Bouchon, Didier

Abstract He.pat.in'co.la. Gr. neut. n. hêpar (gen. hêpatos) , liver; L. masc./fem. n. incola , inhabitant, dweller; N.L. masc. n. Hepatincola , a dweller of the liver. Pseudomonadota / Alphaproteobacteria / Rhodospirilalles / Candidatus Hepatincolaceae / Candidatus Hepatincola Candidatus Hepatincola is a genus of uncultured, obligately host‐associated bacteria originally discovered in the lumen of the midgut glands (hepatopancreas) of terrestrial isopods. In this niche, the bacteria exhibit a characteristic morphology: elongated rods with a stalk‐like appendage at one pole, which can be inserted between the microvilli of the midgut epithelium. All known members of the genus belong to two species ( Ca . Hepatincola porcellionum and a yet to be named Ca . Hepatincola sp.) associated with terrestrial isopods. However, closely related genera from the same family have been observed in numerous aquatic and terrestrial members of the Ecdysozoa (molting invertebrates). Ca . Hepatincola and closely related genera all have reduced genomes of about 1.3 Mb with low DNA G + C content ( c . 30%) and extremely limited biosynthetic capacities but rich in transporters to import essential nutrients and precursors from the environment. In line with its ecological niche (the host gut lumen, a nutrient‐rich environment) and genomic characteristics, Ca . Hepatincola is rather a nutrient scavenger than a nutrient provider for the host. Ca . Hepatincola was initially classified within the order Rickettsiales and later within the Holosporales , but recent phylogenomic analyses support its phylogenetic placement within the order Rhodospirillales . DNA G + C content (mol % ) : 30. Type species : Candidatus Hepatincola porcellionum Wang et al. (2004), CL1.

Abstract The order Holosporales (Alphaproteobacteria) encompasses obligate intracellular bacterial symbionts of diverse Eukaryotes. These bacteria have highly streamlined genomes and can have negative fitness effects on the host. Herein, we present a comparative analysis of the first genome sequences of ‘Ca. Hepatincola porcellionum’, a facultative symbiont occurring extracellularly in the midgut glands of terrestrial isopods. Using a combination of long-read and short-read sequencing, we obtained the complete circular genomes of two Hepatincola strains and an additional metagenome-assembled draft genome. Phylogenomic analysis validated its phylogenetic position as an early-branching family-level clade relative to all other established Holosporales families associated with protists. A 16S rRNA gene survey revealed that this new family encompasses diverse bacteria associated with both marine and terrestrial host species, which expands the host range of Holosporales bacteria from protists to several phyla of the Ecdysozoa (Arthropoda and Priapulida). Hepatincola has a highly streamlined genome with reduced metabolic and biosynthetic capacities as well as a large repertoire of transmembrane transporters. This suggests that this symbiont is rather a nutrient scavenger than a nutrient provider for the host, likely benefitting from a nutrient-rich environment to import all necessary metabolites and precursors. Hepatincola further possesses a different set of bacterial secretion systems compared to protist-associated Holosporales, suggesting different host-symbiont interactions depending on the host organism.