Phylogenomic And Biogeographic Study On The East Asia And North America Disjunct Genus Thuja(Cupressaceae)

Biogeography is one important discipline that closely related to organism's origin,diversification and dispersal based on which people can recognize the nature well.How to understand the differences and connections of one biogeographic distribution pattern,that is,between the flora of the northern hemisphere,especially the flora of east Asia and North America,has still been an important issue for systemists and biogeographers.The effects of both vicariance and frequent dispersal by the Bering Land Bridge(BLB)and the North Atlantic Land Bridge(NALB)have made the biogeographic patterns of Northern Hemisphere complicated and intricate.The Eastern Asian-North American disjunction is particularly of great interest for many years.Previous studies have shown that in angiosperm lineages,east Asian origin and then migrated to North America are extensively mentioned,while the opposite case,examples for species North American originated and then migrated to East Asia,is also abundant.Systematic studies of some gymnosperms suggest that many species experience a similar pattern for origin and migration.Gymnosperms originated much earlier than angiosperms,and,due to the ancient diversification and economical importance,the gymnosperns are regarded as ideal objective for understanding the deep biogeographical pattern esp.those of intercontinental.Thuja is a small genus of Cupressaceae comprising five species,namely T.plicata Donn ex D.Don,T.occidentalis L.,T.koraiensis Nakai,T.standishii(Gordon)Carriere and.sutchuenensis Franch.Species of Thuja are discontinuously distributed across both Eastern Asia and Northern America.Of that,T.plicata and T.occidentalis are endemic to North Anerica,whereas T.koraiensis,T.standishii and T.sutchuenensis are restricted to East Asia,and there are obvious discontinuous and fragmented distributions in the three countries of East Asia.Especially the endemic species.koraiensis in China,which has important academic significance and important economic value.But the species has very few wild populations,which is highly endangered species.Therefore,we choose the taxa for study and screening of molecular markers at the genome level for population genetics analysis,which can provide an important basis for the future conservation for species.Phylogenetic studies of Thuja have been done using different data sets such as data from the field of morphology and anatomy,ecology,nuclear DNA,microsatellites or Single Sequence Repeats(SSRs)and selected few genes from chloroplast DNA.However,not a harmonious conclusion has been reached on the source of these taxa.The complete chloroplast genones(plastomes)have been commouly used for phylogenetic reconstruction with the explosive development of Next-Generation Sequencing(NGS).They can provide vast of informative sites and greatly improve the resolution of phylonetic trees at broaden levels.In addition to that,complete plastome sequences can provide insight into the molecular evolutionary patterns associated with gene rearrangements,duplication and loss.Therefore,in this study,the complete chloroplast genomes were employed to understand the plastome evolution of Thuja species,and reconstruct phylogenetic relationships within this important genus as well as investigate the disjunct distribution of these plants across both Eastern Asia and North America.The main results are as follows:(1)The complete chloroplast genome sizes of all the Thuja taxa as:Thuja plicata to be 131,118 bp,Thuja sutchuenensis,130,668 bp5 Thuja occidentalis 131,602 bp,Thuja koraiensis,130,273 bp and Thuja standishii 130,505 bp.All the plastomes of Thuja lost the IR regions which is consistent with previous studies.Besides,the SSRs were generated for futher population genetic study;(2)Phylogenetic results showed that the monophyly of Thuja was supported with 100%bootstrap support.T.plicata,.sutchuenensis and T.occidentalis were resolved to be the early diverging species within Thuja.The sister relationship of T.standishii and T.koraiensis was well-supported;T.occidendalis was sister to the clade formed by.sutchuenensis and T.plicata.In addition,the sister relationships between Thuja and Thujopsis were also affirmed with strong support;(3)By biogeographic analysis,the origin of the taxa was traced to be from Japan/westem North America.The early biogeographic history of Thuja was likely influenced by both dispersal and vacariance events across the old and new world.Our results revealed hence vital migration scenario at the period between the Miocene and Pliocene(13.03My)and subsequent vicariance events in the Quaternary(1.58My)shaped the current biogeographic distributions of Thuja.In this study,we have chosen the gymnosperms genus Thuja as the research objective,and based on phylogenomic analysis,we have elucidated the phylogenetic relationship within the small ancient group inferred from complete plastome evidences.Besides,we discussed the origin and dispersal routine of Thuja in between East Asia and North America.Our study has provided a new insight of the complex history of the kind of disjunct distribution pattern.