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.