[Genomic instability in picornaviruses].

Picornaviruses are small animal RNA viruses and include wtiological agents of poliomyelitis, foot and mouse disease, hepatitis A, etc. Replication of their genome results in many mutations, which are close in number to a viability threshold. Hence every virus population contains a great variety of genomes and represents a quasispecies. Covalent rearrangements (deletions, insertions, recombination) also contribute to genome variation and arise by replicative and nonreplicative mechanisms, which are still poorly understood. Only a minor fraction of all new changes is fixed during evolution. The fixation is based on two principally different ways of selection: with (positive and negative selection) and without (random selection of nonrepresentative variants) regard to the phenotype. In natural evolution of picornaviruses, the latter way is prevalent, and most fixed mutations are phenotypically neutral. To understand the mechanisms of evolution, it is necessary to evaluate the biological significance of particular genetic changes. Several new approaches to this problem have recently been proposed.