Origin of symbiotic gut spirochetes as key players in the nutrition of termites

Summary Termites harbour symbiotic spirochetes in their hindguts, which have long been considered treponemes, although they represent separate lines of descent from known species of Treponema . ‘Termite gut treponemes’ have a mutualistic relationship with the host termites with their physiological properties including CO 2 ‐reductive acetogenesis, from which the resulting acetate fulfils most of the respiratory requirement of the host. Song and co‐workers showed that a spirochetal isolate (strain RmG30) from a Madeira cockroach represents the earliest branching lineage of extremely diverse termite ( Treponema ) cluster I and was a simple homolactic fermenter, suggesting that CO 2 ‐reductive acetogenesis exhibited by some members of termite cluster I originated via horizontal gene transfer. Phylogenomic and 16S rRNA sequence‐based phylogenetic analyses indicated a deeply‐branched sister clade containing termite cluster I was distinguishable as a family‐level lineage. In this context, a n