Biogeography Of Cupressaceae Sensu Lato

Plate tectonics has greatly impacted the distribution and migration of biota on different continents by changing connectivity between them; it has thus shaped the current biogeographic pattern. Nearly all current continents were united together as the single one, known as the supercontinent "Pangea" during the latest Paleozoic and the early Mesozoic. From the Jurassic on, the northern and southern part of this supercontinent began to drift apart and form two new supercontinents; of which, Laurasia was in the north and mostly comprised the northern hemisphere continents, while Gondwana in the south consisted of the southern hemisphere continents. Laurasia started to break up into plates that gave rise to the current Eurasia and North American since the Jurassic, while Gondwana was gradually disassembled into plates that became the present Africa, South America, Antarctic. Australia and India etc. When two such landmasses separated, biota inhabiting each one began to evolve in different directions, which is the basis of the "Vicariance" theory in the biogeographic studies. Plate tectonics might also change the circulation of atmosphere and ocean currents, which could in turn lead to paleoclimate changes; and through the later, plate tectonics indirectly affected the biogeographic pattern of each continent. In addition, climate oscillations during the Quaternary influenced the distribution pattern of the current biota, especially at the species level. Generally. it is hypothesized that when paleoclimate was warmer, e.g. during interglacial periods, species tended to colonize or move towards high latitude regions or high attitude habitats; and when climate was cooler, e.g. during glacial maxima, these species retreated to lower latitudes and altitudes:glacial refugia provide an example of this.Cupressaceae sensu lato (comprising 32 genera and about 160 species) is of cosmopolitan distribution and its fossil record is long and abundant. This family is ideal for testing biogeographic hypothesis related to vicariance due to paleo-tectonics. As the biggest genus of this family, Juniperus contains 67 species disjuctly distributed in the Northern Hemisphere. The biogeographic history of this genus may be highly correlated with migrations and dispersals of plants through the Land bridges in the Northern Hemisphere. In addition, some species of this genus are distributed in the high altitude regions, for example, the Qinghai-Tibet Plateau (QTP) which was suggested to have experienced extensive glaciation and climate changes during the Quatenary. These species are good models for studying the phylogeographic history on the QTP. In this study, we aimed to study biogeographic patterns of members of Cupressaceae on three different taxonomic levels (family, genus and species) and also at different timescales from the Cretaceous to the Quaternary.First, the biogeographic and diversification histories of the whole family were constructed, Phylogenetic relationships within the Cupressaceae were constructed using 5 cpDNA regions and the diversification history of this family was dated by relaxed molecular clock approach with multiple calibrations. By employing ancestral distribution area reconstruction analysis, a biogeographic history of Cupressaceae was inferred together with fossil records. Cupressaceae and "Taxadoids" (includes Taxaceae, Cephalotaxus) diverged with each other around the Permian/Jurassic boundary. The subfamily Cunninghamioideae split from the other members of Cupressaceae since the middle Jurassic, followed by the subfamily Taiwanioideae, Athrotaxioideae, Sequoioideae and Taxodioideae during the middle to late Jurassic, the late Jurassic, the Jurassic/Cretaceous boundary and the early Cretaceous, respectively. The remaining lineage diverged into the subfamily Cupressoideae (Rich. ex Sweet) and Callitroideae (Saxton) during the middle Early Cretaceous. This vicariance between the monophyletic Cupressioideae (Northern Hemisphere) and Callitroideae (Southern Hemisphere) was estimated to have occurred during the middle Early Cretaceous (125.88±6.89 million yeas ago), which is highly consistent with these new geological evidences for separation betwen Gondwana and Laurasia. The present first independent estimation has a general significance for understanding vicarient patterns of other organisms between the current continents during the Cretaceous.Second, the biogeographic and diversification history of Juniperus was examined in detail. Nine cpDNA fragments (>10000bp) from 51 Juniperus species were determined and phylogenetic relationships were constructed. The diversification timescales of Juniperus were generated by employing three relaxed molecular clock approaches with 8 calibration points. Then, the biogeographic history of this genus was infered based on Bayes-DIVA analysis and fossil records. Juniperus and Cupressus s. lat. are both monophyletic and they diverged with each other around the Cretaceous/Tertiary boundary. Juniperus may have had a wider distribution range including Europe and Asia during the early Tertiary:and it began to diversify into seven lineages and dispersed all over the Northern Hemisphere during the period from the late Eocene to the early Oligocene. Three migrations of junipers from Eurasia to North America were detected; these occurred during the late Eocene, the middle Miocene and the Pliocene, respectively. The only juniper species (J. procera) occurring south of the equator probably migrated from Eurasia to Africa during the late Oligocene. The diversification of this genus was slow during the Oligocene, but since the Middle Miocene, its diversification was accelarated, generating most of the current species. Juniperus species probably comprised a part of the Madrean-Tethyan vegetation that strentched from California (Madrean) to Mediterranean (Tethyan) during the early to middle Tertiary (Paleocene, Eocene and Oligocene). This study provides a detailed example of how one genus in this vegetation belt diversified and migrated.Third, the phylogeographic history of the Juniperus tibetica complex from the Qinghai-Tibet Plateau was studied. Three cpDNA regionsfor 590 individuals from 102 populations of this complex were sequenced. Significant interpopulation differentiation as well as phylogeographic structure were detected (GST= 0.49, NST= 0.72, NST> GST, P< 0.01), indicating limited gene flow among populations. Of 62 haplotypes,40 were restricted to single populations. These private haplotypes and overall degrees of diversity were evenly spread among plateau and edge populations, providing strong evidences for the existence of numerous Last Glacial Maximum (LGM) microrefugia throughout the present distribution range of this complex, most of which were located above 3500 m. These results mark the highest known LGM tree lines, illustrating the potential significance of the high-mountain areas for the glacial refugia. More researches on alpine microrefugia may be triggered by this study.