牦牛的分类学地位及起源研究:mtDNAD-loop序列的分析

Taxonomic placement and origin of yaks:implications from analyses of mtDNA D-loop fragment sequences

牦牛的起源与属级分类学地位至今仍然存在一定的争议。我们测定了家养牦牛和野生牦牛线粒体控制区(D-loop)序列,并以此构建牦牛和牛属、野牛属、水牛属以及非洲水牛属相关种的系统发育树。研究结果表明线粒体D-loop区与Cytb基因序列在构建牛族的系统发育具有同样重要的价值。系统发育关系显示野牛属的灭绝种草原野牛与现存种美洲野牛先聚合为一单系群,然后再和牦牛形成一单系分支,表明牦牛与野牛属的草原野牛、美洲野牛亲缘关系最近,具有最近的共同祖先,而与牛属的其它亚洲物种亲缘关系较远。因此,本研究不支持将牦牛独立为牦牛属———Poephagus,牛属与野牛属在分类上也应合并为一个属。基于上述研究结果和化石证据,我们进一步对牦牛起源的历史背景进行了讨论,认为牦牛与野牛属的分化是由于第四纪气候变化在欧亚大陆发生的,野牛通过白令陆桥进入北美;冰期结束后,由于欧亚大陆其它地区温度升高,牦牛只能局限分布在较为寒冷的青藏高原;而野牛属在北美先后分化为草原野牛和美洲野牛,前者可能是后者的直接祖先。

The origin and generic placement of the yak （ Bos grunniens ） , a large ungulate endemic to Qinghai-Tibetan Plateau, remains unresolved. 637 hp of mitochondrial DNA control region of both wild and domesticated yaks were sequenced. We found that D-loop region sequences were similarly informative to commonly used cytochrome b gene in constructing phylogeny for tribe bovini. Based on the genetic variation of this sequence, we firstly constructed a more discernable phylogeny of Bovini with high support than previous analyses. The analyses suggested that in Bovini, two monophyletic lineages were recovered： one comprising Bubalus and the other consisting of all species of Bos and Bison. Two species of Bison, the extinct species B. priscus and the extant species B, bison, clustered as a monophyletic lineage which is a sister to Bos grunniens. These results suggested that yaks are more closely related to North American Bison than to other species of Bos in Asia, and that yaks and North American bison have a common recent ancestor. Our results did not support a previous suggestion of establishing a monotypic genus （Poephagus） to accommodate yaks. Because species of neither Bos nor Bison clustered as respective monophyletic lineages, these two should be taxonomically treated as one genus. Based on our results and other available fossil evidence, we discuss the historical context regarding the origin of yaks, The divergence between yaks and bison may have occurred in the Eurasia during the Quaternary climatic oscillation, when rising temperature following the ice ages restricted yaks to the cold Qinghai-Tibetan Plateau. Bison migrated to North America through the Bering Land Bridge and differentiated two species. It seems likely that American bison originated directly from extinct steppe bison.