TABLE 2-2 Examples of Viral Evolutionary Mechanisms

Mechanism

Example

Reference

Point mutations

A single amino acid change can affect virulence; lethal chicken influenza

Kawaoka and Webster, 1988

Intramolecular recombination

Insertion of piece of genome; eastern equine encephalitis virus and Sindbis-like virus → western equine encephalitis virus

Hahn et al., 1988

Genetic reassortment

Origin of pandemic influenza viruses of 1957 and 1968; external protein gene(s) from animal virus

Scholtissek et al., 1978

Recombination and mutation

Evolution of live poliovirus vaccine following administration

Kew and Nottay, 1984

Biased hypermutation (uridine to cytosine transitions)

Evolution of SSPE virus from measles virus

Catteneo et al., 1989

Genetic rearrangement

Evolution of rubella virus

Dominguez et al., 1990

Recombination between deletion mutations

Regeneration of functional plant virus genome

Allison et al., 1990

NOTE: SSPE = subacute sclerosing panencephalitis. Adapted from Kilbourne, 1991. Used with permission.

Natural Variation/Mutation

RNA VIRUSES

RNA viruses confront us with a paradox. On the one hand, their mutation rates are extraordinarily high (because unlike DNA viruses, RNA viruses have no mechanisms for correcting errors made during replication). On the other hand, the clinical expressions of the diseases they cause (such as poliomyelitis and measles) have remained constant for centuries. Nevertheless, analysis of RNA virus genomes reveals that each "virus" comprises a heterogenous mixture of mutants in variable proportions. Thus, any given strain or isolate is, in fact, polymorphic or represents a subset of the quasispecies



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