Department of Wetland Ecology, Estación Biológica de Doñana, CSIC, Seville, Spain.
J Evol Biol. 2012 Jul;25(7):1438-47. doi: 10.1111/j.1420-9101.2012.02529.x.
The major histocompatibility complex (MHC), which harbours the most polymorphic vertebrate genes, plays a critical role in the host-pathogen coevolutionary arms race. However, the extent to which MHC diversity determines disease susceptibility and long-term persistence of populations is currently under debate, as recent studies have demonstrated that low MHC variability does not necessarily hamper population viability. However, these studies typically assayed small and decimated populations in species with restricted distribution, thereby making inferences about the evolutionary potential of these populations difficult. Here, we show that MHC impoverishment has not constrained the ecological radiation and flourishing of falcons (Aves: Falconidae) worldwide. We found two remarkably different patterns of MHC variation within the genus Falco. Whereas MHC variation in kestrels (the basal group within the genus) is very high, falcons exhibit ancestrally low intra- and interspecific MHC variability. This pattern is not due to the inadvertent survey of paralogous genes or pseudogenes. Further, patterns of variation in mitochondrial or other nuclear genes do not indicate a generalized low level of genome-wide variability among falcons. Although a relative contribution of genetic drift cannot be completely ruled out, we propose the falcons went through an evolutionary transition, driven and maintained by natural selection, from primarily highly variable towards low polymorphic and slow-evolving MHC genes with a very specific immune function. This study highlights that the importance of MHC diversity cannot be generalized among vertebrates, and hints at the evolution of compensatory immune mechanisms in falcons to cope with emerging and continuously evolving pathogens.
主要组织相容性复合体 (MHC) 是容纳最多脊椎动物多态性基因的复合体，在宿主-病原体协同进化军备竞赛中发挥着关键作用。然而，MHC 多样性在多大程度上决定疾病易感性和种群的长期生存能力目前仍存在争议，因为最近的研究表明，MHC 变异性低不一定会阻碍种群的生存能力。然而，这些研究通常在分布范围有限的物种中检测到小的和减少的种群，从而难以推断这些种群的进化潜力。在这里，我们表明 MHC 贫化并没有限制隼形目鸟类（Falconiformes）在全球范围内的生态辐射和繁荣。我们在隼属（Falco）内发现了两种截然不同的 MHC 变异模式。尽管红隼（Falco tinnunculus）（该属的基础群）的 MHC 变异非常高，但隼属的种间和种内 MHC 变异性却很低。这种模式不是由于偶然调查了旁系基因或假基因。此外，线粒体或其他核基因的变异模式并没有表明隼属的基因组在总体上具有较低的变异性。尽管不能完全排除遗传漂变的相对贡献，但我们提出，隼属经历了一个进化转变，由自然选择驱动和维持，从主要高度可变的 MHC 基因向低多态性和缓慢进化的 MHC 基因转变，具有非常特定的免疫功能。这项研究强调了 MHC 多样性的重要性不能在脊椎动物中普遍化，并暗示了隼属中出现了补偿性免疫机制的进化，以应对新兴和不断进化的病原体。
