sp. Strain B2-EB: Comparative Genomics Reveals Minimal Genomic Features Required by a Cultivable -Related Endofungal Bacterium.

Obligate bacterial endosymbionts are critical to the existence of many eukaryotes. Such endobacteria are usually characterized by reduced genomes and metabolic dependence on the host, which may cause difficulty in isolating them in pure cultures. Family -related endofungal bacteria affiliated with the clade can be associated with the fungal subphyla Mortierellomycotina and Glomeromycotina. In this study, a cultivable endosymbiotic bacterium, sp. strain B2-EB, present in the fungal host was obtained successfully. The B2-EB genome (1.88 Mb) represents the smallest genome among the endofungal bacterium (2.64-2.80 Mb) of and the uncultured endosymbiont " Glomeribacter gigasporarum" (1.37 to 2.36 Mb) of arbuscular mycorrhizal fungi. Despite a reduction in genome size, strain B2-EB displays a high genome completeness, suggesting a nondegenerative reduction in the B2-EB genome. Compared with a large proportion of transposable elements (TEs) in other known genomes (7.2 to 11.5% of the total genome length), TEs accounted for only 2.4% of the B2-EB genome. This pattern, together with a high proportion of single-copy genes in the B2-EB genome, suggests that the B2-EB genome reached a state of relative evolutionary stability. These results represent the most streamlined structure among the cultivable endofungal bacteria and suggest the minimal genome features required by both an endofungal lifestyle and artificial culture. This study allows us to understand the genome evolution of -related endosymbionts and to elucidate microbiological interactions. This study attempted the isolation of a novel endobacterium, sp. B2-EB (JCM 33615), harbored in the fungal host E1425 (JCM 39028). We report the complete genome sequence of this strain, which possesses a reduced genome size with relatively high genome completeness and a streamlined genome structure. The information indicates the minimal genomic features required by both the endofungal lifestyle and artificial cultivation, which furthers our understanding of genome reduction in fungal endosymbionts and extends the culture resources for biotechnological development on engineering synthetic microbiomes.