Foot-and-Mouth Disease Virus Populations Are Quasispecies

Abstract

The term “quasispecies” describes complex distributions of replicating molecules subject to mutation and competitive selection (Eigen 1971; Eigen and Schuster 1979; Eigen and Biebricher 1988). The original theoretical concept of Eigen and colleagues concerned populations of infinite numbers of individual molecules and ideal, steady-state equilibrium conditions (recent review on the theoretical concept in Eigen et al. 1989). It is clear that in spite of their large population size, RNA viruses deviate from such idealized behaviour. However, several key features of RNA viruses such as nucleotide sequence heterogeneity, generally high mutation rates, and potential for very rapid evolution are best understood in the framework of the quasispecies concept (see the chapter by Holland et al.). Viral isolates, either in their natural niche or disturbed by adaptation to growing in cell culture, consist of a multitude of viable and defective mutants termed the “mutant spectrum” of the population. During replication, each genomic distribution is dominated by one (or several) “master sequence (s),” that generally coincides with the average or consensus sequence of the population. With the levels of genetic heterogeneity for foot- and-mouth disease virus (FMDV) documented in the following paragraphs, the master sequence often represents as little as 1% or less of the population of molecules, and it may have a brief life span. Here we review the evidence for the quasispecies structure of FMDV and its biological implications, notably the antigenic diversity of this widespread pathogen.