Recombination between temperature-sensitive () mutants of foot-and-mouth disease (FMD) virus was examined, using an infectious centre technique that was more sensitive (approx. 30-fold) than the conventional virus yield test. The test involved a brief incubation of the mixedly infected cells at the permissive temperature to allow recombination to occur followed by assay at the restrictive temperature to select for those cells in which recombination had occurred. With crosses involving widely separated mutations, as many as 28% of the infected cells produced presumptive recombinant plaques. Since each plaque was the result of an independent event, large numbers of different presumptive recombinants could be isolated for further study. Analysis of presumptive recombinant plaques from a variety of crosses showed that, in general, the virus produced had the properties expected of recombinants. An approximate correlation was found between genetic distance, as determined in the yield recombination test, and the percentage of recombinant infectious centres observed. The phenomenon was very sensitive to the balance between the input multiplicities of the two parent viruses and occurred very early in virus replication. The test has considerable potential for the study of genetic interactions in FMD virus, but it would be surprising if this potential was limited to picornaviruses.


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