Value of the fluctuation test for establishing the origin of agar-resistant L particles emerging in S subclones of ECHO virus 19

Summary

The aim of the authors was to verify the value of the fluctuation test as a criterion to differentiate between mutational versus adaptive origin of variants emerging in genetically pure virus populations.

Mutation from the S to L plaque character in ECHO virus 19 was investigated.

Several tests on S parental virus stock subcultures obtained under single cycle growth conditions in tubes and a single test on 59 subclones of the same virus population representing isolated plaques harvested after 5 days' multiplication were performed.

Nine out of the 11 tests in the first series exhibited a distribution of the number of mutants characteristic of induced adaptive processes. In only two tests of the first series and in the test performed with fiveday old subclones, mutant distribution was of the clonal type and accordingly χ² and F test significant.

The possible explanation of the low incidence of “explosive” mutant clones is discussed and it is suggested that in its present form the fluctuation test as applied to viruses may yield nonsignificant results although true mutation is present. Accordingly, under these conditions the test does not offer a sensitive enough criterion for ascertaining the mutational origin of virus variants.