Leveraging the subgenus category to address monophyletic genus over-splitting: illustration with recently proposed Mycobacteriales genera

Three related circumstances are affecting the stability of prokaryotic taxonomy and nomenclature, with significant implications in the field of pathogenic micro-organisms: (i) the subjective application of genomics-based demarcation criteria to subdivide monophyletic genera, creating an increasing number of new genera; (ii) databases' preference for the latest validly published names; and (iii) the practical irreversibility of new names in databases, even when later taxonomic opinion supports reverting to previous classifications. Due to understandable end-user reluctance to accept name changes affecting well-known pathogens, parallel nomenclatures coexist, causing confusion. To address this issue, we propose using the subgenus category to mitigate the disruptive impact of genus name changes in databases. Specifically, we suggest lowering to subgenus rank those new genera arising from monophyletic genera splits that have limited practical utility and may contribute to taxonomic instability. Under the International Code of Nomenclature of Prokaryotes, the species' generic name would revert to its previous synonym, optionally followed in parentheses by the validly published subgenus name (corresponding to the latest genus synonym used in databases). Because the subgenus is an optional taxonomic category, it may be omitted; however, its use may facilitate the mapping of synonyms in databases and literature. We illustrate this strategy through its application to recent genus splits in the <i>Mycobacteriales</i>, specifically the genus <i>Prescottella</i> nested within the rhodococcal radiation, and the several genera into which <i>Mycobacterium</i> was subdivided.