Beyond the pollen dilemma: a case of heteranthery driven by pollinator attraction in Mamedea pulchella (Malpighiaceae), an oil-rewarding species.

Heteranthery is widespread among angiosperms and is classically linked to nectarless flowers that offer pollen as a reward, where the division of labour in the androecium helps to decrease the consumption of viable pollen grains. However, heteranthery also occurs in lineages with non-pollen-rewarding flowers. This is the case of some neotropical species of Malpighiaceae, which depend on oil-collecting bees for pollination, and offer floral oils. This study analyzes the androecium of Mamedea pulchella to assess the presence of division of labour and whether such specialization functions primarily in pollinator attraction rather than pollen economy. Anther traits from flowers of different populations were measured and analyzed statistically, complemented by anatomical and histochemical studies. Fertile stamens show significantly bigger anther area while sterile stamens bear antherodes with a significantly larger glandular connective. Occasional abortive anthers in usually fertile stamens and fertile anthers in usually sterile stamens were observed. Histochemical reactions indicate that connective glands secrete mainly terpenes. Differences between anthers and antherodes are consistent across populations, suggesting that heteranthery is a stable condition in M. pulchella, with a clear division of labour: anthers of large stamens supply pollen for fertilization whereas antherodes of smaller stamens act as osmophores, emitting scents that attract and guide oil-collecting bees to the elaiophores. This case highlights a potential alternative role for heteranthery in oil-rewarding flowers, where stamen specialization may contribute to pollinator attraction rather than solely mitigating the "pollen dilemma", suggesting that heteranthery can evolve through multiple functional pathways in pollination systems.