白鹭MHCⅡDABⅠ基因第二外显子的多态性与进化

Polymorphism and Evolution of MHCⅡDABⅠ Gene Exon2 in Little Egret(Egretta garzetta)

克隆测序白鹭(Egretta garzetta)5个种群138份个体组织样本的主要组织相容性复合体Ⅱ类B基因(MHCⅡDABⅠ)第二外显子(exon2)序列,分析探讨exon2的多态性、进化选择、系统关系和种群遗传结构.主要结果如下:白鹭MHCⅡDABⅠexon2序列长度为270bp,共计定义了139个等位基因;序列分析显示exon2有101个核苷酸变异位点(37.4%)和31个氨基酸变异位点(34.4%);基于贝叶斯法构建的系统树显示白鹭MHCⅡDABⅠexon2有5个高支持率的谱系;肽结合位点(PBR)、非肽结合位点(non-PBR)的非同义替换率(dN)和同义替换率(dS)比值计算显示,PBR的dN/dS为1.99(p<0.05),而non-PBR的dN/dS则略小于1(p>0.05),表明白鹭MHCⅡDABⅠexon2受到正选择作用;根据等位基因在群体中的分布频率作分子方差分析(AMOVA),得到群体间分化指数(FST)为0.194 1(p<0.000 1),提示白鹭MHCⅡDABⅠexon2存在显著的种群遗传结构分化.

The DNA fragments of major histocompatibility complex class Ⅱ B gene （MHC Ⅱ DABI） exon2 alleles from 138 individual samples in 5 populations of little egret （Egretta garzetta） were cloned and sequenced to investigate their polymorphism,evolution selection,phylogenetic and population genetic structures.The main results were as follows., sequence of MHC II DAB1 exon2 of little egret was 270 bp in length, and a total of 139 alleles were defined in the exon2 ：sequence analyses indicated that exon2 genes had 101 nucleotide acid variation sites （37.4 %） and 31 amino acid variation sites （34.4 %） ;the Bayesian phylogenetic tree showed that there were 5 distinct lineages with high bootstrap values in the exon2;when the proportion of synonymous substitution rate （ds） to non-synonymous substitution rate （d N ） was calculated for peptide binding residues （PBR） or non-PBR of MHC Ⅱ DAB I exon2, the d N/ds in PBR was 1.99 （p 〈0. 05）, whereas the ratio in non-PBR was a bit lower than 1 （p2〉0. 05）,implying that positive selection was acting on the MHC ⅡDAB f exon2 in the little egret;analyses of molecular variance （AMOVA） based on allele distribution frequencies in different populations showed that FsT value was 0. 194 l（p〈0. 000 1） ,suggesting that there was significant population structure differentiation of MHC Ⅱ DAB I exon2 in little egret.