Members of the bacterial order Rickettsiales are obligatorily associated with a wide range of eukaryotic hosts. Their evolutionary trajectories, in particular concerning the origin of shared or differential traits among distant sub‐lineages, are still poorly understood. Here, we characterized a novel Rickettsiales bacterium associated with the ciliate Paramecium tredecaurelia and phylogenetically related to the Rickettsia genus. Its genome encodes significant lineage‐specific features, chiefly the mevalonate pathway gene repertoire, involved in isoprenoid precursor biosynthesis. Not only this pathway has never been described in Rickettsiales, it also is very rare among bacteria, though typical in eukaryotes, thus likely representing a horizontally acquired trait. The presence of these genes could enable an efficient exploitation of host‐derived intermediates for isoprenoid synthesis. Moreover, we hypothesize the reversed reactions could have replaced canonical pathways for producing acetyl‐CoA, essential for phospholipid biosynthesis. Additionally, we detected phylogenetically unrelated mevalonate pathway genes in metagenome‐derived Rickettsiales sequences, likely indicating evolutionary convergent effects of independent horizontal gene transfer events. Accordingly, convergence, involving both gene acquisitions and losses, is highlighted as a relevant evolutionary phenomenon in Rickettsiales, possibly favoured by plasticity and comparable lifestyles, representing a potentially hidden origin of other more nuanced similarities among sub‐lineages.

中文翻译：

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纤毛虫草履虫的“Candidatus Sarmatiella mevalonica”内共生体为立克次氏体的进化可塑性提供了见解

细菌立克次体目的成员必然与范围广泛的真核宿主相关联。他们的进化轨迹，特别是关于远距离亚系之间共享或差异特征的起源，仍然知之甚少。在这里，我们表征了一种与纤毛虫草履虫相关的新型立克次体细菌，并且在系统发育上与立克次体属相关。其基因组编码重要的谱系特异性特征，主要是甲羟戊酸途径基因库，参与类异戊二烯前体生物合成。不仅这条通路从未在立克次氏体中被描述过，它在细菌中也非常罕见，尽管在真核生物中很典型，因此可能代表了水平获得的特征。这些基因的存在可以有效利用宿主衍生的中间体进行类异戊二烯合成。此外，我们假设逆反应可能已经取代了产生乙酰辅酶 A 的经典途径，这是磷脂生物合成所必需的。此外，我们在宏基因组衍生的立克次体序列中检测到与系统发育无关的甲羟戊酸途径基因，这可能表明独立水平基因转移事件的进化趋同效应。因此，在立克次氏体中，融合，涉及基因获取和损失，被强调为相关的进化现象，可能受到可塑性和可比生活方式的青睐，代表了子谱系之间其他更细微相似之处的潜在隐藏起源。