Institute of Infection & Global Health, University of Liverpool, 146 Brownlow Hill, Liverpool L3 5RF, UK; College of Veterinary Medicine, University of Al-Qadisiyah, Diwaniyah 58001, Qadisiyyah Province, Iraq.
Institute of Infection & Global Health, University of Liverpool, 146 Brownlow Hill, Liverpool L3 5RF, UK; School of Life Sciences, University of Lincoln, Brayford Pool, Lincoln LN6 7TS, UK.
Ticks Tick Borne Dis. 2019 Jan;10(1):52-62. doi: 10.1016/j.ttbdis.2018.08.016. Epub 2018 Aug 31.
Ticks have relatively complex microbiomes, but only a small proportion of the bacterial symbionts recorded from ticks are vertically transmitted. Moreover, co-cladogenesis between ticks and their symbionts, indicating an intimate relationship over evolutionary history driven by a mutualistic association, is the exception rather than the rule. One of the most widespread tick symbionts is Candidatus Midichloria, which has been detected in all of the major tick genera of medical and veterinary importance. In some species of Ixodes, such as the sheep tick Ixodes ricinus (infected with Candidatus Midichloria mitochondrii), the symbiont is fixed in wild adult female ticks, suggesting an obligate mutualism. However, almost no information is available on genetic variation in Candidatus M. mitochondrii or possible co-cladogenesis with its host across its geographic range. Here, we report the first survey of Candidatus M. mitochondrii in I. ricinus in Great Britain and a multi-locus sequence typing (MLST) analysis of tick and symbiont between British ticks and those collected in continental Europe. We show that while the prevalence of the symbiont in nymphs collected in England is similar to that reported from the continent, a higher prevalence in nymphs and adult males is apparent in Wales. In general, Candidatus M. mitochondrii exhibits very low levels of sequence diversity, although a consistent signal of host-symbiont coevolution was apparent in Scotland. Moreover, the tick MLST scheme revealed that Scottish specimens form a clade that is partially separated from other British ticks, with almost no contribution of continental sequence types in this north-westerly border of the tick's natural range. The low diversity of Candidatus M. mitochondrii, in contrast with previously reported high rates of polymorphism in I. ricinus mitogenomes, suggests that the symbiont may have swept across Europe recently via a horizontal, rather than vertical, transmission route.
蜱虫的微生物组相对复杂，但记录到的蜱虫共生细菌中只有一小部分是垂直传播的。此外，蜱虫与其共生体之间的共同进化，表明它们在进化历史上存在密切的关系，这是互惠共生关系的结果，而不是普遍现象。最广泛的蜱虫共生体之一是候选中密螺旋体（Candidatus Midichloria），它已在所有具有医学和兽医重要性的主要蜱虫属中被检测到。在某些钝缘蜱属物种（如绵羊蜱（Ixodes ricinus），感染候选中密螺旋体线粒体（Candidatus Midichloria mitochondrii））中，共生体固定在野生成年雌性蜱虫中，这表明这是一种强制性的共生关系。然而，几乎没有关于候选中密螺旋体线粒体在其地理范围内与宿主的遗传变异或可能的共同进化的信息。在这里，我们报告了英国首次对绵羊蜱中的候选中密螺旋体线粒体的调查，以及对英国蜱虫及其在欧洲大陆收集的蜱虫和共生体的多基因座序列分型（MLST）分析。我们表明，尽管在英格兰采集的若虫中该共生体的流行率与大陆报道的相似，但在威尔士，若虫和成年雄性的流行率更高。一般来说，候选中密螺旋体线粒体表现出非常低的序列多样性，尽管在苏格兰明显存在宿主-共生体协同进化的信号。此外，蜱虫 MLST 方案表明，苏格兰标本形成了一个与其他英国蜱虫部分分离的分支，在蜱虫自然分布的这个西北边界几乎没有来自欧洲大陆的序列类型的贡献。候选中密螺旋体线粒体的低多样性，与以前报道的绵羊蜱 mitogenomes 中高多态性形成鲜明对比，这表明该共生体可能最近通过水平而非垂直传播途径在欧洲传播。
