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

Abstract 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-3B R ) 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-3B R 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.