Mutation and replication inUstilago maydis

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Abstract
A method is described which makes it possible to detect induced mutations in the pre-existing gene by determining whether both daughter cells of the first division after mutagenic treatment carry the mutation. The method depends on the use of diploid strains ofUstilago maydiswhich have biochemical markers in both homozygous and heterozygous condition. Cells which are induced to mutate at a homozygous locus will also occasionally undergo mitotic crossing-over. Since only mutant cells are selected, the reciprocal products of such a cross-over will not be detected if the mutation is transmitted to only one daughter cell after treatment. Mutations induced by UV light were examined in three different diploids, and with each, by use of the appropriate selective media and the replica plating technique, it was possible to detect mutant colonies which did consist of the reciprocal products of the exchange. It is deduced that a high proportion of the mutations must occur in the pre-existing gene; and a hypothesis is suggested which attempts to reconcile this conclusion with the current concept of the structure and replication of the gene.