Revisiting the classification of curtoviruses based on genome-wide pairwise identity.

Members of the genus Curtovirus (family Geminiviridae) are important pathogens of many wild and cultivated plant species. Until recently, relatively few full curtovirus genomes have been characterised. However, with the 19 full genome sequences now available in public databases, we revisit the proposed curtovirus species and strain classification criteria. Using pairwise identities coupled with phylogenetic evidence, revised species and strain demarcation guidelines have been instituted. Specifically, we have established 77 % genome-wide pairwise identity as a species demarcation threshold and 94 % genome-wide pairwise identity as a strain demarcation threshold. Hence, whereas curtovirus sequences with >77 % genome-wide pairwise identity would be classified as belonging to the same species, those sharing >94 % identity would be classified as belonging to the same strain. We provide step-by-step guidelines to facilitate the classification of newly discovered curtovirus full genome sequences and a set of defined criteria for naming new species and strains. The revision yields three curtovirus species: Beet curly top virus (BCTV), Spinach severe surly top virus (SpSCTV) and Horseradish curly top virus (HrCTV).