Stamen dimorphism in bird-pollinated flowers – investigating alternative hypotheses on the evolution of heteranthery

Heteranthery, the presence of distinct stamen types within a flower, is commonly explained as functional adaptation to alleviate the ‘pollen dilemma’, defined as the dual and conflicting function of pollen as pollinator food resource and male reproductive agent. A single primary hypothesis, ‘division of labour’, has been central in studies on heteranthery. This hypothesis postulates that one stamen type functions in rewarding pollen-collecting pollinators and the other in reproduction, thereby minimizing pollen loss. Only recently, alternative functions (i.e. staggered pollen release), were proposed, but comparative and experimental investigations are lagging behind.

Here, we use 63 species of the tribe Merianieae (Melastomataceae) to demonstrate that, against theory, heteranthery occurs in flowers offering rewards other than pollen, such as staminal food bodies or nectar. Although shifts in reward type released species from the ‘pollen dilemma’, heteranthery has evolved repeatedly d...