6种鸡形目和雁形目禽类Mx蛋白编码基因的适应性进化

Adaptive Evolution of Six Galliform and Anseriform Avians Mx Protein Coding Genes

为进一步了解禽类Mx基因(myxo-virus resistance gene)进化历程,结构与功能变异及筛选新的潜在抗性位点。根据GenBank中已报道的6种鸡形目(galliform)和雁形目(anseriform)禽类Mx基因编码序列,联配后进行最适核苷酸替代模型筛查,序列重组事件和核苷酸替换的饱和性检验;并采用最大似然法检测各位点承受的选择压力。AIC信息量准则检验结果表明6种鸡形目和雁形目禽类Mx基因的最优核苷酸替代模型为"GTR+G",核苷酸替换的饱和性检测结果表明序列的核苷酸替换并未饱和。单一断裂点(single break-point)和遗传算法扫描(GARD)在联配序列中发现两个断裂点,分别位于162bp和999bp处,将联配序列分为3个独立非重组部分。密码子置换模型的"位点特异模型"检测到禽类Mx基因编码序列承受了正选择作用,似然比检验(LRT)确定模型M2a和M8为正选择位点优先检测模型(P<0.05,P<0.01),分别在3部分非重组序列中检测到4个、1个和3个后验概率大于95%的正选择位点,正选择位点主要分布于禽类Mx蛋白特有的N-端结构域和Mx蛋白抗病毒作用所必须的三联GTP酶结合区,可能与Mx蛋白的抗病毒活性有关。

The objective of this study was to trace the route of avian Mx（myxo-virus resistance） gene evolution,knowledge of its structure and function variations and select new antiviral sites.Mx gene cDNA sequences of six galliform and anseriform avians submitted in GenBank were used in this study.mrModeltest was used to select the best nucleotide substitution model.Datamonkey and DAMBE were used to detect recombination events and test nucleotide substitution saturation.PAML4b were used to test the selective pressure on amino acid sites.The best nucleotide substitution model selected by mrModeltest for avian Mx gene sequences was＂GTR＋G＂.Saturation test did not indicate any sign of substitution saturation.Single breakpoint scanning and genetic algorithm scanning by Datamonkey found two breakpoints,located respectively in 162bp and 999bp,and divided the sequences into three nonrecombination split partitions.The selection test of＂site-specific model＂showed that avian Mx gene sequences had suffered positive selection pressure.Likelihood ratio test suggested M2a and M8 as more advantageous models,and four,one and three positive sites with more than 95% posterior probability were identified in three nonrecombination partitions.The detected positive sites mainly distributed in the avian specific N-ternimal end of Mx protein and GTP-binding domain necessary for antiviral activity,and may be related to the antiviral activity of Mx protein.