|其他摘要||Biogeographic disjunction in Northern Hemisphere is one of the most well-known and complicate patterns in the world. It has always been the hot spot for biogeographic researches and attentions. In this study, 7 representatives from species to family level were selected, i.e., Phryma leptostachys, Kelloggia, Sassafras, Parthenocissus, Toxicodendron, Orontioideae, and Magnoliaceae. Phylogenetic reconstruction, divergence time, and ancestral areas were analyzed for understanding their biogeographic origin, diversification, and migration. Moreover, sythensized analyses with these taxa may throw light on temporal and spatial evolutionary processes and mechansim of eastern Asia – North American disjunction and evolutionary implications on the eastern Asian flora.
Phryma leptostachys: Phrymaceae, monotypic Phryma, and disjunct perennial herbs between eastern Asia and eastern North America. Molecular analysis of ITS, rps16, and trnL-F revealed two clades corresponding to their biogeographic distribution, with a divergence time of 3.68-5.23mya. Phylogeographic and DIVA results suggested their North American origin and migration to Asia via Bering strait. Multivariate analysis based on 23 quantitative morphological characters detected no geographic groups at the intercontinental level. The discordance of the molecular and morphological patterns may be explained by morphological stasis due to ecological similarity in both continents.
Kelloggia: Rubiaceae, only 2 species, disjunct between western North America and northwestern China. Phylogenetic and biogeographic analysis of rbcL, atpB-rbcL, and rps16 supported the monophyly of the genus with a divergence time of 5.42±2.32mya. Ancestral area and DIVA analysis suggested an Old World origin and long distance dispersal to the New World.
Sassafras: Lauraceae, 3 species, disjunct between eastern Asia (S. tzumu and S. randaiense) and eastern North America (S. albidum). Phylogenetic and biogeographic analysis of ITS, rpl16, trnL-F, and psbA-trnH suggested the monophyly of Sassafras with a divergence timeof 13.80-16.69 mya. Sassafras has a relict distribution in the northern hemisphere. The divergence time of the two eastern Asian species is estimated to be 0.61–2.23 mya with Sassafras randaiense from Taiwan most likely derived from an ancestor from continental China.
Parthenocissus: one of the few north temperate genera of Vitaceae, 15 species, disjunct between eastern Asia and eastern North America. Phylogenetic analysis of trnL-F, rps16, atpB_rbcL, and GAI1 supported the monophyly of the genus and two clades corresponding to their distribution. A divergence time of 14.37 estimated for the genus is consist with paleoclimatological and fossil evidence with possible migration via Bering strait.
Toxicodendron: ca. 24 species, one of the few north temperate taxa in Anacardiaceae. Phylogenetic analysis based on ITS, NIA-i3, trnL-F, and ndhF sequences suggested the monophyly of the genus, sect. Griffithii and Toxicodendron, but not sect. Venenata within the genus. Two temperate disjunctions were recognized from section Toxicodendron and T. vernix – T. vernicifluum with divergent times estimated to be 14.55 and 9.24 mya based on Bayesian method, respectively. DIVA results combined with fossil data suggested a migration route from eastern Asia to North America via Beringia. The subtropical – tropical disjunct between Asian section Griffithii and central American T. stratium was first detected with divergent time estimated around 22.55 mya. We proposed a similar Beringia route as the temperate disjuncts with subsequent movement to central America, rather than regular North Atlantic route.
Orontioideae: few of the north temperate taxa in Araceae, including Symplocarpus, Lysichiton, and Orontium. Symplocarpus is disjunct in eastern Asia and eastern North America; Lysichiton has an intercontinental distribution in eastern Asia and northwestern North America; and the monotypic Orontium is restricted to eastern North America. Phylogenetic and biogeographic analyses of the trnL-F and ndhF supported the monophyly of Symplocarpus and Lysichiton with divergence at 4.49-6.88 and 4.02-7.18 mya, respectively. Eastern Asia was suggested to be the ancestral area of the Symplocarpus and Lysichiton with independent migrations via the Beringia in the late Tertiary. The relative rate test showed similar substitution rates of the trnL-F between the proto and the true aroids, although the latter has substantially higher species diversity. The proto Araceae perhaps suffered from a higher rate of extinction in the temperate zone associated with periods of climatic cooling in the Tertiary.
Magnoliaceae: Three nuclear genes (PHYA, LFY, and GAI1) revealed that 12 major groups largely consistent with the recent taxonomic revision at the sectional and subsectional level. However, relationships at deeper nodes of the subfamily Magnolioideae remained poorly resolved, perhaps due to an early radiation of the subfamily. Bayesian dating suggested that a complicated divergent evolution of Magnolioideae began around the early Eocene (54.57mya), concordant with paleoclimatic and fossil evidence. Intercontinental disjunctions of Magnoliaceae in the Northern Hemisphere originated during at least two geologic periods. The more recent one occurred after the middle Miocene, producing two well-recognized temperate disjunct lineages between eastern Asia and eastern North America. The second may have occurred no later than the Oligocene, with ancient separations between or within tropical and relict temperate lineages.
The 7 taxa used in this study were selected from family, subfamily, genus to species levels, including herbs, woods, and vines, and representing temperate and pantropical as well as different subtypes within eastern Asian – North American disjunctions. Synthesized with geological and fossil data, phylogenetic and biogeographic results supported the hypothesis that north temperate disjunctions were relicts of the Boretropical flora once widely distributed in the Northern Hemisphere. The disjunct pattern was formed in multiple times in the whole Tertiary, with migration more relatated to Beringia forboth temperate and pantropical taxa rather than North Alantic land bridge. Herbs usually had a divergence time after middle Miocene, while those of woody taxa no later than the Miocene. North temperate disjuncts are also younger than pantropical ones. Long distance dispersal played a similar important role as varcariance. This study also suggested multiple origins of the eastern Asian flora, represented by both ancient Sino-Japanese and young Sino-Himalayan elements. In the Tertiary, large extinction had been occurred in the Northern Hemisphere. The complex disjunct pattern in eastern Asia – North America might directly lead to the complicate origin and source of Sino-Himalayan elements. Morphological stasis or convergence is common in disjunct taxa, which indicates a long stable and similar habitat among relitct floras in the Northern Hemisphere.|