Selection pressure from parasites is thought to be a major force shaping the extreme polymorphism of the major histocompatibility complex (MHC) genes, but the modes and consequences of selection remain unclear. Here, we analyse MHC class II and microsatellite diversity in 16 guppy (Poecilia reticulata) populations from two islands (Trinidad and Tobago) that have been separated for at least 10 ky. Within-population MHC diversity was high, but allele sharing was limited within islands and even lower between islands, suggesting relatively fast turnover of alleles. Allelic lineages strongly supported in phylogenetic analyses tended to be island-specific, suggesting rapid lineage sorting, and an expansion of an allelic lineage private to Tobago was observed. New alleles appear to be generated locally at a detectably high frequency. We did not detect a consistent signature of local adaptation, but FST outlier analysis suggested that balancing selection may be the more general process behind spatial variation in MHC allele frequencies in this system, particularly within Trinidad. We found no evidence for divergent allele advantage within populations, or for decreased genetic structuring of MHC supertypes compared to MHC alleles. The dynamic and complex nature of MHC evolution we observed in guppies, coupled with some evidence for balancing selection shaping MHC allele frequencies, are consistent with Red Queen processes of host-parasite coevolution.

中文翻译：

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平衡孔雀鱼 MHC II 类基因的选择与等位基因和超型转换

寄生虫的选择压力被认为是塑造主要组织相容性复合体 (MHC) 基因的极端多态性的主要力量，但选择的模式和后果仍不清楚。在这里，我们分析了来自两个岛屿（特立尼达和多巴哥）的 16 只孔雀鱼 (Poecilia reticulata) 种群的 MHC II 类和微卫星多样性，这些岛屿已经分离了至少 10 ky。种群内 MHC 多样性很高，但等位基因在岛屿内的共享有限，岛屿之间的共享率更低，这表明等位基因的更新相对较快。系统发育分析强烈支持的等位基因谱系往往是岛屿特异性的，表明快速的谱系分类，并且观察到多巴哥私有的等位基因谱系的扩展。新的等位基因似乎以可检测的高频率在局部产生。我们没有检测到局部适应的一致特征，但 FST 异常值分析表明，平衡选择可能是该系统中 MHC 等位基因频率空间变异背后更普遍的过程，特别是在特立尼达境内。我们没有发现任何证据表明人群中存在不同的等位基因优势，或者与 MHC 等位基因相比，MHC 超型的遗传结构减少。我们在孔雀鱼中观察到的 MHC 进化的动态和复杂性质，加上一些平衡选择塑造 MHC 等位基因频率的证据，与宿主-寄生虫共同进化的红皇后过程一致。我们没有发现任何证据表明人群中存在不同的等位基因优势，或者与 MHC 等位基因相比，MHC 超型的遗传结构减少。我们在孔雀鱼中观察到的 MHC 进化的动态和复杂性质，加上一些平衡选择塑造 MHC 等位基因频率的证据，与宿主-寄生虫共同进化的红皇后过程一致。我们没有发现任何证据表明人群中存在不同的等位基因优势，或者与 MHC 等位基因相比，MHC 超型的遗传结构减少。我们在孔雀鱼中观察到的 MHC 进化的动态和复杂性质，加上一些平衡选择塑造 MHC 等位基因频率的证据，与宿主-寄生虫共同进化的红皇后过程一致。