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A mutant strain of Neurospora which requires sulfonamides for growth at 35° frequently becomes altered so that it can grow at that temperature without sulfonamides.(1) In each instance analyzed, the developed ability to grow without sulfonamides has resulted from mutation of genes distinct from that responsible for the sulfonamide requirement. Each "reverted" strain has proved to be a heterocaryon, composed of two kinds of nuclei, both of which carry the gene (sfo) for sulfonamide requirement, but one also carries a new "supressor" gene. At least two independent supressor genes have been involved in such reversions. Homocaryotic strains have been isolated, some carrying both sfo and the supressor, others carrying the supressor alone. None of these isolates has the growth characteristics of wild type, but each shows some peculiar relationship to the p-aminobenzoic acid requirement, the nature of which is still obscure. In a few instances, artifically constructed heterocaryons between the sulfonamide-requiring strain and a strain carrying both the sulfonamide-requiring gene and a supressor have resulted in better growth on minimal medium than either strain is capable of by itself, thus reconstituting the situation observed in the original "reverted" heterocaryons.

A Physiological Basis for Some Suppressor Mutations and Possibly for One Gene Heterosis