Genome-based reclassification of Suonthocola fibrivorans as a later heterotypic synonym of Diplocloster agilis.

Both the genera Suonthocola and Diplocloster are members of the family Lachnospiraceae. Their type species, both Suonthocola fibrivorans Sanger_33^T and Diplocloster agilis ASD5720^T, were isolated from human faeces. A comparison of their 16S rRNA gene sequences revealed 100% similarity, suggesting their close relatedness and the possibility of belonging to the same species. To clarify their taxonomic relationship, genome-based analyses were carried out. Overall genomic relatedness indices analyses indicated that S. fibrivorans Sanger_33^T shared average amino acid identity and percentage of conserved proteins prediction values higher than the Lachnospiraceae-specific genus-level thresholds with D. agilis ASD5720^T, as well as the other two existing species of the genus Diplocloster. Additionally, S. fibrivorans Sanger_33^T formed a distinct branch with D. agilis ASD5720^T, clustering with the other two species of the genus Diplocloster into the same clade in both the 16S rRNA gene phylogenetic tree and the core-genome phylogenomic tree. Essentially, both the average nucleotide identity and digital DNA-DNA hybridization prediction values between S. fibrivorans Sanger_33^T and D. agilis ASD5720^T were above the recommended species boundaries. It is thus clear that S. fibrivorans Sanger_33^T and D. agilis ASD5720^T constitute the same species. On the basis of the earliest valid publication, priority is given to D. agilis Chaplin et al. 2022. Based on this, the orphan species S. fibrivorans Hitch et al. 2022 is reclassified as a later heterotypic synonym of D. agilis Chaplin et al. 2022, along with the reclassification of the genus Suonthocola Hitch et al. 2022 as a later synonym of Diplocloster Chaplin et al. 2022.