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

Abstract Background and Aims Plant-pollinator interactions span a continuum from strict specialisation to generalisation 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 at identifying the evolutionary drivers of floral syndromes in the genus Aquilegia. Methods 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. Key results Our results indicate that floral syndromes in Aquilegia are mainly shaped 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. Conclusions 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. In conclusion, floral syndromes in Aquilegia are associated with nectar-collecting pollinators only, maybe because they are more efficient at pollinating, which remains to be tested in field experiments.