Floral syndromes in Aquilegia (Ranunculaceae) are associated with nectar- but not pollen-collecting pollinators

Plant-pollinator interactions span a continuum from strict specialization to generalization and most flowers are visited by more than a single functional group of pollinators. However, one functional group might be more efficient than the others and thus exert stronger selective pressure on floral traits. In this study we aim to identify the evolutionary drivers of floral syndromes in the genus Aquilegia.

We analyse floral syndromes using multivariate statistics, morphospace analyses as well as a machine learning approach (random forests), testing for the association between floral traits and documented pollinators for 28 Aquilegia species. In particular, we test whether pollen-collecting pollinators (small bees, large bees, syrphid flies) and nectar-collecting pollinators (large bees, hummingbirds, hawkmoths) are associated with specific floral traits. Furthermore, we test whether mixed pollination systems are reflected in floral syndrome properties.

Our results indicate that floral syndromes in Aquilegia are shaped mainly by nectar-collecting pollinators (and not by pollen-collecting pollinators). Flowers pollinated by large bees are mostly pendent and short-spurred; hummingbird flowers are red, with constricted spurs and short petal blades; and hawkmoth flowers are erect with long and slender spurs. Flowers pollinated by two groups of nectar-collecting pollinators show syndromes corresponding to only one of their pollinator groups.

Despite their ubiquity, we did not find cues for selection by any of the pollen-collecting pollinators. Nevertheless, selection for traits associated with pollen-collecting pollinators, such as openly accessible stamens and a contrasted yellow floral centre (almost always present in Aquilegia), cannot be ruled out. Floral syndromes in Aquilegia are associated with nectar-collecting pollinators only, perhaps because they are more efficient at pollinating, which remains to be tested in field experiments.