The Genetic Structure And Adaptive Evolution Of Isoetes On Hengduan Mountain Regions Under The Geographic And Climatic Influences

Isoetes L.is the only survival genus of Isoeteceae.It originates from Late Devonian,experiences the whole evolutionary history of vascular plants,and occupies an important position in plant evolution.Nowadays,there are 6 species of Isoetes in China,Isoetes hypsophila,Isoetes shangrilaensis,Isoetes yunguiensis,Isoetes sinensis,Isoetes orientalis and Isoetes taiwanensis.Among them,I.hypsophila and I.shangrilaensis are distributed in Hengduan Mountains.Owing to the geographic and climatic particularity,the Qinghai-Tibetan Plateau(QTP)possesses the diverse and rich ecosystems,and is one of the most popular region to study alpine plants.Moreover,the evolutionary history of Chinese Isoetes is believed to closely relate to the uplift of Tibetan Plateau and the retreat of the old Tethys.In this research,we focused on Isoetes hypsophila and Isoetes shangrilaensis.By using Chloroplast DNA,Restriction site-associated DNA sequencing(RADseq),and Ecological Niche Modeling(ENM),we would like to explore how the geological history and climate changes of the Hengduan Mountain Regions affect the genetic structure and adaptive evolution of Isoetes hypsophila and Isoetes shangrilaensis.Eight chloroplast DNA fragments were selected to amplify.The sequences were grouped by the function or the position in the chloroplast genome and then combined separately.The concatenated sequences were used to construct phylogenetic trees,analyze the sequence characteristics,make AMOVA analysis and conduct neutral tests.The intergenetic spaces can be the best choice to distinguish the variations within the species,because these regions are not under selection and their genetic diversity are large.The introns are relatively conserved and can be an effective fragment to distinguish the interspecific differences.The protein coding genes can be employed to delve into the evolution of gene function.The results showed that the evolutionary rates of the intergenetic spaces and the protein coding genes were higher than that of the intron,and the evolutionary rate of the single copy region was faster than that of the reverse repetition region in both I.hypsophila and I.shangrilaensis.But,in general,the evolutionary rate of chloroplast DNA is still slow.They can make a good distinction between species,nonetheless,the ability to distinguish within species is limited.The SNPs of 33 individuals were obtained using the RADseq.Based on these information,the kinship tree was constructed,PCA(Principle component analysis),genetic structure analysis were conducted and Fst values were calculated.The results showed that the individuals from GH population of I.hypsophila and the samples of I.shangrilaensis were gathered on a branch,while the individuals from the other populations of I.hypsophila were clustered on the other branch,where JL,MK and DR2 located at the relative basal position.In PCA,the individuals from GH population were clearly separated from other Isoetes;all samples from I.shangrilaensis were gathered together;and the other I.hypsophila were grouped together.In the structure analysis,when k = 2,GH and I shangrilaensis were grouped together;when k is greater than 2,GH was always grouped separately.The results of Fst value showed that the genetic variations within I.shangrilaensis were small while the variations between I.shangrilaensis and I.hypsophila were great.The GH was significantly different with the other populations of I.hypsophila,and there existed the apparent genetic separation.Among other populations of I.hypsophila,JL,HY possessed relatively large variations with the rest populations,while the genetic variations were relatively small among BY and DC.It is presumed that the evolution of the Tethyan ocean and the uplift of the QTP may play a significant role on the genetic structure of I.hypsophila and I.shangrilaensis.The apparent genetic separation of GH is likely caused by the formation of modern water system of Jinsha River(the first bend of Yangtze River).The significant gene flow among other populations may be related to the planation during the uplift of the QTP.The distributions of I.hypsophila and I.shangrilaensis in different periods,present,Last Glacial Maximum(LGM)and Last interglacial(LIG)were estimated.The model worked well as the AUC values were greater than 0.75.These two Isoetes first expanded and then contracted in general.In the initial LIG period,the distributions are small,and after a long period of interglacial dispersal,they expand a larger distribution.In the initial LGM period,they reach the maximum range,and then the LGM reduces their distribution.It was found that there was ecological niche overlap between.hypsophila and I.shangrilaensis.The environmental similarity is likely to bring out the evolutionary convergence in their morphology.The populations of GH and the two populations of I.shangrilaensis were similar in Climatic PCA,and in the phylogenetic tree,they were distributed in the same branch(close genetic distance).The JL,HY and DR2 were distinct from other populations in Climatic PC A,and in the phylogenetic tree,they were located in the basal position(far genetic distance).These results indicate that environmental selection may drive the response of the genetic structure.This study lays the foundation for the further exploration of plant adaptive evolution in the Hengduan Mountains,and provides a basis for further examining the mechanism of speciation.