Consistency in patterns of pollen sterility between floral morphs in distylous populations of Erythroxylum (Erythroxylaceae)

Gender specialization is a common feature present in many distylous species of Erythroxylum. In general, short-styled flowers present higher levels of pollen sterility than long-styled flowers. Variation in pollen sterility also occurs within species in a way that some populations display morph-biased sterility, while others do not. The inconsistent patterns of sterility indicate the importance of more studies to understand the evolution and maintenance of morph-biased sterility within the species and populations. Furthermore, the selective mechanisms responsible for the evolution of gender specialization in Erythroxylum species remain unclear. In this study, the levels of pollen sterility in short- and long-styled flowers were quantified in populations of four Erythroxylum species (one population per species) for three years to test if variation in sterility between morphs is consistent over time in the same populations. The study also tested whether variations in natural fruit set and pollen limitation between morphs are related to morph-biased sterility. Consistently, short-styled flowers of E. campestre and E. deciduum displayed higher levels of pollen sterility than long-styled flowers, while E. suberosum and E. tortuosum exhibited no morph-biased sterility. Variations in fruit set and pollen limitation between morphs were not necessarily related to morph-biased sterility. Patterns of pollen sterility between morphs remained consistent within the same populations, and morph-biased sterility in E. campestre and E. deciduum indicates potential for gender specialization in these populations. In contrast, distyly appears to be a stable floral system in E. suberosum and E. tortuosum.