Resistance to extinction of low fitness virus subjected to plaque-to-plaque transfers: diversification by mutation clustering.

Plaque-to-plaque transfers of RNA viruses lead to accumulation of mutations and fitness decrease. To test whether continuing plaque-to-plaque transfers would lead to viral extinction, we have subjected several low fitness foot-and-mouth disease virus (FMDV) clones to up to 130 successive plaque transfers, and have analyzed the evolution of plaque titers and genomic nucleotide sequences. No case of viral extinction could be documented. Some low fitness clones that posses an internal poly(A) tract evaded extinction by modifying the length or base composition of the poly(A) tract. The comparison of entire genomic sequences of FMDV clones at increasing plaque transfer number revealed that mutations accumulated at a uniform rate, and that they were distributed unevenly along the genome. Clusters of mutations were identified at different genomic sites in two plaque transfer lineages. Mutation clustering appears to occur stochastically and could not be related to fixation of compensatory mutations. The results document resistance of viral clones to extinction, and suggest that mutation clustering may be a mechanism of genetic diversification of low fitness virus.