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Monday, February 1, 2010 @ 5:37 PM
Entry02: Classification of Viruses Classifications of viruses involves placing viruses into taxonomic system: Kingdom, Phylum, Class, Order, Family, Genus and Species. Viruses are classified according to morphology, chemical composition and mode of replication. Today, there are more than 30 000 virus isolates known and are grouped in more thwn 3 600 species, in 164 genera and 71 families. Viral morphology aids in grouping viruses into families. A viral family might only consists of viruses that only replicate in vertebrates, invertebrates, plants, or bacteria. Certain families contain viruses that can replicate in more than one of these hosts. International Committe of Taxanomy of Viruses(ICTV) Classification The International Committe of Taxonomy of Viruses(ICTV) began devising and implementing rules for the naming and classification of viruses in the early 1990s which is an effort that continues to the present day. The ICTV Classification system:1. Order -virales 2. Family -viridae 3. Subfamily -virinae 4. Genus -virus 5. Species Currently, the 6 orders that have been established by ICTV are: Caudovirales, Herpesvirales, Mononegavirales, Nidovirales, Picornavirales, and Tymovirales. So far, 6 orders, 87 families, 19 subfamilies, 348 genera, and 2,288 species of virus have been defined. There are several rules that have been set to classification and nomenculture. General Rules: 1. Classification and nomenclature of viruses and sub-viral agents shall be international and applied universally. 2. The universal virus classification system shall employ the hierarchical levels of Order, Family, Subfamily, Genus, and Species . 3. The ICTV is not responsible for classification and nomenclature of taxa below the rank of species. The classification and naming of serotypes, genotypes, strains, variants and isolates of virus species is the responsibility of acknowledged international specialist groups. 4. Artificially created viruses and laboratory hybrid viruses will not be given taxonomic consideration. Their classification will be the responsibility of acknowledged international specialist groups. 5. Taxa will be established only when representative members are sufficiently well characterized and described in the published literature so as to allow them to be identified unambiguously and the taxon to be distinguished from other similar taxa. 6. When it is uncertain how to classify a species into a genus but its classification in a family is clear, it will be classified as an unassigned species of that family. 7. Names will only be accepted if they are linked to taxa at the hierarchical levels described in Rule 2 and which have been approved by the ICTV. Rules Pertaining to Naming Taxa: 8. Names proposed for taxa are "valid names" if they conform to the Rules set out in the Code and they pertain to established taxa. Valid names are "accepted names" if they are recorded as approved international names in the 6th ICTV Report or become "accepted names" by an ICTV vote of approval for a taxonomic proposal. 9. Existing names of taxa and viruses shall be retained whenever feasible. 10. The rule of priority in naming taxa and viruses shall not be observed. 11. No person's name shall be used when devising names for new taxa. 12. Names for taxa shall be easy to use and easy to remember. Euphonious names are preferred. 13. Subscripts, superscripts, hyphens, oblique bars and Greek letters may not be used in devising new names. 14. New names shall not duplicate approved names. New names shall be chosen such that they are not closely similar to names that are in use currently or have been in use in the recent past. 15. Sigla may be accepted as names of taxa, provided that they are meaningful to virologists in the field, normally as represented by Study Groups. 16. In the event of more than one candidate name being proposed, the relevant Subcommittee will make a recommendation to the Executive Committee of the ICTV, which will then decide among the candidates. 17. If no suitable name is proposed for a taxon, the taxon may be approved and the name will be left undecided until the adoption of an acceptable international name when one is proposed to and accepted by ICTV. 18. Names shall be selected such that they, or parts of them, do not convey a meaning for the taxon which would either (1) seem to exclude viruses which lack the character described by the name but which are members of the taxon being named, or (2) seem to exclude viruses which are as yet undescribed but which might belong to the taxon being named, or (3) appear to include within the taxon viruses which are members of different taxa. 19. New names shall be chosen with due regard to national and/or local sensitivities. When names are universally used by virologists in published work, these or derivatives shall be the preferred basis for creating names, irrespective of national origin. 20. Proposals for new names, name changes, establishment of taxa and taxonomic placement of taxa shall be submitted to the Executtive Committee of the ICTV in the form of taxonomic proposals. All relevant ICTV subcommittees and study groups will be consulted prior to a decision being taken. Rules Pertaining to Species: 21. A species is defined as a polythetic class of viruses or sub-viral agents that constitutes a replicating lineage and occupies a particular ecological niche. 22. When an ICTV Subcommittee is uncertain about the taxonomic status of a new species or about the assignment of the new species to an established genus, the new species will be listed as a tentative species in the appropriate genus or family. Names of tentative species, as of taxa generally (Rule 14), shall not duplicate approved names and shall be chosen such that they are not closely similar to names that are in use currently, names that have been in use in the recent past, or names of definitive species. 23. A species name shall consist of as few words as practicable but shall not consist only of a host name and a formal ending such as "virus". 24. A species name, usually together with a strain designation, must provide an appropriately unambiguous identification without mention of the genus or family to which it belongs. 25. Numbers, letters, or combinations thereof may be used as species epithets where such numbers and letters are already widely used. However, newly designated serial numbers, letters or combinations thereof are not acceptable alone as species epithets. If a number or letter series is in existence it may be continued. Rules Pertaining to Genera: 26. A genus is a group of species sharing certain common characters. 27. A name for a genus containing virus species shall be a single word ending in "...Virus"; a name for a genus containing viroid species shall be a single word ending in "...Viroid". 28. Approval of a new genus must be accompanied by the approval of a type species. Rules Pertaining to Subfamilies: 29. 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. 30. A name for a subfamily containing virus genera shall be a single word ending in "...Virinae." Rules Pertaining to Families: 31. A family is a group of genera (whether or not these are organized into subfamilies) sharing certain common characters. 32. A name for a family containing virus genera shall be a single word ending in "...Viridae"; a name for a family containing viriod genera shall be a single word ending in "...Viroidae." Rules Pertaining to Orders: 33. An order is a group of families sharing certain common characters. 34. A name for an order containing virus families shall be a single word ending in "...Virales" Baltimore Classification The Balitmore Classification is based on the genetic contents and the replicaion strategies of the virus. The genetic material that is consistant in all cells is the double-stranded Deoxyribonucleic Acid(DNA), but some viruses uses Ribonucleic Acid(RNA) or single-stranded DNA to containt the gentetic information. According to Baltimore Classification, viruses are classified into 7 classes: 1. Double-stranded DNA viruses 2. Single-stranded DNA viruses 3. Double-stranded RNA viruses 4. (+)-sense Single-stranded RNA viruses 5. (-)-sense Single-stranded RNA viruses 6. RNA reverse transcribing viruses 7. DNA reverse transcribing viruses Labels: #entry |