Large-scale comparative genomics unravels great genomic diversity across the Rickettsia and Ca. Megaira genera and identifies Torix group as an evolutionarily distinct clade.

Members of the genus Rickettsia were originally identified as causative agents of mammalian vector-borne disease. In the last 25 years we have recognised that many Rickettsia are arthropod symbionts, and sit alongside a sister taxon, Ca. Megaira, which are symbiotic associates of microeukaryotes. The lack of genomic information for symbiotic strains affects our ability to determine the evolutionary relationships between strains and understand the biological underpinnings of the different symbioses. We clarify these relationships by assembling 26 genomes of Rickettsia from understudied groups, and the first two Ca. Megaira, from various insects and microeukaryotes. Of note, the accessory genome diversity and broad host range of Torix Rickettsia parallels all other Rickettsia combined. This diversity, alongside the breadth of host species, make the Torix clade an important hidden player in invertebrate biology and physiology. We argue this clade should be given its own genus status, for which we propose Ca. Tisiphia. (Genomes derived from publicly available genomes deposited in PRJNA767332)