Bryja J, Charbonnel N, Berthier K, Galan M, Cosson J-F
Centre de Biologie et Gestion des Populations (UMR 22), INRA, Campus International de Baillarguet, CS 30016, 34988 Montferrier sur Lez Cedex, France.
Mol Ecol. 2007 Dec;16(23):5084-97. doi: 10.1111/j.1365-294X.2007.03584.x. Epub 2007 Oct 22.
Host-pathogen interactions are of particular interest in studies of the interplay between population dynamics and natural selection. The major histocompatibility complex (MHC) genes of demographically fluctuating species are highly suitable markers for such studies, because they are involved in initiating the immune response against pathogens and display a high level of adaptive genetic variation. We investigated whether two MHC class II genes (DQA1, DRB) were subjected to contemporary selection during increases in the density of fossorial water vole (Arvicola terrestris) populations, by comparing the neutral genetic structure of seven populations with that estimated from MHC genes. Tests for heterozygosity excess indicated that DQA1 was subject to intense balancing selection. No such selection operated on neutral markers. This pattern of selection became more marked with increasing abundance. In the low-abundance phase, when populations were geographically isolated, both overall differentiation and isolation-by-distance were more marked for MHC genes than for neutral markers. Model-based simulations identified DQA1 as an outlier (i.e. under selection) in a single population, suggesting the action of local selection in fragmented populations. The differences between MHC and neutral markers gradually disappeared with increasing effective migration between sites. In the high-abundance year, DQA1 displayed significantly lower levels of overall differentiation than the neutral markers. This gene therefore displayed stronger homogenization than observed under drift and migration alone. The observed signs of selection were much weaker for DRB. Spatial and temporal fluctuations in parasite pressure and locus-specific selection are probably the most plausible mechanisms underlying the observed changes in selection pattern during the demographic cycle.
宿主与病原体的相互作用在种群动态与自然选择之间相互作用的研究中尤为引人关注。在种群数量波动的物种中，主要组织相容性复合体（MHC）基因是此类研究的高度合适的标记，因为它们参与启动针对病原体的免疫反应，并表现出高水平的适应性遗传变异。我们通过比较七个种群的中性遗传结构与从MHC基因估计的结构，研究了在穴居水田鼠（Arvicola terrestris）种群密度增加期间，两个MHC II类基因（DQA1、DRB）是否受到当代选择。杂合子过剩检验表明，DQA1受到强烈的平衡选择。中性标记上没有这种选择作用。这种选择模式随着丰度的增加而变得更加明显。在低丰度阶段，当种群在地理上隔离时，MHC基因的总体分化和距离隔离比中性标记更为明显。基于模型的模拟将DQA1确定为单个种群中的异常值（即处于选择之下），表明在碎片化种群中存在局部选择作用。随着位点间有效迁移的增加，MHC和中性标记之间的差异逐渐消失。在高丰度年份，DQA1的总体分化水平明显低于中性标记。因此，该基因显示出比仅在漂变和迁移情况下更强的同质化。观察到的DRB选择迹象要弱得多。寄生虫压力和位点特异性选择的时空波动可能是人口统计学周期中观察到的选择模式变化的最合理机制。
