Shaanxi Key Laboratory for Animal Conservation, College of Life Sciences, Northwest University, Xi'an, China.
Middle School Affiliated to Northwest University, Xi'an, China.
BMC Evol Biol. 2018 Mar 13;18(1):29. doi: 10.1186/s12862-018-1148-7.
Maintaining variation in immune genes, such as those of the major histocompatibility complex (MHC), is important for individuals in small, isolated populations to resist pathogens and parasites. The golden snub-nosed monkey (Rhinopithecus roxellana), an endangered primate endemic to China, has experienced a rapid reduction in numbers and severe population fragmentation over recent years. For this study, we measured the DRB diversity among 122 monkeys from three populations in the Qinling Mountains, and estimated the relative importance of different agents of selection in maintaining variation of DRB genes.
We identified a total of 19 DRB sequences, in which five alleles were novel. We found high DRB variation in R. roxellana and three branches of evidence suggesting that balancing selection has contributed to maintaining MHC polymorphism over the long term in this species: i) different patterns of both genetic diversity and population differentiation were detected at MHC and neutral markers; ii) an excess of non-synonymous substitutions compared to synonymous substitutions at antigen binding sites, and maximum-likelihood-based random-site models, showed significant positive selection; and iii) phylogenetic analyses revealed a pattern of trans-species evolution for DRB genes.
High levels of DRB diversity in these R. roxellana populations may reflect strong selection pressure in this species. Patterns of genetic diversity and population differentiation, positive selection, as well as trans-species evolution, suggest that pathogen-mediated balancing selection has contributed to maintaining MHC polymorphism in R. roxellana over the long term. This study furthers our understanding of the role pathogen-mediated balancing selection has in maintaining variation in MHC genes in small and fragmented populations of free-ranging vertebrates.
维持免疫基因的变异，如主要组织相容性复合体（MHC）的基因，对于小而孤立的种群中的个体抵抗病原体和寄生虫非常重要。金面猴（Rhinopithecus roxellana）是中国特有的一种濒危灵长类动物，近年来数量迅速减少，种群严重破碎化。在这项研究中，我们测量了秦岭山脉三个种群的 122 只猴子的 DRB 多样性，并估计了不同选择因素在维持 DRB 基因变异中的相对重要性。
我们共鉴定出 19 条 DRB 序列，其中 5 个等位基因是新的。我们发现，金面猴的 DRB 变异很高，有三个证据表明，平衡选择在长期以来一直有助于维持该物种 MHC 多态性：i）在 MHC 和中性标记中检测到不同的遗传多样性和种群分化模式；ii）在抗原结合位点检测到非同义替换相对于同义替换的过剩，最大似然随机位点模型显示出显著的正选择；iii）系统发育分析揭示了 DRB 基因的跨物种进化模式。
这些金面猴种群中高水平的 DRB 多样性可能反映了该物种强烈的选择压力。遗传多样性和种群分化模式、正选择以及跨物种进化表明，病原体介导的平衡选择有助于在长期内维持金面猴 MHC 多态性。这项研究增进了我们对病原体介导的平衡选择在维持自由生活脊椎动物小而破碎种群中 MHC 基因变异中的作用的理解。
