Division of labour between dimorphic stamens in Melastoma candidum (Melastomataceae): Role of stamen strength in the biomechanics of pollination

The division of labour hypothesis suggests that plants exhibiting heteranthery utilise different stamen types for distinct purposes; feeding stamens provide pollen as a reward for pollinators, whereas pollinating stamens ensure successful reproduction. However, the biomechanical factors that influence pollinator behaviour in relation to different stamen types remain underexplored. In this study, we investigated Melastoma candidum, a species characterised by dimorphic stamens and poricidal anthers, to elucidate how flower morphology, pollen fertility, and biomechanics of pollinator interactions affect pollination efficiency. We observed the behaviour and stamen preference of multiple bee species visiting the flowers, with a particular focus on the frequency of flower visitation and stigma contact. Among the pollinators studied, Xylocopa flavifrons emerged as the primary pollinator on Okinawajima Island, Japan. Our analysis revealed that while both stamen types exhibit comparable pollen fertility, X. flavifrons preferentially buzzed feeding stamens with the mechanical advantage of pollen collection due to their structural integrity. This preference was further corroborated by biomechanical interactions, wherein the weaker pollinating stamens could not bear the weight of the bee, thereby relegating their function in pollen release. These findings elucidate the significant influence of biomechanical factors on pollinator behaviour and stamen function, providing novel insights into the mechanisms underlying division of labour in plant-pollinator interactions.