Holosporales are an alphaproteobacterial order engaging in obligate and complex associations with eukaryotes, in particular protists. The functional and evolutionary features of those interactions are still largely undisclosed. Here, we sequenced the genomes of two members of the species Bealeia paramacronuclearis (Holosporales, Holosporaceae) intracellularly associated with the ciliate protist Paramecium, which resulted in high correspondence. Consistent with the short-branched early-divergent phylogenetic position, Bealeia presents a larger functional repertoire than other Holosporaceae, comparable to those of other Holosporales families, particularly for energy metabolism and motility. Our analyses indicate that different Holosporales likely experienced at least partly autonomous genome reduction and adaptation to host interactions, for example regarding dependence on host biotin driven by multiple independent horizontal acquisitions of transporters. Among Alphaproteobacteria, this is reminiscent of the convergently evolved Rickettsiales, which however appear more diverse, possibly due to a probably more ancient origin. We identified in Bealeia and other Holosporales the plasmid-encoded putative genetic determinants of R-bodies, which may be involved in a killer trait towards symbiont-free hosts. While it is not clear whether these genes are ancestral or recently horizontally acquired, an intriguing and peculiar role of R-bodies is suggested in the evolution of the interactions of multiple Holosporales with their hosts.

中文翻译：

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比较基因组学对全孢菌目（Alphaproteobacteria）与真核宿主相互作用的新进化见解

全孢菌目是一种α变形菌目，与真核生物，特别是原生生物有专一且复杂的联系。这些相互作用的功能和进化特征在很大程度上仍未公开。在这里，我们对细胞内与纤毛虫原生生物草履虫相关的Bealeia paramacronuclearis（Holosporales，Holosporaceae ）物种的两个成员的基因组进行了测序，结果具有高度一致性。与短分枝的早期分化的系统发育位置一致，Bealeia比其他全孢菌科具有更大的功能库，与其他全孢菌目家族相当，特别是在能量代谢和运动方面。我们的分析表明，不同的全孢菌目可能至少经历了部分自主的基因组减少和对宿主相互作用的适应，例如对由转运蛋白的多个独立水平获取驱动的宿主生物素的依赖。在α变形菌中，这让人想起趋同进化的立克次体目，但它们看起来更加多样化，可能是由于可能更古老的起源。我们在Bealeia和其他Holosporales中鉴定出了R 体的质粒编码的假定遗传决定子，这可能与针对无共生体宿主的杀手性状有关。虽然尚不清楚这些基因是祖先的还是最近水平获得的，但 R 体在多个全孢菌与其宿主相互作用的进化中发挥着有趣且独特的作用。