Limited phenological and pollinator-mediated isolation among selfing and outcrossing Arabidopsis lyrata populations

Mating system transitions from outcrossing to selfing have been a frequent evolutionary shift in angiosperms and clearly play a role in species divergence. However, many questions remain about the initial mechanistic basis of reproductive isolation during the evolution of selfing. For instance, how important are prezygotic pre-pollination mechanisms (e.g., changes in phenology and pollinator visitation) in maintaining reproductive isolation between newly arisen selfing populations and their outcrossing ancestors? To test whether changes in phenology and pollinator visitation reproductively isolate selfing populations of Arabidopsis lyrata from conspecific outcrossing populations, we conducted a common garden experiment in the native range with plants from selfing and outcrossing populations as well as their F1 hybrids. Hybrid plants resulting from crosses between self-incompatible populations had the highest probability of flowering. Time of peak flowering differed among mating systems although the magnitude of the difference was small. Flowering duration and maximum flower number did not strongly differ between mating systems and hybrids, and these phenological traits had little influence on the frequency of pollinator visits of either solitary bees or hoverflies. Additionally, pollinators preferred to visit flowers on the same plant (~50% of the time) rather than exploring nearby plants, regardless of whether they were of an alternative mating system type or F1 hybrids, creating a large opportunity for geitonogamy. Finally, by modelling pollen-transfer probabilities, we found no significant differences in the opportunity for pollinations between selfing and outcrossing plants. Overall, we found that selfing and outcrossing A. lyrata populations show some phenological divergence but are likely only weakly prezygotically isolated via pre-pollination mechanisms.