基于线粒体基因组的沙蜥高海拔适应研究

High-altitude Adaptation of Genus Phrynocephalus Based on Mitochondrial Genome

高海拔地区的物种容易受到低温、低氧、强紫外辐射等极端环境因素的影响。研究这些物种对特殊环境的应对反应,能够为进一步理解适应进化的机制提供重要线索。线粒体是细胞的能量代谢中心,因此线粒体基因组很可能在动物高原适应中起着重要作用。鬣蜥科Agamidae沙蜥属Phrynocephalus物种广泛分布于海拔1000~5300 m范围内,是研究高原适应的良好材料。本研究对2种高海拔沙蜥和6种低海拔沙蜥的线粒体基因组进行了比较研究,检测了可能经历过正选择的蛋白编码基因,探讨了线粒体基因在沙蜥高海拔适应中的作用。结果发现,在不同物种间,高海拔西藏沙蜥Phrynocephalus theobaldi的线粒体基因组中蛋白编码基因的进化速率最快;在不同基因间,ATP8具有最快的进化速率。使用分支-位点模型进行正选择检测,发现ATP8基因在西藏沙蜥中存在明显的正选择信号（P〈0.05,ω〉1）。通过贝叶斯方法进一步计算每个位点的后验概率,发现在ATP8基因上存在2个正选择位点。这些结果说明ATP8基因可能在西藏沙蜥高海拔适应中起到了重要的作用。但在同为高海拔的青海沙蜥Phrynocephalus vlangalii中,却没有发现类似的正选择信号,这揭示不同物种高海拔适应的分子机制可能不同。

Organisms living in high altitudes are exposed to multiple stresses such as low temperature,hypoxia,high UV radiation and other extreme environmental factors. Understanding how high-altitude species cope with the combined effects of environmental factors can provide important insights into the process of adaptive evolution. Mitochondria are the energy factory in cells,and more than 95% of the energy in cells is produced by the mitochondrial oxidative phosphorylation（ OXPHOS）. Therefore,mitochondria likely plays an important role in the process of high-altitude adaptation. Lizard species of the genus Phrynocephalus（ Squamata,Agamidae） are widespread in central Asia and cross a large elevational range from1000 m to 5300 m. Using two high-altitude and six low-altitude Phrynocephalus species,the role of mitochondrial genome in high-altitude adaptation of ectotherms were tested in this study. The branch model analyses revealed that Phrynocephalus theobaldi had the largest ω value among species,followed by P. vlangalii. Among different genes,ATP8 had the largest ωvalue,and in P. theobaldi it was greater than 1,suggesting that ATP8 had experienced positive selection. In the branch-site model analyses,we found evidence of positive selection in ATP8 gene on the P. theobaldi branch（ P 〈0. 05）. Amino-acid residue sites 5 and 40 of the ATP8 genes were inferred as positively selected sites with posterior probabilities greater than95%. Our results suggested that the changes of mitochondrial genes likely played a critical role during the adaptation process to high altitudes for P. theobaldi. Surprisingly,no evidence for positive selection was detected in P. vlangalii,suggesting that species might use different molecular mechanisms in high-altitude adaptation.