Evolutionary history of the widespread association between placozoans and diverse Rickettsiales symbionts

Rickettsiales are an enigmatic clade of strictly host-associated bacteria, of which some are very common in aquatic habitats. Despite a wealth of sequencing-based observations, the hosts for many of these marine Rickettsiales are still unknown. The first molecular data from two closely related placozoan species points to Rickettsiales as abundant symbionts in these simple and early-branching animals. Given the electron microscopy observations of intracellular and Rickettsia-like-organisms in all placozoans examined, we systematically characterized the microbiomes of placozoan’s with a focus on the Rickettsiales . Using single-animal metagenomics, we detected a consistent association with Rickettsiales from the Midichloriaceae and Rickettsiaceae , as well as several other commonly associated bacteria, including representatives from additional intracellular groups such as Chlamydiota as well as Margulisbacteria . Our comparative phylogenetic analyses of placozoans and their symbiotic Rickettsiales revealed a long evolutionary entanglement, with host species-specific replacements with other Rickettsiales clades in several placozoans. The common association across all placozoan lineages suggests a beneficial role. As placozoans gain more and more attention as an emerging model across the life sciences, it is imperative to consider the Rickettsiales symbionts, as they are commonly known as potent manipulators of their hosts’ cellular and organismal biology in other associations.