基于细胞色素b基因的中国岩羊不同地理种群遗传差异分析

Genetic diversity based on cytochrome b gene analysis of different geographic populations of blue sheep in China

为了揭示中国岩羊不同地理种群的遗传差异,探讨岩羊亚种分化的分子机制,采用中国岩羊不同地理种群的细胞色素b(Cyt b)基因的全序列,分析了碱基变异情况、遗传距离以及核苷酸序列差异。用最大似然法和贝叶斯法构建分子系统树并对获得的拓扑结构进行分析。结果发现,西藏亚种与四川亚种Cyt b基因平均序列差异为4.2%(±0.007),处于偶蹄目亚种的序列差异范围内,支持了目前对岩羊西藏亚种的分类地位。四川亚种内部各地理种群之间的遗传距离(0.033±0.0 111)与它们分别到西藏种群的遗传距离(0.042±0.007)差异不显著(t=1.824,P=0.084),说明四川亚种内部各地理种群间已经发生较显著的遗传分化。其中,四川、甘肃和青海种群亲缘关系较近,并与四川亚种内部的其它种群已产生了显著的遗传分化。因此认为四川亚种内部各地理种群的种下分化需要深入研究。

Blue sheep（Pseudois nayaur） are endemic to the Tibetan Plateau and surrounding mountain regions.They are classified as Least Concern in the IUCN Red List of Threatened Species and are included in the list of the second grade nationally protected animals of China.Traditionally,blue sheep in China were classified into two subspecies,the nominate or Tibetan subspecies（P.n.nayaur） which is distributed on the Tibetan Plateau,and the Sichuan subspecies（P.n.szechuanensis） which has a widespread geographical distribution in the western and north-western areas of China.Different geographically distributed populations of the Sichuan subspecies are also found on several isolated mountains in China.Because of the lack of research data,the genetic validity and geographic characteristics of these two subspecies remain unclear.In the present study,we have sequenced the complete mitochondrial DNA cytochrome b（Cyt b） gene from tissue samples of blue sheep originating from seven distinct geographical populations（Qinghai,Sichuan,Gansu,Inner Mongolia,Ningxia,Xinjiang,and Tibet） covering all regions in which the sheep are distributed.Our aim was to measure the genetic divergence among these populations,and to analyze the base variation,genetic distance,and nucleotide sequence variability between the different populations.A total of 45 haplotypes from 59 cyt b gene sequences were identified.Only one of these haplotypes was shared by individuals from the Ningxia and Inner Mongolia populations.A maximum likelihood tree and a Bayesian tree were established using the Kimura2-parameter model to evaluate the genetic distance between samples from each pair of different geographical populations.We found that the genetic distances between the Tibetan populations（nominate subspecies） and the other geographical populations（Sichuan subspecies） were on average,4.2%（±0.007）.This value is higher than the average level of Cyt b sequence divergence among the different populations from the Sichuan subspecies and higher than the reported genetic divergence of five known Artiodactyla animals.Therefore,we propose that,based on the mitochondrial DNA Cyt b gene,the sheep that are distributed on the Tibetan plateau do indeed form a valid subspecies of blue sheep.A molecular clock calculation estimated that the two subspecies might have separated approximately 1.05 million years ago.The average genetic distance within different geographical populations of the Sichuan subspecies（0.033 ± 0.0111） was not significantly different from the average genetic distance（0.042 ± 0.007） between populations of the two subspecies（t=1.824,P=0.084）.This result indicates that significant genetic divergence had occurred among populations of the Sichuan subspecies.For example,the Helan Mountain population which includes individuals from Ningxia and Inner Mongolia,and the Xinjiang population both have significant genetic divergence compared with the populations from Sichuan,Gansu,and Qinghai,which have a much closer genetic relationship within themselves.Phylogenetic analyses between the 45 haplotypes based on Bayesian Inference and Maximum Parsimony revealed different maternal inheritances that corresponded to different geographical distribution regions.The Gansu,Qinghai,and Sichuan populations formed a cluster,indicating a close genetic relationship between them.The Ningxia population and the Inner Mongolia population from the Helan Mountain also grouped together;the Tibetan population formed a monophyletic group.The Xinjiang population and a Tibetan population（RT1） tended to cluster together,which was sister group of the other three groups.Further studies are needed to help improve our understanding of the genetic composition and geographical characteristics of the Sichuan subspecies of blue sheep.