A Systematic Study Of Saussurea Sect. Cyathidium(Asteraceae)

The Qinghai-Tibet Plateau is one of the 34 biological hotspots in the world.It is not only rich in species but also has a high proportion of endemic species.But where and when did these species come from and the relationship between these species and the environmental changes in the Tibetan Plateau remain unclear.Clarifying these issues is a prerequisite for biodiversity conservation in this area,as well as a window to understand the relationship between biological evolution and the environment.A total of 71 species are classified into Saussurea sect.Cyathidium,of which 70 species are distributed in the Qinghai-Tibet Plateau and 53 species are unique to this region.This rare species diversity provides an ideal model to investigate these problems mentioned above.However,recent molecular phylogenetic studies have found that many intra-generic taxa of Saussurea are not natural.At the same time,differences among the species of this group mainly depend on quantitative characters such as plant height and leaf morphology,so it is difficult to delimitate species boundaries.Besides,a large number of species with the domain distribution and other factors make it extremely difficult to complete the classification and identification of this group.We conducted a large number of field investigation of the distribution area of the Saussurea sect.Cyathidium and collected the samples and materials of 33 species from 143 populations.We amplified and sequenced two fragments of the chloroplast genome,psbA-trnH and matK,and the ITS sequence of the nuclear genome.We constructed two sets of data.A group contains 233 ITS sequences,and the full-length is 679 bases,including 226 variation locus and 127 information sites.The other group includes a total of 136 chloroplast sequences,and full length is 1394 bp.The number of variable sites is 84 and the number of informative sites is 29.We used Bayes approach to analyze the two groups of data,and estimate the origin and differentiation time within the group by Beast based on ITS sequences.The main results and conclusions are as follows:1.Saussurea sect.Cyathidium is a polyphyletic group.The analysis of the ITS dataset suggested the members of Saussurea sect.Cyathidium were clustered with a representative of many other subgenus or groups with 99% of the Bayesian posterior probability containing 19 sub-branches,of which 10 sub-branches consisted of two or more species.There is no reliable relationship among different sub-branches.For the sake of later discussion,we defined this one as the generalized sect.Cyathidium sensu lato.Five species of the original Saussurea sect.Cyathidium were clustered,two species were clustered together with one of the other species,and the remaining 25 species were clustered into one,and the Bayes posterior probability was 94%.The sub-branch was analyzed in the chloroplast(96%).Therefore,we temporarily define it as a sect.Cyathidium sensu stricto.According to the principle of branch systematics,sect.Cyathidium sensu stricto should be defined as a group or other subgroups,but this treatment might cause that other sub-branches must be defined as the corresponding group,which might greatly lead to the increase of the number of subgroups and the complexity of the classification system.However,sect.Cyathidium sensu lato should not be defined as a subgroup,because this is composed of many groups or subgenus of Saussurea and the morphological difference is very significant.There are questions in many species of the botanical sector,such as the classification of legumes in the subfamily class.The molecular dataset of the branching analysis showed that the original three subfamilies should be reclassified into 6 to 15 subfamilies,but it is difficult to revise morphological characteristics of the resolution and definition,there is no classification scheme widely recognized.2.The environmental changes of the Qinghai-Tibet Plateau have promoted multiple radiological differentiation of the feather group and the related taxa.There are a large number of divergent branches in group of sect.Cyathidium sensu lato and sect.Cyathidium sensu stricto,revealing that these branches are the result of rapid differentiation.The molecular clock estimated the differentiation time of the two groups,which was 8.4_MYA and 3.6_MYA respectively.There are two intense uplifting in the Qinghai-Tibet Plateau in these two periods.We speculated that the first uplift may promote the differentiation of the various branches in the generalized sect.Cyathidium sensu lato.Because of the earlier occurrence,the morphological differences of these branches are very significant.S.andersonii was members of sect.Depressae because of the lack of the receptacles.S.picridifolium was independent genus because of the two layer feathery pappus.The second uplift promoted the differentiation of sect.Cyathidium sensu stricto.As a result of the late occurrence,there was a lot of controversy over the division of this branch,such as S.kansuensis and S.subulisquama.Leaves of the former are long oval with deep pinnate and serrated lobes edge.Whereas,the latter's leaves are lanceolate and plump,and lobes are full.But there are many transitional types between the two species.We found a number of individuals with different types and characteristics of the transitional species.We found that there were significant genetic differences among different individuals,but the branching methods of both the core ITS and the chloroplast were inconsistent with the morphological identification.There are similar cases in the majority of sect.Cyathidium sensu lato,there is often a transitional type between species,which leads to difficulties in classification and identification.However,it is difficult to use the molecular dataset for classification.For these groups with radiated differentiation,classification revisions might require molecular markers with higher mutation rates or genes that are directly used to identify related species.In summary,we advised that the rich species diversity of the feather group is mainly from the differentiation of the local group,which mainly occurs in two periods and closely related to the uplift of the Qinghai-Tibet Plateau.Radiation differentiation and close differentiation time make it very difficult to classify and identify this group.