Anther modes influence diversification rates in the animal-pollinated species-rich Didymocarpoideae

Stamen traits significantly influence pollen presentation patterns and play a critical role in mating systems, floral evolution, and the diversification of angiosperms. Anthers within a flower can either develop freely or be united, resulting in three primary modes: separated anthers, synandry, and paired-united anthers. The impact of these anther modes on species diversification remains inadequately understood. In this study, we employed 14 plastid and nuclear ribosomal markers from 789 species to reconstruct the phylogenetic relationships of the Old World Didymocarpoideae, which are predominantly bee pollinated. We then investigated the evolutionary patterns of anther modes and assessed their potential role in species diversification. Our findings indicate that the evolution of anther modes and associated floral traits exhibited significant trait conservatism. Paired-united anthers likely represent the ancestral form of Didymocarpoideae, characterized by four fertile stamens with exserted locations for the anthers. Notably, derived synandry was associated with relatively high rates of species diversification, particularly in the species-rich Cyrtandra, Primulina, and Streptocarpus, which could be due to enhanced pollination precision facilitated by aggregations of anthers and pollen grains. This study elucidates the evolutionary transitions of different anther modes while highlighting their influence on diversification rates within Didymocarpoideae.