The evolutionary interplay between self-fertilisation and sex allocation in flowering plants

Abstract

Flowering plants vary in sex allocation (resource division between male and female function) and in selfing rate. These traits should evolve together: selfing changes fitness returns from pollen and ovules, whereas sex allocation changes the pollen available for selfing and export. Yet most models allow only one trait to evolve, leaving their evolutionary feedback unresolved. Here we model the joint evolution of sex allocation, pollen export and maternal control of competition between self- and cross-pollen. We show that selfing and sex allocation strongly influence one another, as selection favours associations between sex allocation and mating-system traits. When maternal control is fixed to recover classical mass-action, evolution can lead to the stable coexistence of selfing hermaphrodites and males specialised in pollen export. This androdioecy persists because hermaphrodites cannot prevent fertilisation by males. When maternal control of pollen competition also evolves, we find that where a mixed-mating equilibrium exists, selection again favours the emergence of androdioecy. As hermaphrodites evolve to favour their own pollen, however, they progressively exclude pollen from males, driving them extinct. The population may then return to mixed mating and repeat the process, or evolve complete selfing. We discuss these results with reference to known variation in the mating and sexual systems of plants.