The Induction of Mitotic Recombination by Mitomycin C in Ustilago and Saccharomyces.

EGETATIVE heterozygous diploid strains of Ustilago maydis undergo sponVtaneous mitotic crossing over at low frequency, but this frequency can be increased up to 100-fold if the cells are treated with ultraviolet light (UV) (HOLLIDAY 1961b, 1962a, b, 1964). There is no indication that the induced crossing over differs in mechanism from that which has been described for spontaneous mitotic crossing over in Aspergillus nidulans ( PONTECORVO and =FER 1958) ; but unlike Aspergillus, Ustilago does not produce haploid somatic segregants, either spontaneously or after UV treatment. Vegetative Saccharomyces diploids do not haploidise either; their somatic segregation occurs as a result of crossing over and is stimulated by UV, but the recombination process involved appears to be distinct from normal mitotic crossing over (WILKIE and LEWIS 1963). On the basis of the results with Ustilago, it has been suggested that UV may promote recombination by bringing about homologous pairing of mitotic chromosomes; and that this might be the consequence of the specific inhibition of DNA synthesis by UV producing a state of unbalanced growth akin to the condition of the meiotic cell ( HOLLIDAY 1961 b) . If UV acts in the way suggested, then other inhibitors of DNA synthesis should also stimulate mitotic crossing over. The antibiotic mitomycin C (MC) has been shown to interfere specifically with DNA synthesis in bacteria (SHIBA, TERAWAKI, TAGUCHI and KAWAMATA 1959), in Saccharomyces (WILLIAMSON and SCOPES 1962) and in animal cells (BEN-PORAT, REISSIG and KAPLAN 1961). In the present paper it is shown that MC stimulates mitotic crossing over in Ustilago with an efficiency comparable to that of UV, and furthermore, that it does not appear to have mutagenic activity under the same conditions of treatment. A similar effect on mitotic recombination in Saccharomyces is also reported. In another study the effect of 5-fluorodeoxyuridine (FUDR) , another inhibitor of DNA synthesis, is examined: it too stimulates mitotic crossing over in Ustilago and appears to be nonmutagenic (EASTON and HOLLIDAY 1964).