The International Committee on Taxonomy of Viruses (ICTV) published their eighth report in 2005. More than 5450 viruses arranged in greater than 2000 species, 287 genera, 73 families, and three orders are described (Table 5.1). While this is a notable achievement, it is not a complete one — the pace of discovery of new viruses and characterization of the genes they encode ensures that the number will change. Further, it is increasingly evident that the very nature of virus replication and association with their hosts leads to complications not found in classification schemes for cell-based life. The rate of genetic change in viruses can be great due to the rapidity and frequency of genome replication with the associated opportunity for error. Viruses can also, however, exchange genetic elements with their hosts and any other genomes present in the same milieu in which the virus is replicating. Such an occurrence can lead to the creation of a new virus species in which some of its genes are derived from one lineage and some from another — clearly its classification will be complicated.
The best generalization that can be made concerning virus classification is that it depends on analysis of a number of features, and the importance of such features may vary depending upon the use being made of the classification. A classification scheme that combines the Baltimore basis along with the nature of the host and detailed genetic characterization of critical viral proteins can be combined to generate a global view of viruses as a virosphere such as shown in Figure 5.5.
The features of viruses discussed in this chapter provide the basis for this comprehensive classification scheme, but they are not complete — for example, diseases caused by viruses cannot be readily listed. Further, relationships between virus families will often transcend the nature of the host — this would require a third dimension to the scheme (appropriate to the concept of a sphere). Since the concept of species in biology has always been a problem, it is no surprise that relationships between viruses pose a number of specific and profound problems. For more distantly related groups, the problems grow. Still, throughout this confusion, virus families made up of related species or types are clear and it is possible to group some major virus families into superfamilies — we will see that this can be done for the herpesviridae and certain bacte-riophage. As a rationalization, it is useful to consider virus families and larger groupings as polythetic — a group whose members always have several properties in common although no single common attribute is present in all of its members. As a result no single property can be used as a defining property of a polythetic group on the basis that it is universally present in all the members and absent in the members of other groups. For viruses, it is impossible to use any one discriminating character for distinguishing related groups and families, because of the inherent variability of the members.
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