Radwan Jacek, Biedrzycka Aleksandra, Babik Wiesław
Institute of Nature Conservation, Polish Academy of Sciences, Al. Mickiewicza 33, 31-120 Kraków, Poland.
Institute of Systematics and Evolution of Animals, Polish Academy of Sciences, Sławkowska 17, 31-016 Kraków, Poland.
Biol Conserv. 2010 Mar;143(3):537-544. doi: 10.1016/j.biocon.2009.07.026. Epub 2009 Sep 11.
Loss of genetic variation may render populations more vulnerable to pathogens due to inbreeding depression and depletion of variation in genes responsible for immunity against parasites. Here we review the evidence for the significance of variation in genes of the Major Histocompatibility Complex (MHC) for conservation efforts. MHC molecules present pathogen-derived antigens to the effector cells of the immune system and thus trigger the adaptive immune response. Some MHC genes are the most variable functional genes in the vertebrate genome. Their variation is clearly of adaptive significance and there is considerable evidence that its maintenance is mainly due to balancing selection imposed by pathogens. However, while the evidence for selection shaping MHC variation on the historical timescale is compelling, a correlation between levels of MHC variation and variation at neutral loci is often observed, indicating that on a shorter timescale drift also substantially affects MHC, leading to depletion of MHC diversity. The evidence that the loss of MHC variation negatively affects population survival is so far equivocal and difficult to separate from effects of general inbreeding. Some species with depleted MHC variation seem to be particularly susceptible to infection, but other species thrive and expand following severe bottlenecks that have drastically limited their MHC variation. However, while the latter demonstrate that MHC variation is not always critical for population survival, these species may in fact represent rare examples of survival despite of the loss of MHC variation. There is clearly a compelling need for data that would disclose the possible consequences of MHC diversity for population viability. In particular, we need more data on the impact of MHC allelic richness on the abundance of parasites or prevalence of disease in populations, while controlling for the role of general inbreeding. Before such evidence accumulates, captive breeding programs and other conservation measures aimed at inbreeding avoidance should be favoured over those protecting only MHC variation, especially since inbreeding avoidance programs would usually conserve both types of genetic diversity simultaneously.
由于近亲繁殖衰退以及负责抵抗寄生虫免疫的基因变异的耗尽，遗传变异的丧失可能使种群更容易受到病原体的影响。在此，我们回顾主要组织相容性复合体（MHC）基因变异对于保护工作重要性的证据。MHC分子将病原体衍生的抗原呈递给免疫系统的效应细胞，从而触发适应性免疫反应。一些MHC基因是脊椎动物基因组中最具变异性的功能基因。它们的变异显然具有适应性意义，并且有大量证据表明其维持主要是由于病原体施加的平衡选择。然而，虽然在历史时间尺度上选择塑造MHC变异的证据很有说服力，但经常观察到MHC变异水平与中性位点变异之间的相关性，这表明在较短时间尺度上，漂变也会对MHC产生重大影响，导致MHC多样性的耗尽。到目前为止，MHC变异丧失对种群生存产生负面影响的证据并不明确，且难以与一般近亲繁殖的影响区分开来。一些MHC变异耗尽的物种似乎特别容易受到感染，但其他物种在经历了严重限制其MHC变异的瓶颈后仍能繁荣发展并扩张。然而，尽管后者表明MHC变异并非总是对种群生存至关重要，但这些物种实际上可能是尽管MHC变异丧失但仍存活的罕见例子。显然迫切需要能够揭示MHC多样性对种群生存能力可能产生的后果的数据。特别是，我们需要更多关于MHC等位基因丰富度对种群中寄生虫丰度或疾病患病率影响的数据，同时控制一般近亲繁殖的作用。在积累此类证据之前，应优先选择旨在避免近亲繁殖的圈养繁殖计划和其他保护措施，而不是仅保护MHC变异的措施，尤其是因为避免近亲繁殖的计划通常会同时保护这两种类型的遗传多样性。
