Factors maintaining floral oil gland polymorphism in a population of Banisteriopsis muricata (Malpighiaceae)

Phenotypic polymorphism within species is often a key stage in evolutionary transitions. In the plant family Malpighiaceae, floral oil glands are typically linked to specialized mutualism with oil-collecting bees, which have contributed to the conservation of floral architecture across the family. However, several Malpighiaceae groups have lost oil glands, and some species present glandular polymorphisms. In one of these species, Banisteriopsis muricata, the eglandular morph is hypothesized to engage in automimicry, attracting pollinators without offering an oil reward. To test this hypothesis, we compared floral visitor diversity and behaviour, corolla size, and pollen production between eglandular and glandular morphs in a Costa Rican population. We also examined the global distribution of the eglandular morph to assess the maintenance of this polymorphism across the range of the species. Contrary to expectations, the main bee pollinator, Monoeca Mexicana, did not attempt oil collection in the eglandular morph. Meliponini bees, the second most frequent bee visitors, collected pollen from both morphs without discrimination. Pollination success in eglandular individuals appeared unaffected by the absence of oil glands. Both morphs received similar visitation rates, shared pollinator species, and exhibited comparable natural fruit set and mating systems. Notably, the eglandular morph produced more pollen, and pollen collection visits outnumbered oil collection visits, potentially explaining the widespread presence of the eglandular morph across the species’ range. These findings suggest a balance in the adaptive value of both morphs, with weak selection against the absence of oil glands. The consistent presence of eglandular individuals, at intermediate frequencies, documented in both historical and recent collections, points to additional selective forces that help maintain this floral polymorphism.