Disassortative pollen transfer in distylous Palicourea padifolia (Rubiaceae), a hummingbird-pollinated shrub

Abstract

Distylous polymorphisms represent a useful model system for studying the functioning of flower morphology because floral morphs have reciprocal flower developmental pathways and co-occur in the same environment such that they share the same pollinators. Distylous plants typically display both reciprocal herkogamy and a heteromorphic incompatibility system, which allows mating only between morphs. In this paper, we document morph-specific patterns in floral morphology and pollen receipt of Palicourea padifolia, a hummingbird-pollinated shrub. Floral traits typically associated with the functioning of distyly, reciprocal herkogamy, and population morph ratios of flowering individuals from 10 populations were measured and quantified. Pollen receipt on stigmas of the same measured flowers was then used to determine the effectiveness of distyly in promoting disassortative pollen transfer. Pollen of long-styled flowers was more effective in reaching legitimate stigmas, while short-styled morphs were more successful in the reception of legitimate pollen across populations. Variation in pollen receipt across populations was not explained by reciprocal herkogamy, which typically promotes legitimate pollination. Natural pollination resulting in a higher proportion of legitimate pollen in short-styled flowers increased with stigma-anther separation (promotion of outcross pollination) and width of corolla entrance (avoidance of self-pollination).