Genetic Diversity And Structure Of Three Species Of Schizothoracinae In Tibet Section Of Nujiang River

The Tibet section of Nujiang River is located in the southeast of Qinghai-Tibet Plateau（QTP）,with complex ecological environment and unique geographical characteristics.The diverse ecological environment and rich waters breed unique aquatic biological resources.Schizothoracinae fishes are one of the main fishes in the QTP,and it is also an important component of the fish in the Tibet section of Nujiang River,and many of them are endemic to China.In recent years,under the background of environmental pollution,hydraulic engineering construction and alien species invasion and other human activities,the fish resources in Nujiang River have been significantly affected.Schizopygopsis thermalis,Ptychobarbus kaznakovi and Schizothorax nukiangensis are important fishes in the Tibet section of Nujiang River.Genetic research will help to understand their genetic diversity and genetic structure,and provide technical support for the protection of fish resources in Nujiang River.The three Schizothoracinae fishes were collected in the Tibet section of Nujiang River from 2017to 2020.The population genetics of these three Schizothoracinae fishes were studied by using mitochondrial control region（CR）,mitochondrial cytochrome c oxidase I gene（COI）and nuclear recombinase activating gene 1/2（RAG1/2）.The main results are as follows:1.Population genetics of Schizopygopsis thermalisA total of 207 Schizopygopsis thermalis were collected from 8 sites（Zuogong,Basu,Mali,Bianba,Zhongnai,Yadong,Naqu and Luoma）in the Tibet section of Nujiang River.Population genetics of these fish were studied by using CR and RAG1sequences.The results showed that 92 haplotypes were defined in 207 CR sequences（710 bp）.The haplotype diversity（h）and nucleotide diversity（π）were 0.968 and0.01021,respectively.A total of 71 haplotypes were defined in 177 RAG1 sequences（1441 bp）with h=0.959 andπ=0.00341.Both markers showed high levels of genetic diversity.Among populations,the genetic diversity of Basu（BS）population was relatively low,and there was little difference in genetic diversity among other populations.Phylogenetic analysis showed that Zuogong（ZG）population,from a tributary,was genetically distinct from the others.Divergence time was estimated to be approximately at 0.71 Mya,which occurred in the Pleistocene and was closely related to the Kunlun-Huanghe movement（1.1～0.6 Mya）.A significant correlation was revealed between the altitude and F_ST for populations from mainstream.Neutrality tests and Bayesian skyline plot（BSP）analyses indicated that population had experienced historically expansion,and the expansion time was estimated at 0.037 Mya（CR based）and 0.028 Mya（RAG1 based）,being in the last glacial period.2.Genetic diversity and genetic structure of Ptychobarbus kaznakoviA total of 210 individuals were collected from 9 sites in the Tibet section of Nujiang River and Chaya of Lancang River,and their population genetics was studied by using CR and COI gene sequences.The results showed that 33 haplotypes were defined in CR（695 bp）and COI（605 bp）combined CR+COI（1300 bp）,and h=0.846,π=00077.Compared with other Schizothoracinae fishes,the genetic diversity of Ptychobarbus kaznakovi was lower.The results of the molecular variance analysis（AMOVA）showed that the total population showed great genetic differentiation.Pairwise F_ST,phylogenetic analysis and median joining network showed that there was a grate genetic differentiation among Zuogong（ZG）population,Chaya（CY）population and 7 populations in the main stream of the Nujiang River,forming three branches with high support rate.Neutrality tests and BSP analysis on each branch showed that the ZG population was declining,while the population of CY was stable.Population expansion occurred in the population in the main stream of the Nujiang River,and the time of expansion was 0.0025 Mya,which was at the end of the Last Glacial Age.3.Genetic diversity and genetic structure of Schizothorax nukiangensisA total of 115 Schizothorax nukiangensis were collected from Chawalong,Basu,Mali,Bianba and Biru in the Tibet section of Nujiang River.The population genetics of Schizothorax nukiangensis was studied by using CR,COI and RAG2 sequences.The results showed that there were 34 haplotypes（h=0.794,π=0.00236）in the 115 combined CR+COI（1317 bp）sequences of CR（746 bp）and COI（571 bp）genes.Six haplotypes were defined in 101 RAG2（1206 bp）sequences and h=0.653,π=0.00071.The genetic diversity of mitochondria and nuclear genes was low.AMOVA showed a certain degree of genetic differentiation,and the phylogenetic tree and median joining network formed two branches with high support,but did not show obvious geographical structure characteristics.Among populations,F_ST was significantly correlated with geographic distance and altitude.Neutrality tests and BSP analyses showed that Schizothorax nukiangensis had a recent population expansion,which was about 0.01 Mya,and was at the end of the Last Glacial Age.Among the three Schizothoracinae fishes,the genetic diversity of Schizopygopsis thermalis was the highest,while Ptychobarbus kaznakovi and Schizothorax nukiangensis were relatively low genetic diversity.The genetic structure analysis of Schizopygopsis thermalis and Ptychobarbus kaznakovi showed that there was significant genetic differentiation between the Zuogong population located in the longest tributary of the Nujiang River and other populations.It was suggested that Zuogong population should be protected as a significant evolutionary unit,and the population and its habitat should be protected.