Major histocompatibility complex class II variation in bottlenose dolphin from Adriatic Sea: inferences about the extent of balancing selection

The bottlenose dolphin (Tursiops truncatus) is the most common cetacean species worldwide and the only marine mammal species resident in the Croatian part of the Adriatic Sea. To gain insight into genetic diversity of bottlenose dolphins at adaptively important loci relevant to conservation, we analysed the polymorphism of major histocompatibility complex (MHC) genes, which play a key role in pathogen confrontation and clearance. Specifically, we examined the diversity of MHC class II DRA, DQA and DQB alleles in 50 bottlenose dolphins from the Adriatic Sea collected between 1997 and 2011 and in 12 animals from other Mediterranean locations. Notable variation in DQA, DQB and three-locus haplotypes was found, with all 10 DQA and 12 DQB alleles encoding unique protein products. Analysis of the ratio of non-synonymous to synonymous substitution rates suggests that positive selection acts at both highly variable loci. Phylogenetic analyses revealed trans-species polymorphism at the DQB locus...

宽吻海豚（Tursiops truncatus）是全球分布最广的鲸类（cetacean）物种，亦是亚得里亚海克罗地亚海域唯一栖息的海洋哺乳动物物种。为探究宽吻海豚在与保护相关的适应性重要基因座上的遗传多样性，我们对主要组织相容性复合体（major histocompatibility complex）基因的多态性展开了分析——该类基因在病原体识别与清除过程中发挥关键作用。具体而言，我们对1997年至2011年间采集自亚得里亚海的50头宽吻海豚，以及来自地中海其他区域的12头个体的MHC II类DRA、DQA和DQB等位基因多样性进行了检测。研究发现DQA、DQB基因座及三基因座单倍型（haplotype）存在显著变异，所有10种DQA等位基因与12种DQB等位基因均编码独特的蛋白质产物。对非同义替换率与同义替换率比值的分析表明，这两个高度变异的基因座均受到正选择作用。系统发育分析显示，DQB基因座存在跨物种多态性（trans-species polymorphism）……