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.