The role of distyly in pollen flow of the hummingbird-pollinated Palicourea rigida (Rubiaceae)

Abstract

Distylous species possess two floral morphs: S-morph with short styles and long stamens, and L-morph with long styles and short stamens. The reciprocal positioning of anthers and stigmas is hypothesized to promote legitimate (between-morph) pollination. Deviations from reciprocity can negatively affect legitimate pollen deposition. The functioning of reciprocal herkogamy occurs if pollinators collect pollen of the L- and S-morph on different parts of the body, and then contact the stigmas of the opposite morph. However, few studies provide experimental evidence that the pollen deposition from the two morphs is stratified on the pollinator's body, especially for hummingbirds. We studied three populations of Palicourea rigida Kunth, an abundant shrub of the Neotropical savanna, to test the function of distyly as a mechanism to promote the differential placement of pollen on different parts of the pollinator's body and facilitate legitimate pollination. For this, we quantified high and low organs reciprocity, pollen dimorphism and pollen flow and developed a method for simulating floral visits by hummingbirds (main pollinators) to test whether pollen deposition occurs in a stratified manner on the bill. We found greater reciprocity in low organs, which explains the greater legitimate pollen deposition on S than L stigmas in two populations. In the other population, there was no difference in legitimate pollen deposition between morphs. Our findings corroborate the evolutionary models which have proposed that distyly promotes pollen deposition on specific parts of the pollinator's body. Pollen from S anthers was more deposited at the base of the bill, while pollen from L anthers appeared on the middle section of the bill. This stratified pollen deposition on the hummingbird's bill corresponded to the anther height of the pollen-donor morph and stigma height of the opposite morph, facilitating the reception of legitimate pollen, thus demonstrating the ecological functioning of distyly.