School of Biological and Chemical Sciences, Queen Mary University of London, London, E1 4NS, UK.
Department of Human and Animal Physiology, Faculty of Biology, National and Kapodistrian University of Athens, Panepistimioupoli Zografou, 15784, Athens, Greece.
Heredity (Edinb). 2019 Apr;122(4):468-484. doi: 10.1038/s41437-018-0146-2. Epub 2018 Sep 26.
The major histocompatibility complex (MHC) has long served as a model for the evolution of adaptive genetic diversity in wild populations. Pathogen-mediated selection is thought to be a main driver of MHC diversity, but it remains elusive to what degree selection shapes MHC diversity in complex biogeographical scenarios where other evolutionary processes (e.g. genetic drift and introgression) may also be acting. Here we focus on two closely related green lizard species, Lacerta trilineata and L. viridis, to address the evolutionary forces acting on MHC diversity in populations with different biogeographic structure. We characterized MHC class I exon 2 and exon 3, and neutral diversity (microsatellites), to study the relative importance of selection, drift, and introgression in shaping MHC diversity. As expected, positive selection was a significant force shaping the high diversity of MHC genes in both species. Moreover, introgression significantly increased MHC diversity in mainland populations, with a primary direction of gene flow from L. viridis to L. trilineata. Finally, we found significantly fewer MHC alleles in island populations, but maintained MHC sequence and functional diversity, suggesting that positive selection counteracted the effect of drift. Overall, our data support that different evolutionary processes govern MHC diversity in different biogeographical scenarios: positive selection occurs broadly while introgression acts in sympatry and drift when the population sizes decrease.
主要组织相容性复合体（MHC）长期以来一直是研究野生种群中适应性遗传多样性进化的模型。病原体介导的选择被认为是 MHC 多样性的主要驱动因素，但在其他进化过程（如遗传漂变和基因渗入）也可能起作用的复杂生物地理情景下，选择在多大程度上塑造 MHC 多样性仍然难以捉摸。在这里，我们专注于两种密切相关的绿蜥蜴物种，三线石龙子（Lacerta trilineata）和绿蜥蜴（L. viridis），以解决在具有不同生物地理结构的种群中对 MHC 多样性起作用的进化力量。我们对 MHC 类 I 外显子 2 和外显子 3 以及中性多样性（微卫星）进行了特征描述，以研究选择、漂变和基因渗入在塑造 MHC 多样性方面的相对重要性。正如预期的那样，正选择是塑造这两个物种 MHC 基因高多样性的重要力量。此外，基因渗入显著增加了大陆种群的 MHC 多样性，基因流的主要方向是从绿蜥蜴到三线石龙子。最后，我们发现岛屿种群中的 MHC 等位基因明显较少，但维持了 MHC 序列和功能多样性，这表明正选择抵消了漂变的影响。总的来说，我们的数据支持不同的进化过程在不同的生物地理情景中控制 MHC 多样性：正选择广泛发生，而基因渗入在种群规模减小时在同域发生，而漂变则在种群规模减小时发生。
