Phylogenetics And Evolutionary History Of Eastern Asian Astragalus L.(Fabaceae)

Astragalus L.(Fabaceae),as the largest genus of the flowering plants,is well-known for its high species richness(3000 spp.)and morphological diversity as well as wide distribution.Some of Astragalus species have high economic value,such as A.mongholicus Bunge and A.membranaceus(Fisch.)Bunge are well-known Chinese medicine.A.sinicus L.is used as green manure and forage.Also,Astragalus is extensively studied and attention by the taxonomists and desert ecology research scientists because of ecological adaptability.Eastern Asia as a distribution center of Astragalus,is also a hotspot for intercontinental biogeography research,which is very important for the evolution of Astragalus and biogeographical research.Nowadays,the Bunge's ten subgenera is widely accepted but the previous studies suggested that many of the subgenera of Astragalus are not monophyletic and the phylogenetic relationships within the genus are still poorly known.In order to explore the above issues,this study uses the chloroplast genome and nrDNA sequences data to study the phylogenetic relationship biogeography of eastern Asian Astragalus.The main results obtained are as follows:(1)Phylogenetic analysis of eastern Asian Astragalus based on plastid genome and nrDNA sequencesIn this study,we sampled 117 taxa of Astragalus and its close relatives,covering 55 sections of the genus plus 30 outgroup taxa to recover the main clades of eastern Asian Astragalus based on sequences of the whole chloroplast genome and chloroplast CDSs.Astragalus is supported to be monophyletic and it is sister to the Oxytropis+Coluteoid clade.Within Astragalus,we recovered ten clades,and the ten clades differ substantially from Bunge's subgenera.The former genus Astracantha which segregated by morphology characters,is also monophyletic,but embedded within Astragalus s.str.,supporting the merge of the former spiny genus Astracantha with Astragalus.We detected the atpF intron losses in the chloroplast genome of the Oxytropis+Coluteoid clade,i.e.,the sister clade to Astragalus.We also obtained 122 nrDNA sequences for the systematics analysis which concatenate the ITS,26S,18S and 5.8S sequences.The phylogenetic results are similar to the CDS tree except the first clade the Glottis clade.The Glottis clade is in different position between ML tree and BI tree.Furthermore,the genera of Astragalus and Oxytropis form a sister clade with low supportive values which need to have a further study of nuclear sequences.Chloroplast genome and nrDNA phylogenetic trees showed that the Astragalus species distributed in America nested in the Old World species,and originated several times which support the Barneby's hypothesis:the species in North America originated from Eurasian via multiple paths.(2)Character evolution of medifixed hair and annual habit groups of Astragalus Here,we got every sample's trichome type by Stereomicroscope and micromorphology of trichome data from representative species.Furthermore,we estimated the ancestral states of the trichome morphology and habit via the Bayesian Binary Method.The medifixed hair type is inferred to have developed at least five times and the annual habit originated at least six times.(3)Divergence times of eastern Asian AstragalusBased on the chloroplast genome data,we estimated the divergence time calibrated the published root age of Hologalegina and IRLC by BEAST program.Astragalus is estimated to have originated in the mid Miocene(stem age,16.09 Ma,95%HPD:12.46-20.50 Ma).The divergence times of the medifixed hair groups ranged from 4.03 to 0.87 Ma,mostly 2-1 Ma,which are correlated with the estimated phased uplifts of the Qinghai-Tibetan Plateau(QTP).We hypothesize that the uplifts of the QTP,which contributed to aridification in eastern Asia and the adjacent regions,may have accelerated the rapid speciation of Astragalus,especially the xerophilous groups(i.e.the medifixed hair groups).In addition,we repeated the BEAST program with nrDNA dataset and got similar results.The environment niche modeling results of representative medifixed hair species A.scaberrimus Bunge based on 291 occurrence records,which indicated the distribution of current expanded a lot compared with the LIG(Last inter-glacial)potential regions.Therefore,the environment niche modeling results also support the hypothesis mentioned above.(4)Astragalus verus Olivier(Fabaceae)-A New Record for the Flora of ChinaAstragalus verus Olivier is a new record collected from Diyag,Zhada County of Xizang Autonomous Region based on the morphology characters and molecular studies.This species is close to the species A.oplites Benth.ex R.Parker which distributed in China in morphology while the former species is different in some characters such as:densely woolly white hairs on calyx and lobes linear with five parts vs tube calyx with few white hairs;the panduriform standards vs the obovate standards.Based on the framework of plastid genome and nrDNA sequences,A.verus belonged to the Astracantha clade which is the belonged to Subg.Astracantha Bunge with high supportive value but far from the species A.oplites.It is also the first Astracantha clade species record founded in China.In summary,in this study we have reconstructed a high supportive phylogenetic tree with chloroplast genome and nrDNA sequences.On this basis,we estimated the divergence time of Astragalus and Hypoglottis clade;simulated the potential distribution of A.scaberrimus Bunge in the past,current and future by the environment niche modeling;estimated the ancestral states of the trichome morphology and habit.Furthermore,we assumed the last uplifting of QTP may accelerated the rapid speciation of Astragalus,especially the xerophilous groups.