A double heterozygous + /+ diploid strain of , phenotypically wild-type, was treated with -methyl-′-nitro--nitrosoguanidine. A total of 5500 colonies isolated on rich medium from the mutagenized cells were replicated to minimal medium. Nine auxotrophs (0·16%) were isolated: six required -aminobenzoic acid, two required arginine and one required both -aminobenzoic acid and nicotinamide. On the basis of their cell volume, mating-type and DNA content, all the auxotrophs appeared to be diploids. However, analysis of crosses between the auxotrophs and the haploid wild-type strain suggested that the auxotrophs are probably aneuploids. This conclusion is also supported by the slow growth, heterogeneity in colony size and reduced pigmentation of certain of the auxotrophs cultivated on supplemented medium. The same conclusion is supported by control experiments in which 0·10% auxotrophs were recovered. One auxotroph, however, apparently resulted from mitotic crossing-over at the four-strand stage. Under our experimental conditions, the mutagen does not seem to play a significant role in the induction of somatic segregation.


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