The significance of recent diversification in the Northern Hemisphere in shaping the modern global flora revealed from the herbaceous tribe of Rubieae (Rubiaceae)

Abstract

The global herbaceous flora is probably shaped by both ancient and/or recent diversification, companied with the impacts from geographic differences between the Northern and Southern Hemispheres. Therefore, its biogeographic pattern with respect to temporal and spatial divergence is far from full understanding. Tribe Rubieae, the largest herbaceous tribe in the woody-dominant Rubiaceae, provides an excellent opportunity for studying the macroevolution of worldwide colonization. Here, we aim to reconstruct the evolutionary history of Rubieae with regard to climate fluctuation and geological history in the Cenozoic. A total of 204 samples of Rubieae representing all the distribution areas of the tribe were used to infer its phylogenetic and biogeographic histories based on two nrDNA and six cpDNA regions. The ancestral area of Rubieae was reconstructed using a time-calibrated phylogeny in RASP and diversification rates were inferred using Bayesian analysis of macroevolutionary mixtures (BAMM). Our results show Rubieae probably originated in European region during the middle Oligocene, with the two subtribes separating at 26.8 million years ago (Ma). All the genera in Rubieae formed separate clades between 24.79 and 6.23 Ma. The ancestral area of the subtribe Rubiinae was the Madrean-Tethyan plant belt and the North Atlantic land bridge (NALB) provided passage between North America and Europe for Rubiinae. The subtribe Galiinae clade originated in Europe/central Asia during the late Oligocene. Two diversification shifts were detected within Rubieae in the late Neogene. Most extant Rubieae species diverged recently during the Neogene within clades that generally were established during the late Paleogene. The tribe shows complex migration/dispersal patterns within the North Hemisphere combined with multiple recent dispersals into Southern Hemisphere. Our results highlighted the important role of recent biogeographic diversification in the Northern Hemisphere in shaping the modern global herbaceous flora during the latest and rapid worldwide expansion in the Neogene.