Quantitative and qualitative aspects of spontaneous specific-locus mutation in the ad-3 region of heterokaryon 12 of Neurospora crassa.

The data from forward-mutation experiments to obtain specific-locus mutations at 2 closely linked loci in the adenine-3 (ad-3) region of heterokaryon 12 (H-12) of Neurospora crassa have been tabulated to determine the frequency of spontaneous ad-3 mutations and to determine the percentages resulting from each of the 2 major genotypic classes: gene/point mutations and multilocus deletion mutations. Gene/point mutations at the ad-3B locus (ad-3BR) have been characterized to determine the percentage showing allelic complementation to obtain a presumptive identification of the genetic alteration in each mutation at the molecular level. Data from experiments performed at 2 different laboratories have been compared to assess the interlaboratory reproducibility of quantitative data on H-12. No difference was found between the frequencies of spontaneous specific-locus mutations in the ad-3 region. Genetic analysis of 172 ad-3 mutants demonstrated that specific-locus mutations in the ad-3 region result from both gene/point mutations (82.0% [141/172]), and multilocus deletion mutations (14.5% [25/172]). Heterokaryon tests for allelic complementation demonstrated that 52.5% (53/101) spontaneous ad-3BR mutants show allelic complementation, and result from single base-pair alterations. In addition, 100% (25/25) of the spontaneous multilocus deletion mutations result from the 3 smallest sized genotypic subclasses. The implications of the present experimental data for the validation of the ad-3 specific-locus assay system in Neurospora are discussed.