, allowing us to focus in inbreeding effects. Our results confirm a tradeoff between male and female allocation in M. annua, as assumed in sex-allocation theory. The individual selfing rates increased with male allocation, greatly altering the female and male fitness gain curves under strong inbreeding depression. There was strong disruptive selection on sex allocation due to its interaction with the mating system. Taken together, our study demonstrates that inbreeding avoidance on its own can lead to disruptive selection on sex allocation and thus the selection and maintenance of dioecy under wind pollination.