Classification of viruses

As we saw at the beginning of this chapter, viruses are not considered to be strictly living, and their classification is a complex issue. As with true organisms we have species, genera, families and orders of viruses, but none of the higher groupings (class, phylum, kingdom). Latin binomials (e.g. Homo sapiens, Escherichia coli), familiar

Figure 10.6 An enveloped virus. The envelope derives from the host cell's membrane, and includes virus-encoded proteins. Some viral envelopes contain projecting spikes, which may assist in attachment to the host

from conventional biological taxonomy, are not used for viruses; however, a proposal for non-Latinised viral binomials has been proposed. Originally, no attempt was made to draw up any sort of phylogenetic relationship between the viruses, but more recent developments in sequencing of viral genomes has meant that insights are being gained in this area.

Factors taken into account in the classification of viruses include:

• host range (vertebrate/invertebrate, plant, algae/fungi, bacteria)

• morphology (capsid symmetry, enveloped/non-enveloped, capsomer number)

• genome type/mode of replication (see Figure 10.3).

In 1971, David Baltimore proposed a scheme that orders the viruses with respect to the strategies used for mRNA production. This results in seven major groupings (Table 10.2). The most recent meeting (2005) of the International Commission on Taxonomy of Viruses (ICTV, established in 1973) produced a report which recognises three orders, 73 families, 287 genera and more than 1900 species of virus. Countless others, undiscovered or insufficiently characterised, also exist.

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