Data from: Very high MHC Class IIB diversity without spatial differentiation in the Mediterranean population of Greater Flamingos

Background: Selective pressure from pathogens is thought to shape the allelic diversity of major histocompatibility complex (MHC) genes in vertebrates. In particular, both local adaptation to pathogens and gene flow are thought to explain a large part of the intraspecific variation observed in MHC allelic diversity. To date, however, evidence that adaptation to locally prevalent pathogens maintains MHC variation is limited to species with limited dispersal and, hence, reduced gene flow. On the one hand high gene flow can disrupt local adaptation in species with high dispersal rates, on the other hand such species are much more likely to experience spatial variation in pathogen pressure, suggesting that there may be intense pathogen mediated selection pressure operating across breeding sites in panmictic species. Such pathogen mediated selection pressure operating across breeding sites should therefore be sufficient to maintain high MHC diversity in high dispersing species in the absence of local adaptation mechanisms. We used the Greater Flamingo, Phoenicopterus roseus, a long-lived colonial bird showing a homogeneous genetic structure of neutral markers at the scale of the Mediterranean region, to test the prediction that higher MHC allelic diversity with no population structure should occur in large panmictic populations of long-distance dispersing birds than in other resident species. Results: We assessed the level of allelic diversity at the MHC Class IIB exon 2 from 116 individuals born in four different breeding colonies of Greater Flamingo in the Mediterranean region. We found one of the highest allelic diversity (109 alleles, 2 loci) of any non-passerine avian species investigated so far relative to the number of individuals and loci genotyped. There was no evidence of population structure between the four major Mediterranean breeding colonies. Conclusion: Our results suggest that local adaptation at MHC Class IIB in Greater Flamingos is constrained by high gene flow and high MHC diversity appears to be maintained by population wide pathogen-mediated selection rather than local pathogen-mediated selection. Further understanding of how pathogens vary across space and time will be crucial to further elucidate the mechanisms maintaining MHC diversity in species with large panmictic populations and high dispersal rates.研究背景：病原体施加的选择压力被认为塑造了脊椎动物主要组织相容性复合体（major histocompatibility complex, MHC）基因的等位基因多样性。具体而言，针对病原体的局部适应与基因流，可解释MHC等位基因多样性中观测到的大部分种内变异。然而迄今为止，关于针对本地流行病原体的适应能够维持MHC多样性的相关证据，仅局限于扩散能力有限、因此基因流较弱的物种。一方面，高扩散率物种的基因流较强，可能破坏其局部适应；另一方面，这类物种更易遭遇病原体压力的空间异质性，这意味着泛交种群物种的繁殖位点间可能存在较强的病原体介导选择压力。若不存在局部适应机制，这种跨繁殖位点的病原体介导选择压力，足以在高扩散物种中维持较高的MHC多样性。本研究以大红鹳（Phoenicopterus roseus）为研究对象——这是一种长寿的群居鸟类，在地中海尺度下其中性标记的遗传结构均一——来验证如下预测：相较于其他居留型鸟类，长距离扩散的大型泛交种群鸟类应具备更高的MHC等位基因多样性，且无种群结构特征。 研究结果：我们对地中海地区4个不同繁殖集群的116只大红鹳个体的MHC IIB类外显子2的等位基因多样性水平进行了评估。相较于已完成基因分型的个体数与位点数量，本研究观测到的等位基因多样性（109个等位基因，2个位点）位列目前已研究的非雀形目鸟类前列。且地中海地区的4个主要繁殖集群之间未发现种群结构特征。 研究结论：本研究结果表明，大红鹳MHC IIB类基因的局部适应受到强基因流的制约，其较高的MHC多样性似乎由种群层面的病原体介导选择维持，而非局部病原体介导选择。未来若要进一步阐明大型泛交种群、高扩散率物种的MHC多样性维持机制，明确病原体在空间与时间上的变化规律将至关重要。