Nomenclature and Classification of Virus

Table of Contents

History of Virus Nomenclature

The ICTV Nomenclature

Rules of Classification and Nomenclature

Baltimore Classification

Classification

DNA Viruses

RNA Viruses

Reverse Transcribing Viruses

Frequently Asked Questions

The process of virus nomenclature and classification includes naming the viruses and placing them in taxonomic hierarchy, similar to other cellular organisms. Humans find a need to classify natural entities so that they can analyse the similar characteristics and properties of thousands of organisms in a straight manner. Since there are no fossil records available for viral particles, the taxonomy of viruses is entirely a result of logical and observational process.

History of Virus Nomenclature

The classification of viruses is a relatively new exercise because it was not till the 1890s that the viruses were discovered. It was in 1927 that Jonhson, a plant virologist first drew attention to the need for the classification of viruses. The classification of viruses then began by considering their biological, ecological and pathogenic characteristics. In 1939, Holmes divided the plant viruses based on their host reactions and host species, but could only describe 89 viruses.

The electron microscopic studies were advanced in the 1950s and thus came the classification of viruses based on their virion properties, such as Herpesvirus, Myxovirus and Poxvirus. At that time new viruses were being discovered every other day and many scientists tried to give a way of classification, but none of them could succeed.

In 1966, at the International Congress for Microbiology, held in Moscow, 43 virologists came forward to form a committee and named it International Committee on Nomenclature of Viruses (ICNV). The main objective of creating this committee was to create a taxonomic and nomenclature system of viruses that was accepted worldwide. The name of this committee was changed to the International Committee on Taxonomy of Viruses (ICTV) in 1974.

The ICTV Nomenclature

According to the latest meeting of ICTV in March 2021, there are 10434 species, 2606 genuses, 168 subfamilies, 233 families and 65 orders of viruses. The ICTV began the process of naming and classification of viruses in the 1970s and the effort continues till today. The objectives of ICTV are:

• to develop an internationally agreed taxonomy for viruses and other mobile genetic elements (MGEs) that are part of the virosphere;
• to develop internationally agreed names for virus taxa;
• to communicate taxonomic decisions to the international community of virologists;
• to maintain an Index of agreed names of virus taxa.

Rules of Classification and Nomenclature

• The nomenclature of viruses is independent of other biological nomenclatures. It is an exception in the International Code of Bionomenclature.
• The hierarchical levels of viral classification with their taxonomic suffixes are as follows:

• The ICTV does not classify ranks below species such as variants, subspecies or serotypes.
• A person’s name can be used for naming a taxa by obtaining a written consent form him or her. The name is further approved by committees of ICTV.
• The names of the taxa should be such that they are easy to remember. Euphonious names are a preference.
• Subscripts, superscripts, punctuation marks, and other non-Latin letters are prohibited for use in naming a taxon.
• Any new names that are given to a taxa should not be remotely similar to the existing names.
• Species is the lowest taxonomic rank that is classified by ICTV. A species is classified as such, so that it can be distinguished from other species on multiple criteria such as vector specificity, antigenicity, and genome.
• The species name is composed of two words separated by a space. The first word is the same as the name of the genus and begins with a capital letter. The second word does not have a fixed suffix and the entire name is italicised.

An example of gill-associated virus (GAV) is depicted based on ICTV nomenclature for your better understanding.

Realm:Riboviria

Kingdom:Orthornavirae

Phylum:Pisuviricota

Class: Pisoniviricetes

Order:Nidovirales

Suborder: Ronidovirineae

Family:Roniviridae

Subfamily:Okanivirinae

Genus:Okavirus

Subgenus:Tipravirus

Species:Gill-associated virus

Baltimore Classification

In 1971, David Baltimore, an American virologist gave a system of classification of viruses based on their manner of mRNA synthesis. He described seven groups of viruses that explained whether the genome was DNA or RNA, single or double stranded and the sense of single stranded RNA genome.

This classification is used along with the standard mode of classification by a majority of virologists.

Classification

DNA Viruses

In DNA viruses, the genome is made up of either double stranded or single stranded DNA. Four realms belong to this category: Duplodnaviria, Adnaviria, Varidnaviria and Monodnaviria. The two groups under this classification are:

• Group I: double-stranded DNA viruses: Example: Herpesvirus, Poxvirus, Adenovirus
• Group II: single-stranded DNA viruses: Example: Parvovirus

RNA Viruses

The genome of RNA viruses is made up of RNA and consists of three groups. A majority of RNA viruses are classified in Riboviria realm under kingdom Orthornavirae.

• Group III: double-stranded RNA viruses: Example: Reovirus
• Group IV: positive sense single-stranded RNA viruses: Example: Coronavirus, Togavirus, Picornavirus
• Group V: negative sense single-stranded RNA viruses: Example: Rhabdoviruses, Orthomyxoviruses

Reverse Transcribing Viruses

The reverse transcribing (RT) viruses are made up of either DNA or RNA that replicate by reverse transcription. These viruses are classified in the realm Riboviria under kingdom Pararnavirae. The two groups in this category are:

• Group VI: single-stranded RNA viruses with a DNA intermediate: Example: Retrovirus
• Group VII: double-stranded DNA viruses with an RNA intermediate: Example: Hepadnavirus

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Also Read:

• A Detailed Overview of Virology
• Viral Diseases – Causes, Symptoms and Treatment
• Virus Life Cycle: Introduction, Life Cycle, FAQs

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