Toll-like receptor variation in the bottlenecked population of the Seychelles warbler: computer simulations see the "ghost of selection past' and quantify the "drift debt'
Toll-like receptor variation in the bottlenecked population of the Seychelles warbler: computer simulations see the "ghost of selection past' and quantify the "drift debt'
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DOI：
10.1111/jeb.13077
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发表时间：
2017-07-01
影响因子：
2.1
通讯作者：
van Oosterhout, C.
中科院分区：
文献类型：
--
作者：
Gilroy, D. L.;Phillips, K. P.;van Oosterhout, C.
平衡选择可以维持宿主种群内的免疫遗传变异，但由于漂移的潜在压倒效应，在后瓶颈种群中检测其信号是具有挑战性的。Toll样受体基因(TLRs)在脊椎动物的免疫防御中起着基础性的作用，并被预测为处于平衡选择之下。我们以前描述了塞舌尔林莺(Acrocephalus Sechellensis)TLR基因座的变异，这是一种经历了历史瓶颈的地方性雀类。7个TLR基因座中有5个是多态的，这与观察到的低全基因组变异形成鲜明对比。然而，标准的群体遗传统计方法未能在任何TLR基因座检测到选择的当代特征。我们检验了观察到的TLR多态是否可以用中性进化来解释，并在DIYABC软件中模拟了种群的人口统计学。这表明，突变率的后验分布必须是不切实际的高，才能解释观察到的遗传变异。然后，我们用基于代理的模型对突变率的典型值进行了模拟，这表明弱平衡选择作用于这三个TLR基因。该模型能够检测到过去的选择提高了前瓶颈人群中TLR多态的证据，但无法辨别出在当代人群中平衡选择的任何影响。我们的结果表明，漂移是形成当代塞舌尔林莺种群TLR变异的压倒一切的进化力量，所观察到的TLR多态可能只是过去选择的幽灵。预测模型预测，在缺乏当代平衡选择的情况下，该物种的免疫遗传变异将继续受到侵蚀。当基因库还没有达到其新的多态平衡水平时，这种漂移债务就会发生，这种损失可能会对许多最近出现瓶颈的种群构成重要威胁。
Balancing selection can maintain immunogenetic variation within host populations, but detecting its signal in a postbottlenecked population is challenging due to the potentially overriding effects of drift. Toll-like receptor genes (TLRs) play a fundamental role in vertebrate immune defence and are predicted to be under balancing selection. We previously characterized variation at TLR loci in the Seychelles warbler (Acrocephalus sechellensis), an endemic passerine that has undergone a historical bottleneck. Five of seven TLR loci were polymorphic, which is in sharp contrast to the low genomewide variation observed. However, standard population genetic statistical methods failed to detect a contemporary signature of selection at any TLR locus. We examined whether the observed TLR polymorphism could be explained by neutral evolution, simulating the population's demography in the software DIYABC. This showed that the posterior distributions of mutation rates had to be unrealistically high to explain the observed genetic variation. We then conducted simulations with an agent-based model using typical values for the mutation rate, which indicated that weak balancing selection has acted on the three TLR genes. The model was able to detect evidence of past selection elevating TLR polymorphism in the prebottleneck populations, but was unable to discern any effects of balancing selection in the contemporary population. Our results show drift is the overriding evolutionary force that has shaped TLR variation in the contemporary Seychelles warbler population, and the observed TLR polymorphisms might be merely the ghost of selection past'. Forecast models predict immunogenetic variation in this species will continue to be eroded in the absence of contemporary balancing selection. Such drift debt' occurs when a gene pool has not yet reached its new equilibrium level of polymorphism, and this loss could be an important threat to many recently bottlenecked populations.