Proposal to emend rules 50a and 50b of the international code of nomenclature of prokaryotes

Rules 50a and 50b of the International Code of Nomenclature of Prokaryotes respectively regulate the elevation of a subspecies to the rank of a species and the lowering of a species to the rank of subspecies. The Code does not indicate that the resulting new names must be considered new combinations, as the cases described in Rules 50a and 50b are not covered by Rule 34a. Based on the rules of the Code, new combination events are applicable only at the identical rank, and therefore new combination events and new species/subspecies events are mutually exclusive. In spite of this there have been at least 44 cases in which the new names were described as comb. nov. during elevation in rank from subspecies to species and at least 30 such cases during lowering in rank from species to subspecies. To prevent confusion in the future we propose adding notes to Rules 50a and 50b to clarify the issue.