Refining the taxonomy of the order () based on whole genome comparisons of over 130 type strains.

The alphaproteobacterial order consists of 38 families comprising at least 152 validly published genera as of January 2024. The order was first described in 1957 and underwent important revisions in 2020. However, we show that several inconsistencies in the taxonomy of this order remain and we argue that there is a need for a consistent framework for defining families within the order. We propose a common genome-based framework for defining families within the order , suggesting that families represent monophyletic groups in core-genome phylogenies that share pairwise average amino acid identity values above ~75 % when calculated from a core set of 59 proteins. Applying this framework, we propose the formation of four new families and to reassign the genera , , and into fam. nov., fam. nov., and fam. nov., respectively, and the genera , , , and into fam. nov. We further propose to unify the families , , , and as ; the families and as ; the families and as ; and the families and as . Lastly, we propose to reassign several genera to existing families. Specifically, we propose to reassign the genus to the family ; the genera , , and to the family ; the genus to the emended family ; the genus to the family ; and the genus to the family . Our data also support the recent proposal to reassign the genus to the family .