Adaptive function of duodichogamy: Why do chestnut trees have two pollen emission peaks?

In cosexual plants, the advantages and drawbacks of mating facilitation between sexes remain poorly understood. To cast some light on this topic, we focused on duodichogamy, a rare flowering system designating plants that consecutively produce three batches of flowers in the sequence male-female-male. We explore the adaptive advantages of this flowering strategy using chestnuts (Castanea spp.), the first duodichogamous species described, as models. These mass-flowering insect-pollinated trees have two types of male catkins, one type producing most pollen, and another type, closely associated with the rewardless female flowers, producing much smaller amounts of pollen. We surveyed insect visits on 11 chestnut trees during the entire flowering period. More insects visited trees during the first major staminate phase. However, there were more visits to female flowers during the second minor staminate phase, suggesting that the attractiveness of male catkins triggers visits to nearby female flowers when trees have already released most of their pollen. This should limit self-pollination and its negative consequences on fruit set. We found 22 animal-pollinated duodichogamous species described in the literature. All of them are also mass-flowering woody plants. In 21 out of 22 cases, gynoecia (female flower parts) are associated with androecia (male flower parts), whereas androecia are often not associated with gynoecia. Moreover, the association generally involves the androecia responsible for the second minor staminate phase, as observed in chestnuts. This supports the hypothesis that duodichogamy was selected to facilitate pollen deposition on stigmas by pollinators while limiting self-pollination, not just to increase pollen emission.