Resolving the phylogenetic relationships and evolutionary history of the East Asian endemic genus Rodgersia (Saxifragaceae) using multilocus data

Abstract

The East Asian endemic genus Rodgersia comprises five species and several variants. These species exhibit variation in ploidy level and a fragmented distribution across the whole of East Asia. In order to explain their present distribution pattern under a robust phylogeny, we employed multiple copy nuclear genes (ITS and ETS), cpDNA (rpl32-trnL, trnV-ndhC, ndhF-rpl32) and a low copy nuclear gene (GBSSI) to reconstruct the species’ relationships within Rodgersia. Identification of ploidy levels was also carried out for most of samples. The early-diverging positions of R. nepalensis and R. podophylla were confirmed in different gene trees. Much topological incongruence among gene trees was found in the shallower branches, which comprised the species of central-southwest China. Both incomplete lineage sorting and ancient introgression may be responsible for the incongruent pattern observed. Ancient introgressions may have occurred between diploid species and the diploid progenitors of tetraploid species in different areas of southwest China. Rodgersia probably originated in Northeast Asia during the Pliocene. This genus is a rare example in which two early-diverging species occur at opposite ends of the distribution area, likely resulting from successive vicariance events. Unlike many vicariance events shaped simply by forest fragmentation during the Pleistocene, the contemporary biogeographic pattern of Rodgersia is more the result of rapid sympatric evolution (reticulate events and polyploidization) of the taxa in the central area.