Do reciprocal herkogamy and pollinators affect legitimate pollen flow in distylous species of Rubiaceae?

Abstract

The function of the reciprocal position of anthers and stigmas of the floral morphs in distylous flowers is to promote cross-pollination between morphs (legitimate pollination). Deviations from reciprocity can negatively affect the rates of legitimate pollen deposition, but other floral traits or the pollination environment might contribute to the function of distyly. In populations of four species of Palicourea and Psychotria, we quantified reciprocity using adaptive inaccuracy and measured the stigmatic lobe length to estimate how these traits influenced legitimate pollen deposition. We described the functional groups of pollinators and compared legitimate pollen flow between morphs in populations visited by different groups. Variations in reciprocity and stigmatic lobe length correlated with the rates of legitimate pollen depositions, with more reciprocal stigmas and longer stigmatic lobes presenting proportionally more legitimate pollen. Populations most frequently visited by bees or with generalized pollination displayed asymmetries in pollen flow between morphs, but with higher legitimate pollination in the S-morph. In contrast, L- and S-flowers showed similar legitimate pollen deposition in the population predominantly visited by hummingbirds. Our results suggest that reciprocity affects the function of distyly; however, floral traits such as stigmatic lobe length and the pollination environment can also influence the rates of legitimate pollen.