A listing of virus families and genera with some discriminatory features
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A listing of virus families and genera with some discriminatory features M. A. Mayo Scottish Crop Research Institute, Invergowrie, U.K.
For viruses, as for other biological entities, classification and nomenclature are necessary prerequisites for meaningful comparative study and discussion. The way viruses are classified and their taxa are named (i.e. taxonomy) is regulated by the International Committee on the Taxonomy of Viruses (ICTV) which publishes up-dates on previous taxonomies at intervals. The latest taxonomy is the 6th Report of ICTV [3] which describes 165 genera, of which 139 are classified among 50 families. The differences between this and the previous taxonomy [1] were agreed at the plenary meeting of ICTV in 1993 and have been described briefly by Pringle [4] and Mayo and Martelli [2]. The classification scheme is necessarily complex as it deals with viruses which range between the large, complicated poxviruses with virions c. 250 nm x 350 nm which contain up to 400 kbp of ds DNA and simple viruses such as leviviruses and tombusviruses with 25 to 30 nm diameter virions which contain c. 4 kb of ssRNA. However, a Key to the Placement of Viruses in Taxa is given in Murphy et al. [3] which, by using relatively few criteria, distinguishes every genus from every other genus. A further simplification, based on the Key and designed to illustrate virus diversity in as succinct a manner as possible, is shown in Table 1 (see pp. 1338-1340). Viruses have been divided into 6 categories depending on the nature of their genomes (dsDNA, ssDNA etc.). Below this level a few criteria distinguish genera either grouped into families or individually when no family exists. The criteria are nested from the top left of each genome category. A version of this Table will be placed on the Internet when ICTV have organized a home page. In the interim, copies can be obtained from the author.
Table 1
The da DNA Viruses Host a prokaryote Host a bacterium; virion with a contractile tall Host a bacterium; virion with a > 60 nm long non-contractile tail Host a bacterium; virlon with a < 30 nm long non-contractile tail Host a bacterium; virion without tails or envelopes DNA linear > 10 kbp DNA circular < 10 kbp Host a mycoplasma; virion enveloped Host an archaebacterium; virion rod-shaped Host an archaebactedum; virion lemon-shaped Host a eukaryote Virion contains multiple DNA molecules Virion contains a single large (> 9Okbp) DNA molecule DNA > 300 kbp; virion not enveloped; host an alga DNA usually < 300 kbp; virion enveloped; host an animal Genome covalently closed circular DNA Genome linear DNA; nucleocapsid not rod-shaped; Virlen ovoid or brick-shaped
Myoviridae : Siphoviridae : Podoviridae :
"T4-1ike phages', "L-like phages" "T7-1ike phages"
Tectiviridae : Corticoviridae : Piasmaviridae : Lipothrixviridae : Fuselloviridae :
Tectivirus Cotticovirus Plasmavirus Lipothrixvirus Fusellovirus
Polydnat4ridae :
Ichnovirus Bracovirus
Phycodnaviridae : Phycodnavirus Baculoviridae :
Nucteopolyhedrovirus Granutovirus
Poxv
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