Techniques for the quantitative study of mutation in plant viruses

Hardly any other virus is chemically and ultramicroscopically as well known as TMV. It is not possible to perform genetic recombinations with this object. The phenomenon of mutation is, however, known and an analysis of the dosis-effect relationship was possible by using the characters "chlorotic" versus "necrotic" primary symptoms. Taking into account the phenomenon of interference (mutual exclusion), i.e., comparing the induced mutation frequency with that of a control virus sample diluted to the same level of infectivity, on can perform quantitative analyses. In this way the first chemical mutagensis in the test tube was demonstrated 10 years ago with nitrous acid as mutagenic agent. The criticism raised byBAWDEN to the first publication ofMUNDRY andGIERER was already inappropriate at that time. In the meantime it has been demonstrated byWITTMANN-LIEBOLD andWITTMANN through analysis of amino acid exchanges in spontaneous mutants and in those isolated after incubation with HNO2 that the "difference" between spontaneous and induced mutants demanded byBAWDEN, which cannot be postulated for symptoms in plants, lies, as expected, in amino acid exchanges of the protein coat.