Evolutionary Origin And Demographic History Of An Ancient Conifer(Juniperus Microsperma) In The Qinghai-Tihetan Plateau

Due to complex topography and various climate conditions in the Qinghai-Tibetan Plateau (QTP), this region harbors abundant species diversity and a considerable amount of these species are endemic. However, most of QTP endemic species are assumed to have originated recently, although very rare species most likely diverged early. These ancient species provide an excellent model to examine the origin and evolution of QTP endemic plants in response to the QTP uplifts and the climate changes that followed in this high altitude region. In this study, we examined these hypotheses by employing sequence variation from multiple nuclear and chloroplast DNA of 239 individuals of Juniperus microsperma and its five congeners (J. sabina, J. semiglobosa, J. convallium, J. tibetica and J.saltuaria). As a QTP endemic species, J. microsperma occurs in the Parlung Zangpo valley only; among its congeners, J. convallium, J. tibetica and J. saltuaria are also distributed in QTP, whereas J. sabina and J. semiglobosa occur distantly away from J. microsperma. However, both phylogenetic and population genetic analyses revealed that J. microsperma is closer to J. sabina and J. semiglobosa, this QTP endemic diverged from its sister clade comprising these two species with long isolation around the Early Miocene, which corresponds to early QTP uplift. Demographic modeling and coalescent tests suggest that J. microsperma experienced an obvious bottleneck event during the Quaternary when the global climate greatly oscillated. The results presented here support the hypotheses that the QTP uplifts and Quaternary climate changes played important roles in shaping the evolutionary history of this rare juniper.