两种鹤科动物MHC-Ⅰ基因高变区的对比分析

主要组织相容性复合体（MHC）是脊椎动物基因组中高度多态的基因家族，其编码产物在脊椎动物免疫系统中起着重要作用。受湿地生境污染和破坏等因素的影响，全球现存鹤科（Gruidae）动物大都已处于受胁状态。为了解鹤科动物MHC-I基因序列信息，本研究设计通用引物对灰鹤（Grus grus）和肉垂鹤（Bugeranus carunculatus）MHC-I基因进行分离。结果从1只灰鹤和1只肉垂鹤血液基因组中分别分离到2条和3条长约1 500 bp的序列片段，这暗示鹤科动物至少存在两个MHC-I基因座位。分离到的核苷酸序列均可翻译成正常的氨基酸，表明它们具有一定的生物学功能。MHC-I抗原结合区的核苷酸和氨基酸变异率，灰鹤分别为5.0%和9.6%，肉垂鹤为9.1%和14.6%。两种鹤抗原肽结合位点的非同义替代率与同义替代率的比值分别为7.348 8和2.145 2，表明其受到强烈的正选择作用。贝叶斯系统发生树显示，鹤科动物MHC-I基因并未按物种聚类，暗示MHC-I基因跨物种多态性的存在。本研究获得的MHC-I基因通用引物及序列信息，可为今后濒危鹤科动物的保护遗传学研究奠定基础。

Comparative Analysis of the Hypervariable Region of MHC-I Gene in Two Crane Species

Major histocompatibility complex (MHC) is a highly polymorphic gene family in the vertebrate genome, and its coding products play an important role in the vertebrate immune system. Most of the world''s surviving crane (Gruidae) species are threatened due to habitat pollution and destruction. To understand the information of MHC-I sequences of cranes, universal primers were designed to isolate and clone MHC-I sequences from genomic DNA extractions from blood of a common crane (Grus grus) and a wattled crane (Bugeranus carunculatus) in captivity. Two and three sequences of 1 500 bp were isolated respectively from the genomes of G. grus and B. carunculatus, suggesting the existence of at least two MHC-I loci in cranes. All nucleotide sequences could be translated into normal amino acid sequences, indicating that they might execute certain functions (Fig. 1). The mutation rates of nucleotide and amino acid in the antigen binding region were 5.0%, 9.6% for G. grus and 9.1%, 14.6% for B. carunculatus, respectively (Table 2). The antigen binding sites were subject to strong positive selection, as the values of dN/dS were 7.348 8 and 2.145 2 for G. grus and B. carunculatus, respectively (Table 3). Bayesian trees showed that MHC-I sequences did not cluster according to crane species, indicating trans-species polymorphism of MHC-I genes in cranes (Fig. 2). The universal primers and sequences of MHC-I genes obtained here could lay a foundation for further study of conservation genetics of endangered cranes in the future.