Davies Charli S, Taylor Martin I, Hammers Martijn, Burke Terry, Komdeur Jan, Dugdale Hannah L, Richardson David S
School of Biological Sciences, University of East Anglia, Norwich Research Park, Norfolk, UK.
Groningen Institute for Evolutionary Life Sciences (GELIFES), University of Groningen, Groningen, The Netherlands.
Mol Ecol. 2021 Jun;30(11):2528-2542. doi: 10.1111/mec.15914. Epub 2021 May 5.
Understanding where genetic variation exists, and how it influences fitness within populations is important from an evolutionary and conservation perspective. Signatures of past selection suggest that pathogen-mediated balancing selection is a key driver of immunogenetic variation, but studies tracking contemporary evolution are needed to help resolve the evolutionary forces and mechanism at play. Previous work in a bottlenecked population of Seychelles warblers (Acrocephalus sechellensis) show that functional variation has been maintained at the viral-sensing Toll-like receptor 3 (TLR3) gene, including one nonsynonymous SNP, resulting in two alleles. Here, we characterise evolution at this TLR3 locus over a 25-year period within the original remnant population of the Seychelles warbler, and in four other derived, populations. Results show a significant and consistent temporal decline in the frequency of the TLR3 allele in the original population, and that similar declines in the TLR3 allele frequency occurred in all the derived populations. Individuals (of both sexes) with the TLR3 genotype had lower survival, and males - but not females - that carry the TLR3 allele had significantly lower lifetime reproductive success than those with only the TLR3 allele. These results indicate that positive selection on the TLR3 allele, caused by an as yet unknown agent, is driving TLR3 evolution in the Seychelles warbler. No evidence of heterozygote advantage was detected. However, whether the positive selection observed is part of a longer-term pattern of balancing selection (through fluctuating selection or rare-allele advantage) cannot be resolved without tracking the TLR3 allele over an extended time period.
从进化和保护的角度来看，了解遗传变异存在的位置以及它如何影响种群内的适应性非常重要。过去选择的特征表明，病原体介导的平衡选择是免疫遗传变异的关键驱动因素，但需要进行追踪当代进化的研究来帮助解析正在起作用的进化力量和机制。先前对塞舌尔莺（Acrocephalus sechellensis）瓶颈种群的研究表明，病毒感应Toll样受体3（TLR3）基因的功能变异得以维持，包括一个非同义单核苷酸多态性，产生了两个等位基因。在这里，我们描述了在塞舌尔莺原始残余种群以及其他四个衍生种群中，该TLR3基因座在25年期间的进化情况。结果显示，原始种群中TLR3等位基因的频率出现了显著且持续的时间性下降，并且在所有衍生种群中TLR3等位基因频率也出现了类似的下降。具有TLR3基因型的个体（无论雌雄）存活率较低，携带TLR3等位基因的雄性——而非雌性——的终生繁殖成功率显著低于仅携带TLR3等位基因的个体。这些结果表明，由一种未知因素引起的对TLR3等位基因的正选择正在推动塞舌尔莺的TLR3进化。未检测到杂合子优势的证据。然而，如果不长期追踪TLR3等位基因，就无法确定观察到的正选择是否是平衡选择长期模式的一部分（通过波动选择或稀有等位基因优势）。
