Compensating for the corolla? Pollen exposure is not associated with pollen-collecting hair length

Secondary pollen presentation, the relocation of pollen from the anthers to elsewhere on the flower, has evolved multiple times across many plant families. While hypotheses suggest it evolved to promote outcrossing, a by-product of relocation may be protection of pollen from loss due to abiotic factors. In Campanulaceae pollen is presented on pollen-collecting hairs along the style and the hairs retract over time and release pollen for transfer. Campanulaceae taxa vary in the degree to which pollen is exposed to environmental factors due to variation in the corolla shape and size. We tested the protective function of pollen-collecting hairs by assessing whether there was a trade-off between the protection provided by the corolla and the pollen-collecting hairs.

We used phylogenetic comparative methods to test for associations between pollen-collecting hair length, floral shape and size metrics, and pollen exposure traits across 39 species.

We anticipated longer pollen-collecting hairs in taxa with more exposed pollen presentation but found there was no relationship between estimates of pollen exposure and pollen-collecting hair length. However, pollen-collecting hair length scaled allometrically with floral size, and variation in pollen-collecting hairs, as well as most floral traits, was phylogenetically structured.

These results indicate that variation in pollen exposure across species does not structure variation in the pollen-collecting hairs, rather hair length scales allometrically and is phylogenetically constrained; therefore pollen-collecting hairs are unlikely to facilitate protection from environmental pressures.