The Effect of a Suppressor on Allelic Inositolless Mutants in Neurospora Crossa

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Abstract
A reversion of an inositolless mutant strain 37401 following u.-v. irradiation grew more slowly on minimal medium than usual reversion types, and when crossed with wild, gave segregation for inositolless types, indicating that the mutant was due to a mutation at another locus (suppressor mutant). Growth curves for asci giving 6:2, 8:0 and 4:4 segregations for inositol-independence vs. inositol-dependence, were obtained by growing cultures on minimal medium and minimal + 4[mu]g. inositol per ml. The four genotypes (inositolless, wild type, inositolless + suppressor and wild type + supressor) were phenotypically distinguishable. The suppressor only partly suppresses the inositolless mutation, since growth in the absence of inositol is considerably less than that of the inositol-independent type and added inosltol stimulates growth of the suppressor. When the suppressor is present with wild type there is a marked growth lag which is not overcome by adding inositol. Quantitative assays for meso-inositol activity indicated that the suppressor genotype had only 1/10 of the mycelial concn. of inositol present in wild type. Crosses between the suppressor combined with wild type and 7 different inositolless mutants (all allelic or possibly pseudoallelic to 37401) showed that the suppressor had no effect on these mutants whereas in simultaneous tests it did suppress strain 37401. In order to confirm the presence of the suppressor, the 4 ascus cultures derived from the cross with strain 89601 were each crossed to 37401. The suppressor was re-extracted from 89601 and again suppressed 37401, although it had no effect on 89601.

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