Metagenome-assembled genomes of three provide insights into isopod-mollicute symbiosis.

The digestive organs of terrestrial isopods harbour bacteria of the recently proposed mollicute family . The only complete genome available so far for is that of ' Hepatoplasma crinochetorum'. The scarcity of genome sequences has hampered our understanding of the symbiotic relationship between isopods and mollicutes. Here, we present four complete metagenome-assembled genomes (MAGs) of uncultured members identified from shotgun sequencing data of isopods. We propose genomospecies names for three MAGs that show substantial sequence divergence from any previously known members: ' Tyloplasma litorale' identified from the semiterrestrial isopod , ' Hepatoplasma vulgare' identified from the common pill bug , and ' Hepatoplasma scabrum' identified from the common rough woodlouse . Phylogenomic analysis of 155 mollicutes confirmed that is a sister clade of in the order . The 16S ribosomal RNA gene sequences and phylogenomic analysis showed that ' Tyloplasma litorale' and other semiterrestrial isopod-associated mollicutes represent the placeholder genus 'g_Bg2' in the r214 release of the Genome Taxonomy Database, warranting their assignment to a novel genus. Our analysis also revealed that lack major metabolic pathways but has a likely intact type IIA CRISPR-Cas9 machinery. Although the localization of the members have not been verified microscopically in this study, these genomic characteristics are compatible with the idea that these mollicutes have an ectosymbiotic lifestyle with high nutritional dependence on their host, as has been demonstrated for other members of the family. We could not find evidence that encode polysaccharide-degrading enzymes that aid host digestion. If they are to provide nutritional benefits, it may be through extra-copy nucleases, peptidases, and a patatin-like lipase. Exploration of potential host-symbiont interaction-associated genes revealed large, repetitive open reading frames harbouring beta-sandwich domains, possibly involved with host cell adhesion. Overall, genomic analyses suggest that isopod-mollicute symbiosis is not characterized by carbohydrate degradation, and we speculate on their potential role as defensive symbionts through spatial competition with pathogens to prevent infection.