Classification of Viruses

Introduction

Virus classification involves naming and placing viruses into a taxonomic system. It is a subject of ongoing debate and proposals, largely due to the pseudo-living nature of viruses, which are not yet definitively living or non-living. As such, they do not fit neatly into the established biological classification system in place for cellular organisms.


Virus classification is based mainly on phenotypic characteristics, including morphology, nucleic acid type, mode of replication, host organisms, and the type of disease they cause. And also the chemical composition, the site of capsid assembly, the site of envelopment in enveloped viruses, the configuration of the nucleic acid and whether the genome is monopartite (meaning all genes are found within one segment of nucleic acid) or multipartite.

The genomic RNA strand of single-stranded RNA viruses is called sense (positive sense, plus sense) in orientation if it can serve as mRNA, and antisense (negative sense, minus sense) if a complementary strand synthesized by a viral RNA transcriptase serves as mRNA.


A combination of two main schemes is currently in widespread use for the classification of viruses. David Baltimore, a Nobel Prize-winning biologist, devised the Baltimore classification system, which places viruses into one of seven groups. These groups are designated by Roman numerals and separate viruses based on their mode of replication, and genome type.
Accompanying this broad method of classification are specific naming conventions and further classification guidelines set out by the International Committee on Taxonomy of Viruses.




Virosphere 2005



ICTV Classification


The International Committee on Taxonomy of Viruses began to devise and implement rules for the naming and classification of viruses early in the 1990s, an effort that continues to the present day. The ICTV is the only body charged by the International Union of Microbiological Societies (IUMS) with the task of developing, refining, and maintaining a universal virus taxonomy. The system shares many features with the classification system of cellular organisms, such as taxon structure. Viral classification starts at the level of order and follows as thus, with the taxon suffixes given in italics:


Order (-virales) - An order is a group of families sharing certain common characters.

Family (-viridae) - A family is a group of genera, whether or not these are organized into subfamilies, sharing certain common characters.

Subfamily (-virinae) - A subfamily is a group of genera sharing certain common characters. The taxon shall be used only when it is needed to solve a complex hierarchical problem.

Genus (-virus) - A virus genus is a group of related species that share some significant properties and often only differ in host range and virulence. Approval of a new genus must be accompanied by the approval of a type species.

Species - A species name shall consist of as few words as practicable but must not consist only of a host name and the word virus. A species name must provide an appropriately unambiguous identification of the species.



So far, six orders have been established by the ICTV: the Caudovirales, Herpesvirales, Mononegavirales, Nidovirales, Picornavirales, and Tymovirales.
These orders span viruses with varying host ranges.


- Caudovirales are tailed dsDNA (group I) bacteriophages
- Herpesvirales contains large eukaryotic dsDNA viruses
- Mononegavirales includes non-segmented (-) strand ssRNA (Group V) plant and animal viruses
- Nidovirales is composed of (+) strand ssRNA (Group IV) viruses with vertebrate hosts
- Picornavirales contains small (+) strand ssRNA viruses that infect a variety of plant, insect, and animal hosts
- Tymovirales contains monopartite ssRNA viruses that infect plants.

The establishment of an order is based on the inference that the virus families contained within a single order have most likely evolved from a common ancestor. The majority of virus families remain unplaced. Currently (2009) 6 orders, 87 families, 19 subfamilies, 348 genera, and 2,288 species of virus have been defined.



Baltimore Classification

Baltimore classification is a classification system that places viruses into one of seven groups depending on a combination of their nucleic acid (DNA or RNA), strandedness (single-stranded or double-stranded), Sense, and method of replication.


Picture for better understanding.


Classifying viruses according to their genome means that those in a given category will all behave in a similar fashion, offering some indication of how to proceed with further research.
Viruses can be placed in one of the seven following groups:


I: dsDNA viruses - viruses possess double-stranded DNA.(e.g. Adenoviruses, Herpesviruses, Poxviruses)

II: ssDNA viruses (+)sense DNA - viruses possess single-stranded DNA.
(e.g. Parvoviruses)

III: dsRNA viruses - viruses possess double-stranded RNA genomes, which are always segmented (e.g. Reoviruses)

IV: (+)ssRNA viruses (+)sense RNA - viruses possess positive-sense single-stranded RNA genomes. Many well known viruses are found in this group.
(e.g. picornaviruses, SARS virus, hepatitis C virus, yellow fever virus, and rubella virus.)

V: (−)ssRNA viruses (−)sense RNA - viruses possess negative-sense single-stranded RNA genomes.(e.g. Orthomyxoviruses, Rhabdoviruses, Ebola and Marburg viruses, influenza virus, measles, mumps and rabies)

VI: ssRNA-RT viruses (+)sense RNA with DNA intermediate in life-cycle - viruses possess single-stranded RNA genomes and replicate using reverse transcriptase. (e.g. Retroviruses)

VII: dsDNA-RT viruses - viruses possess double-stranded DNA genomes and replicate using reverse transcriptase.(e.g. Hepadnaviruses, Hepatitis B)









References:
http://en.wikipedia.org/wiki/Virus_classification
http://en.wikipedia.org/wiki/Baltimore_classification
http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=mmed&part=A2252