Genes of the major histocompatibility complex (MHC) play a key role in the adaptive immunity of vertebrates, as they encode receptors responsible for antigen recognition. Evolutionary history of the MHC proceeded through numerous gene duplications, which increase the spectrum of pathogens recognized by individuals. Although pathogen-mediated selection is believed to be a primary driver of MHC expansion over evolutionary times, empirical evidence for this association is virtually lacking. Here, we used an extensive dataset on MHC class II copy number variation in non-passerine birds to test for an evolutionary correlation with helminth parasite richness. As expected, our phylogenetically-informed modelling revealed a positive association between MHC copy number and total helminth richness, even after controlling for a broad spectrum of ecological and life-history traits. Thus, total helminth richness appears to be the most important correlate of MHC copy number, supporting a leading role of pathogen-mediated selection in the evolution of MHC in birds. Our results provide some of the first, although correlative, evidence linking parasitism to interspecific variation in MHC copy number among birds.

主要组织相容性复合体（MHC）的基因在脊椎动物的适应性免疫中起关键作用，因为它们编码负责抗原识别的受体。MHC的进化历史是通过许多基因重复进行的，这增加了个体识别的病原体的范围。尽管据信病原体介导的选择是进化过程中MHC扩展的主要驱动力，但实际上缺乏这种关联的经验证据。在这里，我们使用了广泛的数据集，研究了非雀形目鸟类中MHC II类拷贝数变异的情况，以测试与蠕虫寄生虫丰富度的进化相关性。不出所料，我们的系统发育信息学模型显示MHC拷贝数与总蠕虫丰富度呈正相关，即使控制了广泛的生态和生命历史特征。因此，总蠕虫的丰富度似乎是MHC拷贝数的最重要关联，支持病原体介导的选择在鸟类MHC进化中的主导作用。我们的研究结果提供了一些首个尽管相关的证据，但也将寄生与家禽中MHC拷贝数的种间变异相关联。