Generalized pollination maintains disassortative pollen transfer in a distylous tropical shrub

Distylous plants are expected to promote accurate intermorph pollen transfer through reciprocal positioning of sexual organs, yet many species are visited by taxonomically diverse pollinator assemblages. Whether generalized pollination compromises mating accuracy in such systems remains unclear.

Single-visit experiments were conducted to quantify pollen transfer in the distylous Caribbean shrub Morinda royoc (Rubiaceae). Pollen-size dimorphism was used to distinguish morph-specific pollen and to estimate both total pollen deposition and the proportion of disassortative (reciprocal) pollen delivered by different visitor groups.

Multiple pollinator taxa, including butterflies, hummingbirds and bees, deposited predominantly intermorph pollen on stigmas. Although pollination effectiveness was unevenly distributed across the visitor assemblage and concentrated in a subset of species, the directionality of pollen flow remained consistently biased toward intermorph transfer across taxa. Variation in floral traits, including corolla tube dimensions, anther height, style length and stigmatic surface area, did not explain differences in the proportion of reciprocal pollen deposited.

These findings indicate that reciprocal floral architecture can maintain mating accuracy under generalized pollination. While quantitative contributions to pollination vary among visitors, the qualitative component of pollen transfer appears robust to differences in pollinator identity. Structural reciprocity may therefore stabilize reproductive processes in distylous systems exposed to variable pollinator assemblages.