Floral polymorphism and heteromorphic incompatibility in the distylous tropical shrub Waltheria rotundifolia (Malvaceae)

The genus Waltheria is known for having a high percentage of distylous species (ca. 85%). However, distyly has only been comprehensively studied in a few Waltheria species. The study of distyly is of particular ecological relevance given the occurrence of exotic pollinators in natural ecosystems nowadays, which could potentially affect legitimate pollen transfer between floral morphs. In the present study, we characterized the sexual reciprocity between floral morphs in the tropical shrub Waltheria rotundifolia, we tested if the pollen transfer patterns were in accordance with proper distyly, and we tested its incompatibility system. Reciprocity was characterized by measuring sexual traits, while pollen traits were measured to evaluate the presence of ancillary polymorphisms. The efficiency of distyly to promote legitimate pollen transfer was determined by quantifying pollen loads deposited on stigmas and on the body of Apis mellifera, the main floral visitor of W. rotundifolia. Finally, hand pollinations were performed to determine the strength of the incompatibility system. Flowers exhibit a reciprocal arrangement of sexual organs and pollen ancillary features typical of distylous plants. We observed a differential placement of pollen on the body of A. mellifera. However, intermorph differences in the pollen loads and legitimate pollen deposited on stigmas suggest inefficient pollen transfer by A. mellifera. Hand pollinations revealed the presence of a heteromorphic incompatibility system. Overall, our results show that although factors for disassortative pollen transfer are present in the studied population, W. rotundifolia exhibits asymmetric pollen deposition probably due to A. mellifera foraging behavior. Around 83% of the species recorded in the genus Waltheria are distylous; however, information about the functioning of this polymorphism is scarce so far. The aim of the present study was to characterize the floral morphology, the incompatibility system, and the distyly in the tropical shrub Waltheria rotundifolia. Our results show that although the factors for disassortative pollen transfer were present, W. rotundifolia exhibits asymmetric pollen deposition. Nevertheless, the morph frequencies were in equilibrium in the studied population, probably because of the heteromorphic incompatibility system of this species.