Lebedeva, Natalia

Summary Holosporales are an alphaproteobacterial lineage encompassing bacteria obligatorily associated with multiple diverse eukaryotes. For most representatives, little is known on the interactions with their hosts. In this study, we characterized a novel Holosporales symbiont of the ciliate Paramecium polycaryum . This bacterium inhabits the host cytoplasm, frequently forming quite large aggregates. Possibly due to such aggregates, host cells sometimes displayed lethal division defects. The symbiont was also able to experimentally stably infect another Paramecium polycaryum strain. The bacterium is phylogenetically related with symbionts of other ciliates and diplonemids, forming a putatively fast‐evolving clade within the family Holosporaceae . Similarly to many close relatives, it presents a very small genome (<600 kbp), and, accordingly, a limited predicted metabolism, implying a heavy dependence on Paramecium , thanks also to some specialized membrane transporters. Characterized features, including the presence of specific secretion systems, are overall suggestive of a mild parasitic effect on the host. From an evolutionary perspective, a potential ancestral trend towards pronounced genome reduction and possibly linked to parasitism could be inferred, at least among fast‐evolving Holosporaceae , with some lineage‐specific traits. Interestingly, similar convergent features could be observed in other host‐associated lineages, in particular Rickettsiales among Alphaproteobacteria .

Abstract Rickettsiales are a lineage of obligate intracellular Alphaproteobacteria, encompassing important human pathogens, manipulators of host reproduction, and mutualists. Here we report the discovery of a novel Rickettsiales bacterium associated with Paramecium, displaying a unique extracellular lifestyle, including the ability to replicate outside host cells. Genomic analyses show that the bacterium possesses a higher capability to synthesise amino acids, compared to all investigated Rickettsiales. Considering these observations, phylogenetic and phylogenomic reconstructions, and re-evaluating the different means of interaction of Rickettsiales bacteria with eukaryotic cells, we propose an alternative scenario for the evolution of intracellularity in Rickettsiales. According to our reconstruction, the Rickettsiales ancestor would have been an extracellular and metabolically versatile bacterium, while obligate intracellularity would have evolved later, in parallel and independently, in different sub-lineages. The proposed new scenario could impact on the open debate on the lifestyle of the last common ancestor of mitochondria within Alphaproteobacteria.

AbstractMembers of the order Rickettsiales are often found in association with ciliated protists. An interesting case is the bacterial endosymbiont “Candidatus Megaira”, which is phylogenetically closely related to the pathogen Rickettsia. “Candidatus Megaira” was first described as an intracellular bacterium in several ciliate species. Since then it has been found in association with diverse evolutionary distantly-related hosts, among them other unicellular eukaryotes, and also algae, and metazoa, such as cnidarians. We provide the characterization of several new strains of the type species “Candidatus Megaira polyxenophila”, and the multidisciplinary description of a novel species, “Candidatus Megaira venefica”, presenting peculiar features, which highlight the diversity and variability of these widespread bacterial endosymbionts. Screening of the 16S rRNA gene short amplicon database and phylogenetic analysis of 16S rRNA gene hypervariable regions revealed the presence of further hidden lineages, and provided hints on the possibility that these bacteria may be horizontally transmitted among aquatic protists and metazoa. The phylogenetic reconstruction supports the existence of at least five different separate species-level clades of “Candidatus Megaira”, and we designed a set of specific probes allowing easy recognition of the four major clades of the genus.