Molecular phylogeny and biogeography of Pseudotsuga (Pinaceae): insights into the floristic relationship between Taiwan and its adjacent areas.

Climatic oscillations and geological events play major roles in shaping species diversity and the distribution of plants. The mechanisms underlying the high level of plant species diversity in eastern Asia are hotly debated. In this study, five cpDNA regions, two mtDNA fragments and one nuclear gene (LEAFY) were employed to investigate species diversification and the historical biogeography of Pseudotsuga (Pinaceae), a genus with a typical eastern Asia and western North America disjunct distribution. Both the nuclear LEAFY gene and cpDNA phylogenies strongly suggest that eastern Asian and North American species are monophyletic, respectively. Within the eastern Asia clade, the cpDNA tree placed P. japonica as sister to the rest of the Asian species, but the LEAFY gene tree showed a sister relationship between P. japonica-P. sinensis-P. gaussenii and P. brevifolia-P. forrestii. Molecular dating indicated that the Asian species last shared a common ancestor 20.26+/-5.84 mya and the species diversification of Pseudotsuga was correlated with the Tertiary climatic and tectonic changes. These results, together with the fossil evidence, suggest that Pseudotsuga might have originated from North America and then migrated to eastern Asia by the Bering land bridge during the early Miocene. The Taiwanese species P. wilsoniana harbored two divergent types of LEAFY sequences, which implies that this species might have originated by hybridization between P. brevifolia or its ancestor and the ancestor of P. japonica-P. sinensis-P. gaussenii. Our study also suggests that Taiwan is closely related to both southwest and east China in flora.