Molecular Ecology and Evolution Program, Department of Genetics, University of Pretoria, Private bag X20, Hatfield 0028, South Africa.
BMC Evol Biol. 2012 Aug 15;12:143. doi: 10.1186/1471-2148-12-143.
The major histocompatibility complex (MHC) is an important component of the vertebrate immune system and is frequently used to characterise adaptive variation in wild populations due to its co-evolution with pathogens. Passerine birds have an exceptionally diverse MHC with multiple gene copies and large numbers of alleles compared to other avian taxa. The Nesospiza bunting species complex (two species on Nightingale Island; one species with three sub-species on Inaccessible Island) represents a rapid adaptive radiation at a small, isolated archipelago, and is thus an excellent model for the study of adaptation and speciation. In this first study of MHC in Nesospiza buntings, we aim to characterize MHCIIß variation, determine the strength of selection acting at this gene region and assess the level of shared polymorphism between the Nesospiza species complex and its putative sister taxon, Rowettia goughensis, from Gough Island.
In total, 23 unique alleles were found in 14 Nesospiza and 2 R. goughensis individuals encoding at least four presumably functional loci and two pseudogenes. There was no evidence of ongoing selection on the peptide binding region (PBR). Of the 23 alleles, 15 were found on both the islands inhabited by Nesospiza species, and seven in both Nesospiza and Rowettia; indications of shared, ancestral polymorphism. A gene tree of Nesospiza MHCIIß alleles with several other passerine birds shows three highly supported Nesospiza-specific groups. All R. goughensis alleles were shared with Nesospiza, and these alleles were found in all three Nesospiza sequence groups in the gene tree, suggesting that most of the observed variation predates their phylogenetic split.
Lack of evidence of selection on the PBR, together with shared polymorphism across the gene tree, suggests that population variation of MHCIIß among Nesospiza and Rowettia is due to ancestral polymorphism rather than local selective forces. Weak or no selection pressure could be attributed to low parasite load at these isolated Atlantic islands. The deep divergence between the highly supported Nesospiza-specific sequence Groups 2 and 3, and the clustering of Group 3 close to the distantly related passerines, provide strong support for preserved ancestral polymorphism, and present evidence of one of the rare cases of extensive ancestral polymorphism in birds.
主要组织相容性复合体（MHC）是脊椎动物免疫系统的重要组成部分，由于其与病原体的共同进化，常被用于描述野生种群的适应性变化。与其他鸟类相比，雀形目鸟类的 MHC 具有高度多样性，包含多个基因拷贝和大量等位基因。Nesospiza 雀种复合体（在尼格利陀岛上有两个物种；在不可接近岛上有一个物种，分为三个亚种）代表了一个在小而孤立的群岛上的快速适应性辐射，因此是研究适应和物种形成的绝佳模型。在对 Nesospiza 雀进行的 MHC 首次研究中，我们旨在描述 MHCIIß 的变异情况，确定该基因区域的选择强度，并评估 Nesospiza 种复合体与其假定的姊妹分类群 Rowettia goughensis（来自戈夫岛）之间的共享多态性水平。
在 14 只 Nesospiza 和 2 只 R. goughensis 个体中发现了 23 个独特的等位基因，这些等位基因至少编码四个假定的功能基因座和两个假基因。在肽结合区（PBR）没有发现正在进行的选择证据。在这 23 个等位基因中，有 15 个在 Nesospiza 物种栖息的两个岛屿上发现，有 7 个在 Nesospiza 和 Rowettia 上都发现；这表明存在共享的祖先多态性。Nesospiza MHCIIß 等位基因与其他几种雀形目鸟类的基因树显示，有三个高度支持的 Nesospiza 特异性群体。所有的 R. goughensis 等位基因都与 Nesospiza 共享，并且这些等位基因在基因树中的三个 Nesospiza 序列组中都有发现，这表明观察到的大部分变异发生在它们的系统发生分裂之前。
PBR 上缺乏选择的证据，以及基因树上共享的多态性表明，Nesospiza 和 Rowettia 之间 MHCIIß 的种群变异是由于祖先多态性而不是局部选择压力造成的。在这些孤立的大西洋岛屿上，寄生虫负荷较低，可能导致选择压力较弱或不存在。高度支持的 Nesospiza 特异性序列组 2 和 3 之间的深度分歧，以及组 3 与远亲雀形目鸟类的聚类，为保存的祖先多态性提供了强有力的支持，并为鸟类中罕见的广泛祖先多态性案例之一提供了证据。
