Trait specialization facilitates autonomous selfing ability in a mixed‐mating plant

Premise Transitions from outcrossing to selfing often drive the evolution of floral traits in a predictable way. However, these expectations are not as straightforward for mixed‐mating systems. In this study, we examine variation in pollen‐collecting hairs, a floral structure involved in secondary pollen presentation within Campanulaceae. While secondary pollen presentation is hypothesized to have evolved to promote outcrossing, we evaluate the association of pollen‐collecting hairs with selfing ability. Methods We characterized pollen‐collecting hair morphology and retraction phenology in 15 populations of Campanula americana with known variation in self‐fertilization ability using time‐series collections and automated image analysis of pollen‐collecting hair length. Results There was two‐fold variation in the length of pollen‐collecting hairs across populations that was associated with a population's within‐flower selfing ability. Retraction rate of pollen‐collecting hairs also varied among populations and was associated with selfing ability. Populations with greater selfing ability had longer hairs that retracted quickly early in floral anthesis. Conclusions We show pollen‐collecting hairs, a trait thought to have evolved to promote outcrossing, is associated with within‐flower selfing ability. Through developmental changes in length, pollen‐collecting hairs appear to be a plastic phenotype that is both associated with autonomous selfing and with outcrossing in C. americana . This provides support for trait specialization rather than trade‐offs, and for the ‘best of both worlds’ hypothesis of mixed mating‐system evolution.