Smith Todd A, Driscoll Timothy, Gillespie Joseph J, Raghavan Rahul
Department of Biology and Center for Life in Extreme Environments, Portland State University.
Department of Biology, Western Carolina University.
Genome Biol Evol. 2015 Jan 23;7(3):831-8. doi: 10.1093/gbe/evv016.
Amblyomma americanum (Lone star tick) is an important disease vector in the United States. It transmits several human pathogens, including the agents of human monocytic ehrlichiosis, tularemia, and southern tick-associated rash illness. Blood-feeding insects (Class Insecta) depend on bacterial endosymbionts to provide vitamins and cofactors that are scarce in blood. It is unclear how this deficiency is compensated in ticks (Class Arachnida) that feed exclusively on mammalian blood. A bacterium related to Coxiella burnetii, the agent of human Q fever, has been observed previously within cells of A. americanum. Eliminating this bacterium (CLEAA, Coxiella-like endosymbiont of A. americanum) with antibiotics reduced tick fecundity, indicating that it is an essential endosymbiont. In an effort to determine its role within this symbiosis, we sequenced the CLEAA genome. While highly reduced (656,901 bp) compared with C. burnetii (1,995,281 bp), the CLEAA genome encodes most major vitamin and cofactor biosynthesis pathways, implicating CLEAA as a vitamin provisioning endosymbiont. In contrast, CLEAA lacks any recognizable virulence genes, indicating that it is not a pathogen despite its presence in tick salivary glands. As both C. burnetii and numerous "Coxiella-like bacteria" have been reported from several species of ticks, we determined the evolutionary relationship between the two bacteria. Phylogeny estimation revealed that CLEAA is a close relative of C. burnetii, but was not derived from it. Our results are important for strategies geared toward controlling A. americanum and the pathogens it vectors, and also contribute novel information regarding the metabolic interdependencies of ticks and their nutrient-provisioning endosymbionts.
美洲钝眼蜱（孤星蜱）是美国一种重要的疾病传播媒介。它传播多种人类病原体，包括人类单核细胞埃立克体病、兔热病和南方蜱相关皮疹病的病原体。吸血昆虫（昆虫纲）依赖细菌内共生体来提供血液中稀缺的维生素和辅因子。目前尚不清楚仅以哺乳动物血液为食的蜱（蛛形纲）是如何弥补这种营养缺乏的。此前在美洲钝眼蜱细胞内观察到一种与人类Q热病原体伯氏考克斯体相关的细菌。用抗生素消除这种细菌（美洲钝眼蜱类考克斯体样内共生体，CLEAA）会降低蜱的繁殖力，这表明它是一种必需的内共生体。为了确定其在这种共生关系中的作用，我们对CLEAA基因组进行了测序。与伯氏考克斯体（1,995,281 bp）相比，CLEAA基因组高度精简（656,901 bp），但仍编码了大多数主要的维生素和辅因子生物合成途径，这表明CLEAA是一种提供维生素的内共生体。相比之下，CLEAA缺乏任何可识别的毒力基因，这表明尽管它存在于蜱的唾液腺中，但它不是病原体。由于在几种蜱类中都报道了伯氏考克斯体和许多“类考克斯体细菌”，我们确定了这两种细菌之间的进化关系。系统发育估计表明，CLEAA是伯氏考克斯体的近亲，但并非由其衍生而来。我们的研究结果对于控制美洲钝眼蜱及其传播的病原体的策略具有重要意义，同时也为蜱与其提供营养的内共生体之间的代谢相互依存关系提供了新的信息。
