Distinctiveness, speciation and demographic history of the rare endemic conifer Juniperus erectopatens in the eastern Qinghai-Tibet Plateau

The unique geography and climate of the Qinghai-Tibet plateau (QTP) have impacted the formation of species in this area, with a large number of relatively young endemic species associated with the uplift of the QTP and Quaternary climatic changes. In our study, we used two chloroplast DNA fragments and seven nuclear genes to investigate the population genetic structure, species divergence and demographic history of two QTP endemic species, Juniperus erectopatens and J. microsperma as well as their close relatives, J. sabina and J. semiglobosa. Furthermore, we assessed the correlation between their divergence and the historical environmental changes of the QTP. *BEAST analysis revealed that a clade containing J. erectopatens and J. microsperma separated from the clade containing J. sabina and J. semiglobosa about 2.72 million years ago (Mya), whereas the divergence between J. erectopatens and J. microsperma occurred ~ 0.64 Mya. In contrast, the Isolation and Migration model suggested that J. erectopatens diverged from J. microsperma due to geographic isolation during the early to middle Pleistocene about 1.24 Mya, which corresponds to the Quaternary climate oscillations. Approximate Bayesian computation (ABC) suggested that J. erectopatens underwent a population expansion ~ 726 thousand years ago (Kya) followed by a decline ~ 188 Kya. These results suggest that the Quaternary climate oscillations had a profound impact on the demographic history of the QTP endemic J. erectopatens as well as on the divergence between J. erectopatens and J. microsperma. Furthermore, they suggest that some QTP endemic woody species might be younger than previously thought.