Spatiotemporal evolution of the global species diversity of Rhododendron

Evolutionary radiation is a widely recognized mode of species
diversification, but its underlying mechanisms have not been unambiguously
resolved for species-rich cosmopolitan plant genera. In particular, it
remains largely unknown how biological and environmental factors have
jointly driven its occurrence in specific regions. Here we use
Rhododendron, the largest genus of woody plants in the Northern
Hemisphere, to investigate how geographic and climatic factors, as well as
functional traits, worked together to trigger plant evolutionary
radiations and shape the global patterns of species richness based on a
solid species phylogeny. Using 3437 orthologous nuclear genes, we
reconstructed the first highly supported and dated backbone phylogeny of
Rhododendron comprising 200 species that represent all subgenera,
sections, and nearly all multi-species subsections, and found that most
extant species originated by evolutionary radiations when the genus
migrated southwards from circumboreal areas to tropical/subtropical
mountains, showing rapid increases of both net diversification rate and
evolutionary rate of environmental factors in the Miocene. We also found
that the geographically uneven diversification of Rhododendron led to a
much higher diversity in Asia than in other continents, which was mainly
driven by two environmental variables, i.e., elevation range and annual
precipitation, and was further strengthened by the adaptation of leaf
functional traits. Our study provides a good example of integrating
phylogenomic and ecological analyses in deciphering the mechanisms of
plant evolutionary radiations and sheds new light on how the
intensification of the Asian monsoon has driven evolutionary radiations in
large plant genera of the Himalaya-Hengduan Mountains.