VIRUS TAXONOMY: PHILOSOPHY AND PRACTICE OF BINOMIAL NOMENCLATURE

The International Committee on Taxonomy of Viruses (ICTV), formerly the International Committee on Nomenclature of Viruses, founded in 1966, is responsible for developing and maintaining an internationally agreed system of hierarchical classification of viruses and naming of taxa. The ICTV also oversees the classification of other mobile genetic elements including satellite nucleic acids, viriforms and viroids. Over time the taxonomic framework has been adapted to the current knowledge on the evolution of virosphere. In particular, the ICTV now allows the classification of viruses known only from genome sequences obtained by metagenomic analyses. Taxonomic ranks have been expanded beyond orders up to realms, to include eight primary ranks (realm, kingdom, phylum, class, order, family, genus and species) and seven secondary ranks (subrealm, subkingdom, subphylum, subclass, suborder, subfamily and subgenus), paving the way to comprehensive studies on evolutionary connections of viruses. The expansion of taxonomic ranks brings virus taxonomy closer to other biological taxonomies, however a standardized format for virus species names was still lacking. Species naming in all these taxonomies follow a Latinized binomial format (i.e., binomial nomenclature) first introduced by Carl Linnaeus in 1753, consisting of two italicized words indicating the genus (“genus name”) and the species (“specific name/species epithet”), respectively. Typical examples of binomial species names are Arabidopsis thaliana, Saccharomyces cerevisiae, Homo sapiens, and Escherichia coli. The adoption of the binomial nomenclature allows to clearly distinguish the species (e.g. Homo sapiens, Saccharomyces cerevisiae) from physical entities (a human, yeast). In contrast, viral species names did not follow a uniform format, except for the requirement to be italicized and to have the first letter of the first word capitalized. The distinction between the virus species and the virus name based mainly on the font style has been for years cause of confusion between two substantially different concepts. In fact, “virus species” is a taxonomic category, whereas “virus” is referred to the physical entity, which can infect a host, or can be studied in laboratories. The ICTV recognized the need for a standardized nomenclature for virus species a long time ago and, after years of internal debates, in 2020 launched a call for public discussion and feedback on adopting a uniform binomial nomenclature. Following this, a binomial nomenclature for virus species was ratified by the ICTV in the 2021 vote. Thus, a virus species name will consist of two italicized words, the first one being the genus name and the second one consisting of a “freeform” species epithet. In this framework, Linnaean-style, Latinized virus species are permitted, but not mandated. The species names can be composed using the standard Latin-script English alphabet containing 26 letters and/or Arabic numbers. Starting 2021, newly created virus species names adopted a binomial format. The process of renaming all current species has been completed and ratified early in 2024. Very importantly, this process involves virus species names only. The common virus name will remain unchanged. Examples of how to correctly write virus species names and the corresponding virus names are given, with reference to the plant virus family Tombusviridae.