Virus taxonomy and the role of the International Committee on Taxonomy of Viruses (ICTV)

Abstract

The taxonomy of viruses is developed and overseen by the International Committee on Taxonomy of Viruses (ICTV), which scrutinizes, approves and ratifies taxonomic proposals, and maintains a list of virus taxa with approved names (https://ictv.global). The ICTV has approximately 180 members who vote by simple majority. Taxon-specific Study Groups established by the ICTV have a combined membership of over 600 scientists from the wider virology community; they provide comprehensive expertise across the range of known viruses and are major contributors to the creation and evaluation of taxonomic proposals. Proposals can be submitted by anyone and will be considered by the ICTV irrespective of Study Group support. Thus, virus taxonomy is developed from within the virology community and realized by a democratic decision-making process. The ICTV upholds the distinction between a virus or replicating genetic element as a physical entity and the taxon category to which it is assigned. This is reflected by the nomenclature of the virus species taxon, which is now mandated by the ICTV to be in a binomial format (genus + species epithet) and is typographically distinct from the names of viruses. Classification of viruses below the rank of species (such as, genotypes or strains) is not within the remit of the ICTV. This article, authored by the ICTV Executive Committee, explains the principles of virus taxonomy and the organization, function, processes and resources of the ICTV, with the aim of encouraging greater understanding and interaction among the wider virology community.

Keywords: ICTV; International Committee on Taxonomy of Viruses; virus classification; virus nomenclature; virus taxonomy.

Fig. 1.

The organization of the ICTV. ICTV members (yellow circle) include the Officers (dark orange circle), Elected Members including Subcommittee chairs (light orange circle), Life Members, National Members and Study Group chairs. The Bacterial Viruses Subcommittee and Fungal and Protist Virus Subcommittee also include ICTV members who are not Study Group chairs. The Officers, Elected Members, and Life Members are elected by a vote of the full ICTV membership. National Members are nominated by Member Societies of the Virology Division of the International Union of Microbiological Societies (IUMS). Members of Virus Subcommittees are appointed by the Virus Subcommittee chairs (who are themselves appointed from the Elected members by the Executive Committee) and include all Study Group chairs. An unlimited number of Study Group members are assembled by the Study Group chairs for their expertise on viruses of a particular taxon but are not part of the ICTV. The names of the many hundreds of Study Group Members are published on the ICTV website to recognize their valuable contribution to virus taxonomy (https://ictv.global/sc/dna, https://ictv.global/sc/rna-m, https://ictv.global/sc/rnas, https://ictv.global/sc/archaeal, https://ictv.global/sc/bacterial, https://ictv.global/sc/fungal-protist, https://ictv.global/sc/plants).

Fig. 2.

The ICTV taxonomic rank hierarchy. (a) The ICTV-authorized 15-rank hierarchal taxonomy of viruses as ratified in 2023. Taxonomic ranks are shown with an idealized distribution pattern of taxa; actual numbers of taxa are shown in brackets. When the ranks are described as a hierarchy, the species rank is often referred to as the lowest rank and the realm rank as the highest rank. When the ranks are used as phylogenetic terms, the realm rank may be described as basal, and the species rank as apical. (b) Taxonomic structure of the realm Adnaviria [45]. Species taxon names are as currently assigned, but will be revised to binomial format in 2023.

Fig. 3.

The number of taxa in primary ranks. (a) The number of taxa in the species, genus, family, and order ranks, 1971–2023. The drop in species taxa numbers in 1999 was due to some closely related viruses being consolidated into single species taxa at the same time as the systematic introduction of virus species taxa. (b) The number of taxa in the class, phylum, kingdom and realm ranks, 2018–2023. Note that the MSL in which these changes occurred generally has the previous calendar year as an identifier, indicating the year of the EC meeting at which the taxonomic changes were proposed.