Distributional Dynamics Of Psammochloa Villosa (Poaceae) In Response To Climate Change

The hydrology and climate in northwestern China have been profoundly shaped throughout the Cenozoic by geological and historical processes such as the uplift of the Qinghai-Tibet Plateau and other orogenic movements as well as Quaternary climatic oscillations.In particular,these processes have dramatically accelerated aridification and desert expansion,which have subsequently affected the spatial genetic structures,geographical distribution patterns,and evolutionary histories of plants in the region.However,the specific responses of desert plants in the arid regions of northwest China to mountain building and Quaternary climatic change remains unclear due to the lack of fossils,pollen,and other related materials.Therefore,I used the grass species,Psammochloa villosa（Poaceae）,as a model to investigate the relationship between species distribution and geological and climatic processes on an evolutionary time scale,and to provide a scientific basis for ecological restoration,control of desertification,and future development of wild forage germplasm resources for the arid desert region of northwestern China.Psammochloa villosa is a desert herb endemic to the Inner Mongolian Plateau and its adjacent regions,and it exhibits strong adaptation to its arid environment.Thus,this species is an ideal system for studying the evolutionary history of adaptation of plants to the dynamic climatic and geological history of these arid environments,which are relatively geologically young,and to mine genes related to drought resistance.In this ecologically keystone species,I took 45 populations of P.villosa to analyzing the distribution of genetic diversity,spatial genetic structure,evolutionary history,potential glacial refugia,migration routes,and modern organization of biodiversity based on amplified fragment length polymorphism（AFLP）,multiple sets of nuclear gene loci（n DNA）,microsatellites（SSRs）,and ecological niche modeling.The main results are as follows:⑴Effects of environmental factors on genetic diversity of P.villosaIn this part of my study,I analyzed the genetic diversity and genetic differentiation of P.villosa using AFLPs and low copy nuclear genes.Based on these,I found that the Nei’s genetic diversity index（h=0.21）of P.villosa was significantly higher than the average value of monocotyledons（h=0.19）,indicating that the species had rich genetic diversity and strong adaptability to its environment.The total genetic diversity（Ht）and intra-population genetic diversity（Hs）of P.villosa showed that its genetic diversity existed within populations,and this many be linked to its clonal mode of propagation and its growth characteristics.Subsequently,the genetic differentiation coefficients(F_ST)among populations of P.villosa were 0.358 and0.754,and were higher than 0.25,indicating that there was high genetic differentiation.Ecological factors had a certain impact on the genetic diversity of P.villosa.There was a significant positive correlation between altitude and geographical distance and genetic differentiation coefficient(F_ST)（r=0.37,P=0.02;r=0.360,P=0.01）.It was considered that altitude and geographical distance were the main factors affecting the genetic structure of P.villosa.⑵Geographic distribution patterns and evolutionary history of P.villosaThe genetic structure of P.villosa was analyzed with the method for UPGMA,SAMOVA,PCo A and Structure.It was found that there was an obvious pedigree geographical structure(G_ST<N_ST,P<0.05),and was divided into two clusters（Group1 and Group 2）bounded by Yinshan Mountains.The reason for this result was the unique geographical location,altitude and mountain trend of Yinshan Mountains,there are significant differences in climate on both sides of the mountains,resulting in significant differences in flora and vegetation distribution on both sides of Yinshan Mountains.In addition,the detection of gene flow barrier found that gene flow among populations of P.villosa flowed from Group 2 to Group 1,and the genetic barriers were mainly caused by the Yinshan Mountains,Helan Mountains,Ordos Plateau and Yabulai Mountains.The current effective population size of P.villosa was 0.50 times that of its ancestors（r<1）.The mismatch distribution curve showed a bimodal curve,which was consistent with the results obtained from Tajima’s D（2.922,P<0.01）,Fu and Li’s D*（2.425,P<0.01）,Fu and Li’s F*（3.198,P<0.01）,indicating that the population of P.villosa might contracted in history.We speculated that the uplift of the Qinghai-Tibet Plateau caused the changes of mountains and deserts,such as Yin Mountain,Helan Mountain,Mu Us Sandy Land,Tengger Desert and Badain Jaran Desert,in the region to form spatial isolation,and the narrowing of population distribution and habitat fragmentation caused by Quaternary climate oscillation.⑶Effects of paleoclimatic and geological historical events on the evolutionary history of P.villosaIt was inferred that the divergence of Group 2 from the ancestral population occurred 6.00 Ma ago,while the divergence of Groups 1 and 2 occurred 0.38 Ma ago.My results suggest that this species might have originated in Vallesian,and populations might have begun to diverge during the Pleistocene.The reason for this result was that the sharp changes in the natural environment caused by the violent climate turbulence since the end of the Tertiary or Quaternary,which led to the initial differentiation of the continuous distribution of the ancestral population of P.villosa.Along with the uplift of Qinghai-Tibet Plateau and the formation of monsoon climate,the northwest arid climate began to strengthen in the period of about 7 Ma and 2～4Ma,geological and historical events such as desert expansion and habitat fragmentation prompted new selective mutations of P.villosa to adapt to changes in the living environment.⑷Responses of P.villosa to climate changeWe analyzed the potential distribution area of P.villosa under different climatic conditions based on the Max Ent model in this study.The results showed that the contribution rate of altitude to the potential distribution of P.villosa was the largest（40%）,which was considered to be the most important environmental factor affecting the potential distribution area of this species.During the last interglacial period（LIG）,P.villosa was continuously distributed in the arid and semi-arid regions of Northwest China.However,the most suitable distribution areas of P.villosa had continuously shrunk from the last interglacial period（LIG）to the present.The most suitable distribution areas were mainly located in the Badain Jaran Desert,Tengger Desert,Mu Us Sandy Land,Hunshandake Desert and the western edge of the Tianshan Mountains,etc.,which were consistent with the research results using molecular data.The geometric center of the suitable distribution area of P.villosa migrated to the southeast from the last interglacial period to the present,indicating that this species might have experienced a dynamic trend of continuous contraction in the history.Supplemented with the evidence of principal component analysis（PCA）for environmental variables and niche consistency,it was found that there was significant niche differentiation between Group 1 and Group 2（P<0.01）.Altitude was the main factors leading to niche differentiation between the two geographical groups of P.villosa.In this study,the conclusion of this study supported that the third uplift of the Qinghai-Tibet Plateau has exacerbated the drought and desert expansion in Northwest China macroscopically,while the complex climate change of Quaternary has caused significant indigenous differences in the natural environment of arid areas in Northwest China microscopically.This provides a typical case for revealing the impact of Quaternary climate change on the genetic diversity and evolutionary history of herbaceous in the arid desert regions of Northwest China in the future.