Temperature-sensitive mutants of foot-and-mouth disease virus: the isolation of mutants and observations on their properties and genetic recombination.

A number of temperature-sensitive mutants were isolated from two strains of foot-and-mouth disease virus (FMDV). Various properties of the mutants were examined including comparative growth curves at permissive and restrictive temperatures, cut-off temperatures, thermal lability and pH sensitivity. Recombination was observed between various pairs of mutants of FMDV strain Pacheco. It occurred early in the growth cycle and the proportion of recombinants remained constant thereafter. Maximum recombination was achieved if the input multiplicity of each virus was 6 p.f.u./cell or greater, provided the ratio of the input multiplicities did not vary by more than a factor of two. Day-to-day variations could be substantially reduced by normalizing recombination frequencies in terms of a standard cross. The results suggested that genetic mapping was possible with two-or three-factor crosses.