Classification into major groups called families, and the subdivision of families into genera, has now reached a position of substantial international agreement Recently three families (Pmaniyxovmdae, Rhabdovindae, and Filovindac)
were grouped into a higher taxon, the order Mono>iegavira!cs, on the basis of the similarity in genome structure and strategy of replication of the member viruses. It is likely that in the future other families will be grouped to form orders.
The primary criteria for delineation of families are (1) the kind of nucleic acid that constitutes the genome (see Table 1-2), (2) the strategy of viral replication, and (3) the morphology of the virion. Subdivision of families into genera is based on criteria that vary for different families Genera, usually defined by substantial differences in their genomes, contain from one to over a hundred species. The definition of species is more arbitrary, and virologists continue to argue about the criteria for the designation of species and about their nomenclature
Serology, more recently strengthened by the use of monoclonal antibodies, is of great value in the differentiation ot viruses below the species level, namely, types, subtypes, strains, and variants—terms that have no generally agreed taxonomic status. Characterization of the nucleic acid of the viral genome, as revealed by such techniques as nucleotide sequence analysis (partial or complete genome sequencing), restriction endonuclease mapping, electrophoresis in gels (especially useful for RIMA viruses with segmented genomes), oligonucleotide fingerprinting, and molecular hybridization, is being used more and more to identify viruses and to distinguish differences between strains and variants.
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