QUANTITATIVE AND QUALITATIVE COMPARISON OF SPONTANEOUS AND CHEMICAL-INDUCED SPECIFIC-LOCUS MUTATION IN THE AD-3 REGION OF HETEROKARYON-12 OF NEUROSPORA-CRASSA

The data from forward-mutation experiments to obtain specific-locus mu tations at two closely linked loci in the adenine-3 (ad-3) region of h eterokaryon 12 (H-12) of Neurospora crassa have been tabulated to dete rmine the relative frequencies and mutational spectra of ad-3 mutants occurring spontaneously and those induced by 22 different chemical tre atments. Previous studies have demonstrated that specific-locus mutati ons at these two loci result from 5 major genotypic classes, namely tw o classes of gene/point mutations (ad-3A(R) and ad-3B(R)), and 3 class es of multilocus deletion mutations ([ad-3A](IR), [ad-3B](IR) and [ad- 3A ad-3B](IR)). In addition, prior studies have demonstrated that some chemical mutagens induced ad-3 mutants exclusively, or almost exclusi vely, by gene/point mutation and other chemical mutagens by gene/point mutation and multilocus deletion mutation. In the latter cases, there was wide variation in the percentages of ad-3 mutants in these 5 majo r genotypic classes. Two comparative methods of analysis that also wer e used to compare spontaneous and chemical-induced ad-3 mutational spe ctra included chi(2)-tests on the numbers of ad-3 mutants resulting in the following two sets of ratios: (1) gene/point mutations and multil ocus deletion mutations; and (2) complementing and non-complementing a d-3B(R) mutants. Combination of the p-values from chi(2)-tests for the se two methods of comparison demonstrated that all 22 chemicals induce a spectrum of ad-3 mutants that is qualitatively different from that occurring spontaneously. In addition, these same two methods of compar ison have been used to compare the mutagenic effects of each of the 22 chemical treatments with each other. Combination of the data from the se two methods of comparison has demonstrated that 93.1% (215/231) of the pairwise combinations of these 22 chemicals were different from ea ch other. The implication of these experimental data on the induction of specific-locus mutations in somatic cells of Neurospora for genetic risk assessment exercises is discussed.