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

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 f