Impact of capitulum structure on reproductive success in the declining species Centaurea cyanus (Asteraceae): small to self and big to flirt?

Attracting pollinators and achieving successful reproduction is essential to flowering plant species, which evolved different strategies to cope with unpredictable pollination service. The ability of selfing is most widespread and represents a reproductive insurance under varying conditions. In this study, we investigated reproductive success in Centaurea cyanus, a self-incompatible declining Asteraceae species. We measured seed set under outcrossing and autonomous selfing and assessed the impact of capitulum structure (i.e., the number of disc florets) on reproductive success. We report that the incompatibility system is either flexible or evolving a breakdown in this species, since autonomous selfing often resulted in production of few seeds. We also show that capitulum structure has a strong impact on reproduction, with smaller inflorescences presenting a better ability to self than larger ones, while larger inflorescences performed better than smaller ones when cross-pollinated. Variable capitulum structure in this Asteraceae species may therefore represent a reproductive strategy to achieve efficient reproduction under diverse pollination environments. Our results also suggest that this declining species might be disrupting its auto-incompatibility system in response to reduced habitats and declining population sizes.