Wolbachia is one of the most abundant endosymbionts on earth, with a wide distribution especially in arthropods. Effective maternal transmission and the induction of various phenotypes in their hosts are two key features of this bacterium. Here, we review our current understanding of another central aspect of Wolbachia's success: their ability to switch from one host species to another. We build on the proposal that Wolbachia host shifts occur in four main steps: (i) physical transfer to a new species; (ii) proliferation within that host; (iii) successful maternal transmission; and (iv) spread within the host species. Host shift can fail at each of these steps, and the likelihood of ultimate success is influenced by many factors. Some stem from traits of Wolbachia (different strains have different abilities for host switching), others on host features such as genetic resemblance (e.g. host shifting is likely to be easier between closely related species), ecological connections (the donor and recipient host need to interact), or the resident microbiota. Host shifts have enabled Wolbachia to reach its enormous current incidence and global distribution among arthropods in an epidemiological process shaped by loss and acquisition events across host species. The ability of Wolbachia to transfer between species also forms the basis of ongoing endeavours to control pests and disease vectors, following artificial introduction into uninfected hosts such as mosquitoes. Throughout, we emphasise the many knowledge gaps in our understanding of Wolbachia host shifts, and question the effectiveness of current methodology to detect these events. We conclude by discussing an apparent paradox: how can Wolbachia maintain its ability to undergo host shifts given that its biology seems dominated by vertical transmission?

中文翻译：

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Wolbachia 宿主转变：途径、机制、约束和进化后果

Wolbachia 是地球上最丰富的内共生体之一，分布广泛，尤其是在节肢动物中。有效的母体传播和在宿主中诱导各种表型是这种细菌的两个关键特征。在这里，我们回顾了我们目前对 Wolbachia 成功的另一个核心方面的理解：它们从一种宿主物种转换到另一种宿主物种的能力。我们基于 Wolbachia 宿主转变发生在四个主要步骤的提议：（i）物理转移到新物种；(ii) 在该宿主内扩散；(iii) 成功的母体传播；(iv) 在宿主物种内传播。主机转移可能会在每个步骤中失败，最终成功的可能性受许多因素的影响。一些源于沃尔巴克氏体的特征（不同的菌株具有不同的宿主转换能力），其他宿主特征，如遗传相似性（例如，密切相关物种之间的宿主转移可能更容易）、生态联系（供体和受体宿主需要相互作用）或常驻微生物群。宿主转移使 Wolbachia 能够在由宿主物种的丢失和获取事件形成的流行病学过程中达到其目前在节肢动物中的巨大发病率和全球分布。在人工引入蚊子等未受感染的宿主后，沃尔巴克氏体在物种之间转移的能力也构成了控制害虫和疾病媒介的持续努力的基础。在整个过程中，我们强调了我们对 Wolbachia 宿主转变的理解中的许多知识差距，并质疑当前检测这些事件的方法的有效性。