Włodarczyk Radosław, Těšický Martin, Vinkler Michal, Novotný Marian, Remisiewicz Magdalena, Janiszewski Tomasz, Minias Piotr
Department of Biodiversity Studies and Bioeducation, Faculty of Biology and Environmental Protection, University of Łódź, Banacha 1/3, 90-237, Łódź, Poland.
Charles University, Faculty of Science, Department of Zoology, Viničná 7, 128 43, Prague, Czech Republic.
Dev Comp Immunol. 2023 Jul;144:104704. doi: 10.1016/j.dci.2023.104704. Epub 2023 Apr 3.
Toll-like receptors (TLRs) form a key component of animal innate immunity, being responsible for recognition of conserved microbial structures. As such, TLRs may be subject to diversifying and balancing selection, which maintains allelic variation both within and between populations. However, most research on TLRs in non-model avian species is focused on bottlenecked populations with depleted genetic variation. Here, we assessed variation at the extracellular domains of three TLR genes (TLR1LA, TLR3, TLR4) across eleven species from two passerine families of buntings (Emberizidae) and finches (Fringillidae), all having large breeding population sizes (millions of individuals). We found extraordinary TLR polymorphism in our study taxa, with >100 alleles detected at TLR1LA and TLR4 across species and high haplotype diversity (>0.75) in several species. Despite recent species divergence, no nucleotide allelic variants were shared between species, suggesting rapid TLR evolution. Higher variation at TLR1LA and TLR4 than TLR3 was associated with a stronger signal of diversifying selection, as measured with nucleotide substitutions rates and the number of positively selected sites (PSS). Structural protein modelling of TLRs showed that some PSS detected within TLR1LA and TLR4 were previously recognized as functionally important sites or were located in their proximity, possibly affecting ligand recognition. Furthermore, we identified PSS responsible for major surface electrostatic charge clustering, which may indicate their adaptive importance. Our study provides compelling evidence for the divergent evolution of TLR genes in buntings and finches and indicates that high TLR variation may be adaptively maintained via diversifying selection acting on functional ligand binding sites.
Toll样受体（TLRs）构成动物先天免疫的关键组成部分，负责识别保守的微生物结构。因此，TLRs可能会受到多样化和平衡选择，从而在种群内部和种群之间维持等位基因变异。然而，大多数关于非模式鸟类物种TLRs的研究都集中在遗传变异匮乏的瓶颈种群上。在这里，我们评估了来自鹀科（Emberizidae）和雀科（Fringillidae）两个雀形目科的11个物种的三个TLR基因（TLR1LA、TLR3、TLR4）细胞外结构域的变异情况，所有这些物种都有庞大的繁殖种群规模（数百万个体）。我们在研究的分类群中发现了异常的TLR多态性，在TLR1LA和TLR4跨物种检测到超过100个等位基因，并且在几个物种中具有高单倍型多样性（>0.75）。尽管物种分化时间较近，但物种之间没有共享核苷酸等位基因变体，这表明TLR进化迅速。与TLR3相比，TLR1LA和TLR4具有更高的变异，这与多样化选择的更强信号相关，这是通过核苷酸替代率和正选择位点（PSS）数量来衡量的。TLRs的结构蛋白建模表明，在TLR1LA和TLR4中检测到的一些PSS以前被认为是功能重要位点或位于其附近，可能影响配体识别。此外，我们确定了负责主要表面静电电荷聚集的PSS，这可能表明它们的适应性重要性。我们的研究为鹀类和雀类中TLR基因的趋异进化提供了令人信服的证据，并表明高TLR变异可能通过作用于功能性配体结合位点的多样化选择而适应性地维持。
