Kuwahara, Hirokazu

Abstract Several Trichonympha protist species in the termite gut have independently acquired Desulfovibrio ectosymbionts in apparently different stages of symbiosis. Here, we obtained the near-complete genome sequence of Desulfovibrio phylotype ZnDsv-02, which attaches to the surface of Trichonympha collaris cells, and compared it with a previously obtained genome sequence of ‘Candidatus Desulfovibrio trichonymphae’ phylotype Rs-N31, which is almost completely embedded in the cytoplasm of Trichonympha agilis. Single-nucleotide polymorphism analysis indicated that although Rs-N31 is almost clonal, the ZnDsv-02 population on a single host cell is heterogeneous. Despite these differences, the genome of ZnDsv-02 has been reduced to 1.6 Mb, which is comparable to that of Rs-N31 (1.4 Mb), but unlike other known ectosymbionts of protists with a genome similar in size to their free-living relatives. Except for the presence of a lactate utilization pathway, cell-adhesion components and anti-phage defense systems in ZnDsv-02, the overall gene-loss pattern between the two genomes is very similar, including the loss of genes responsive to environmental changes. Our study suggests that genome reduction can occur in ectosymbionts, even when they can be transmitted horizontally and obtain genes via lateral transfer, and that the symbiont genome size depends heavily on their role in the symbiotic system.

Abstract We investigated the phylogenetic diversity, localisation and metabolism of an uncultured bacterial clade, Termite Group 2 (TG2), or ZB3, in the termite gut, which belongs to the candidate phylum ‘Margulisbacteria’. We performed 16S rRNA amplicon sequencing analysis and detected TG2/ZB3 sequences in 40 out of 72 termite and cockroach species, which exclusively constituted a monophyletic cluster in the TG2/ZB3 clade. Fluorescence in situ hybridisation analysis in lower termites revealed that these bacteria are specifically attached to ectosymbiotic spirochetes of oxymonad gut protists. Draft genomes of four TG2/ZB3 phylotypes from a small number of bacterial cells were reconstructed, and functional genome analysis suggested that these bacteria hydrolyse and ferment cellulose/cellobiose to H2, CO2, acetate and ethanol. We also assembled a draft genome for a partner Treponema spirochete and found that it encoded genes for reductive acetogenesis from H2 and CO2. We hypothesise that the TG2/ZB3 bacteria we report here are commensal or mutualistic symbionts of the spirochetes, exploiting the spirochetes as H2 sinks. For these bacteria, we propose a novel genus, ‘Candidatus Termititenax’, which represents a hitherto uncharacterised class-level clade in ‘Margulisbacteria’. Our findings add another layer, i.e., cellular association between bacteria, to the multi-layered symbiotic system in the termite gut.