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Virus classification
Virus classification is the process of naming viruses and placing them into a taxonomic system similar to the classification systems used for cellular organisms.
Viruses are classified by phenotypic characteristics, such as morphology, nucleic acid type, mode of replication, host organisms, and the type of disease they cause. The formal taxonomic classification of viruses is the responsibility of the International Committee on Taxonomy of Viruses (ICTV) system, although the Baltimore classification system can be used to place viruses into one of seven groups based on their manner of mRNA synthesis. Specific naming conventions and further classification guidelines are set out by the ICTV.
In 2021, the ICTV changed the International Code of Virus Classification and Nomenclature (ICVCN) to mandate a binomial format (genus|| ||species) for naming new viral species similar to that used for cellular organisms; the names of species coined prior to 2021 are gradually being converted to the new format, a process planned for completion by the end of 2023.[needs update]
As of 2022, the ICTV taxonomy listed 11,273 named virus species (including some classed as satellite viruses and others as viroids) in 2,818 genera, 264 families, 72 orders, 40 classes, 17 phyla, 9 kingdoms and 6 realms. However, the number of named viruses considerably exceeds the number of named virus species since, by contrast to the classification systems used elsewhere in biology, a virus "species" is a collective name for a group of (presumably related) viruses sharing certain common features (see below). Also, the use of the term "kingdom" in virology does not equate to its usage in other biological groups, where it reflects high level groupings that separate completely different kinds of organisms (see Kingdom (biology)).
The currently accepted and formal definition of a 'virus' was accepted by the ICTV Executive Committee in November 2020 and ratified in March 2021, and is as follows:
Viruses sensu stricto are defined operationally by the ICTV as a type of MGE that encodes at least one protein that is a major component of the virion encasing the nucleic acid of the respective MGE and therefore the gene encoding the major virion protein itself or MGEs that are clearly demonstrable to be members of a line of evolutionary descent of such major virion protein-encoding entities. Any monophyletic group of MGEs that originates from a virion protein-encoding ancestor should be classified as a group of viruses.
Species form the basis for any biological classification system. Before 1982, it was thought that viruses could not be made to fit Ernst Mayr's reproductive concept of species, and so were not amenable to such treatment. In 1982, the ICTV started to define a species as "a cluster of strains" with unique identifying qualities. In 1991, the more specific principle that a virus species is a polythetic class of viruses that constitutes a replicating lineage and occupies a particular ecological niche was adopted.
As at 2021 (the latest edition of the ICVCN), the ICTV definition of species states: "A species is the lowest taxonomic level in the hierarchy approved by the ICTV. A species is a monophyletic group of MGEs (mobile genetic elements) whose properties can be distinguished from those of other species by multiple criteria", with the comment "The criteria by which different species within a genus are distinguished shall be established by the appropriate Study Group. These criteria may include, but are not limited to, natural and experimental host range, cell and tissue tropism, pathogenicity, vector specificity, antigenicity, and the degree of relatedness of their genomes or genes. The criteria used should be published in the relevant section of the ICTV Report and reviewed periodically by the appropriate Study Group."
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Virus classification
Virus classification is the process of naming viruses and placing them into a taxonomic system similar to the classification systems used for cellular organisms.
Viruses are classified by phenotypic characteristics, such as morphology, nucleic acid type, mode of replication, host organisms, and the type of disease they cause. The formal taxonomic classification of viruses is the responsibility of the International Committee on Taxonomy of Viruses (ICTV) system, although the Baltimore classification system can be used to place viruses into one of seven groups based on their manner of mRNA synthesis. Specific naming conventions and further classification guidelines are set out by the ICTV.
In 2021, the ICTV changed the International Code of Virus Classification and Nomenclature (ICVCN) to mandate a binomial format (genus|| ||species) for naming new viral species similar to that used for cellular organisms; the names of species coined prior to 2021 are gradually being converted to the new format, a process planned for completion by the end of 2023.[needs update]
As of 2022, the ICTV taxonomy listed 11,273 named virus species (including some classed as satellite viruses and others as viroids) in 2,818 genera, 264 families, 72 orders, 40 classes, 17 phyla, 9 kingdoms and 6 realms. However, the number of named viruses considerably exceeds the number of named virus species since, by contrast to the classification systems used elsewhere in biology, a virus "species" is a collective name for a group of (presumably related) viruses sharing certain common features (see below). Also, the use of the term "kingdom" in virology does not equate to its usage in other biological groups, where it reflects high level groupings that separate completely different kinds of organisms (see Kingdom (biology)).
The currently accepted and formal definition of a 'virus' was accepted by the ICTV Executive Committee in November 2020 and ratified in March 2021, and is as follows:
Viruses sensu stricto are defined operationally by the ICTV as a type of MGE that encodes at least one protein that is a major component of the virion encasing the nucleic acid of the respective MGE and therefore the gene encoding the major virion protein itself or MGEs that are clearly demonstrable to be members of a line of evolutionary descent of such major virion protein-encoding entities. Any monophyletic group of MGEs that originates from a virion protein-encoding ancestor should be classified as a group of viruses.
Species form the basis for any biological classification system. Before 1982, it was thought that viruses could not be made to fit Ernst Mayr's reproductive concept of species, and so were not amenable to such treatment. In 1982, the ICTV started to define a species as "a cluster of strains" with unique identifying qualities. In 1991, the more specific principle that a virus species is a polythetic class of viruses that constitutes a replicating lineage and occupies a particular ecological niche was adopted.
As at 2021 (the latest edition of the ICVCN), the ICTV definition of species states: "A species is the lowest taxonomic level in the hierarchy approved by the ICTV. A species is a monophyletic group of MGEs (mobile genetic elements) whose properties can be distinguished from those of other species by multiple criteria", with the comment "The criteria by which different species within a genus are distinguished shall be established by the appropriate Study Group. These criteria may include, but are not limited to, natural and experimental host range, cell and tissue tropism, pathogenicity, vector specificity, antigenicity, and the degree of relatedness of their genomes or genes. The criteria used should be published in the relevant section of the ICTV Report and reviewed periodically by the appropriate Study Group."