Faculty of Veterinary Medicine, University of Zaragoza, Spain.
Institute of Infection and Immunity, School of Medicine, Cardiff University, CF14 4XN, UK.
Infect Genet Evol. 2018 Dec;66:308-318. doi: 10.1016/j.meegid.2018.03.023. Epub 2018 Apr 12.
The life cycle of spirochetes of the genus Borrelia includes complex networks of vertebrates and ticks. The tripartite association of Borrelia-vertebrate-tick has proved ecologically successful for these bacteria, which have become some of the most prominent tick-borne pathogens in the northern hemisphere. To keep evolutionary pace with its double-host life history, Borrelia must adapt to the evolutionary pressures exerted by both sets of hosts. In this review, we attempt to reconcile functional, phylogenetic, and ecological perspectives to propose a coherent scenario of Borrelia evolution. Available empirical information supports that the association of Borrelia with ticks is very old. The major split between the tick families Argasidae-Ixodidae (dated some 230-290 Mya) resulted in most relapsing fever (Rf) species being restricted to Argasidae and few associated with Ixodidae. A further key event produced the diversification of the Lyme borreliosis (Lb) species: the radiation of ticks of the genus Ixodes from the primitive stock of Ixodidae (around 217 Mya). The ecological interactions of Borrelia demonstrate that Argasidae-transmitted Rf species remain restricted to small niches of one tick species and few vertebrates. The evolutionary pressures on this group are consequently low, and speciation processes seem to be driven by geographical isolation. In contrast to Rf, Lb species circulate in nested networks of dozens of tick species and hundreds of vertebrate species. This greater variety confers a remarkably variable pool of evolutionary pressures, resulting in large speciation of the Lb group, where different species adapt to circulate through different groups of vertebrates. Available data, based on ospA and multilocus sequence typing (including eight concatenated in-house genes) phylogenetic trees, suggest that ticks could constitute a secondary bottleneck that contributes to Lb specialization. Both sets of adaptive pressures contribute to the resilience of highly adaptable meta-populations of bacteria.
螺旋体属的螺旋体的生命周期包括脊椎动物和蜱虫的复杂网络。伯氏疏螺旋体-脊椎动物-蜱虫的三方共生关系已被证明对这些细菌在生态上是成功的，它们已成为北半球最著名的一些蜱传病原体。为了与双宿主生活史保持进化同步，伯氏疏螺旋体必须适应两组宿主施加的进化压力。在这篇综述中，我们试图协调功能、系统发育和生态观点，提出一个连贯的伯氏疏螺旋体进化情景。现有的经验信息支持伯氏疏螺旋体与蜱虫的共生关系非常古老。蜱科的蜱科-虱科（约 2.3-2.9 亿年前）之间的主要分裂导致大多数回归热（Rf）物种局限于蜱科，很少与虱科有关。另一个关键事件产生了莱姆病（Lb）物种的多样化：硬蜱属的蜱虫从原始的蜱科（约 2.17 亿年前）辐射。伯氏疏螺旋体的生态相互作用表明，蜱科传播的 Rf 物种仍然局限于一种蜱虫和少数几种脊椎动物的小生态位。因此，该群体受到的进化压力较低，物种形成过程似乎是由地理隔离驱动的。与 Rf 相反，Lb 物种在数十种蜱种和数百种脊椎动物的嵌套网络中循环。这种多样性赋予了进化压力一个显著可变的池，导致 Lb 群体的大规模物种形成，其中不同的物种适应在不同的脊椎动物群体中循环。基于 ospA 和多位点序列分型（包括 8 个串联的内部基因）系统发育树的现有数据表明，蜱虫可能构成导致 Lb 专业化的次要瓶颈。两组适应性压力都有助于高度适应性的细菌混合种群的恢复力。
