Data from: Contrasting evolutionary histories of MHC class I and class II loci in grouse - effects of selection and gene conversion

Genes of the major histocompatibility complex (MHC) encode receptor molecules that are responsible for recognition of intra- and extra-cellular pathogens (class I and class II genes, respectively) in vertebrates. Given the different roles of class I and II MHC genes, one might expect the strength of selection to differ between these two classes. Different selective pressures may also promote different rates of gene conversion at each class. Despite these predictions, surprisingly few studies have looked at differences between class I and II genes in terms of both selection and gene conversion. Here, we investigated the molecular evolution of MHC class I and II genes in five closely related species of prairie grouse (Centrocercus and Tympanuchus) that possess one class I and two class II loci. We found striking differences in the strength of balancing selection acting on MHC class I versus class II genes. More than half of the putative antigen-binding sites (ABS) of class II were under positive or episodic diversifying selection, compared with only 10% at class I. We also found that gene conversion played a stronger role in shaping the evolution of MHC class II than class I. Overall, the combination of strong positive (balancing) selection and frequent gene conversion has maintained higher diversity of MHC class II than class I in prairie grouse. This is one of the first studies clearly demonstrating that macroevolutionary mechanisms can act differently on genes involved in the immune response against intra- and extra-cellular pathogens.

主要组织相容性复合体（major histocompatibility complex, MHC）基因编码受体分子，负责识别脊椎动物体内的胞内与胞外病原体（其中I类基因对应胞内病原体、II类基因对应胞外病原体）。鉴于I类与II类MHC基因的功能存在差异，我们可预期两类基因所承受的选择压力强度有所不同。不同的选择压力也可能推动两类基因的基因转换（gene conversion）速率产生差异。尽管存在上述预测，但目前同时从选择作用与基因转换角度探究I类与II类MHC基因差异的研究少之又少，这令人颇感意外。本研究针对5种亲缘关系密切的草原松鸡属（Centrocercus和Tympanuchus）物种开展了分子演化分析，这类物种均携带1个I类MHC基因位点与2个II类MHC基因位点。我们发现，作用于MHC I类与II类基因的平衡选择强度存在显著差异。II类基因中超过半数的推定抗原结合位点（antigen-binding sites, ABS）处于正向选择或间歇性多样化选择之下，而I类基因中仅10%的位点符合该情况。我们还发现，相较于I类MHC基因，基因转换在塑造II类MHC基因的演化过程中起到了更强的作用。总体而言，强烈的正向（平衡）选择与频繁的基因转换共同维持了草原松鸡体内MHC II类基因的多样性高于I类基因。本研究是首批明确证实宏观演化机制可对参与胞内、胞外病原体免疫应答的基因产生差异化作用的研究之一。