Quantitative and qualitative comparisons of spontaneous and radiation-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 mutations at two closely linked loci in the adenine-3 (ad-3) region of heterokaryon 12 (H-12) of Neurospora crassa have been used to determine the relative frequencies and mutational spectra of ad-3 mutants occurring spontaneously and those induced by 7 different radiation treatments. Previous studies have demonstrated that specific-locus mutants at these two loci result from 5 major genotypic classes, namely two classes of gene/point mutations (ad-3AR and ad-3BR), and 3 classes of multilocus deletion mutations ([ad-3A]IR, [ad-3B]IR and [ad-3A ad-3B]IR). Two different approaches were used to compare spontaneous mutation in the ad-3 region with that induced by 7 different radiation treatments (UV, 32P, 447 MeV protons, 85Sr, 250 kVp X-rays, 39 MeV helium ions, and 101 MeV carbon ions). These comparisons included X2-tests on the numbers of ad-3 mutants resulting in the following two sets of ratios: (1) gene/point mutations and multilocus deletion mutations; and (2) complementing and non-complementing ad-3BR mutants. Combination of the data from these two methods of comparison has demonstrated that each of the 7 radiation treatments induced a spectrum of ad-3 mutants that is statistically different from the spontaneous spectrum. In addition, these same two methods of comparison have been used to compare the mutagenic effects of each of the 7 radiation treatments with each other. Combination of the data from these two methods of comparison demonstrated that the majority of radiation-induced specific-locus mutations: (90.5% (19/21 of the pairwise combinations)) are qualitatively different from each other. We conclude that the mechanisms by which various radiations modify DNA tend to exhibit fundamental differences from each other and from the processes involved in spontaneous mutation.

X-ray-induced specific-locus mutations in the ad-3 region of two-component heterokaryons of Neurospora crassa. VI. Induction kinetics of gene/point mutations, multilocus deletions and multiple-locus mutations

Genetic fine-structure analysis of X-ray-induced specific-locus mutants in the ad-3 region of two-component heterokaryons of Neurospora crassa has shown that gene/point mutations, multilocus deletions and multiple-locus mutations are induced. When the dose-response curves for these classes of ad-3 mutants were plotted, it was demonstrated that X-ray-induced gene/point mutations (ad-3R) increased linearly with X-ray dose and X-ray-induced multilocus deletions increased as the square of the X-ray dose. However, all classes of multiple-locus mutations, which would be expected to result from 3 to 8 hits on the basis of target theory (Lea, 1955), were found to increase as the square of the dose. Target theory assumes that the DNA of individual chromosomes is distributed randomly throughout the interphase nucleus. A model of eukaryotic interphase chromosome structure in which the DNA of individual chromosomes presents a nonrandom target to X-rays [Pinkel et al., Proc. Natl. Acad. Sci. (U.S.A.) 83 (1986), 2934-2938] provides a possible explanation for the high frequency and dose-squared induction kinetics of the multiple-locus mutants induced by X-rays in the ad-3 region.

X-ray-induced specific-locus mutations in the ad-3 region of two-component heterokaryons of Neurospora crassa. V. Irreparable mutants of genotype ad-3A ad-3B, ad-3A ad-3B nic-2, and ad-3B nic-2 result from multilocus deletion and an unexpectedly high frequency of multiple-locus mutations

More extensive complementation tests than those performed initially (Webber and de Serres, 1965) on a series of 832 X-ray-induced specific-locus mutations in the adenine-3 (ad-3) region of a two-component heterokaryon (H-12) of Neurospora crassa (de Serres, 1989a) showed that unexpectedly high frequencies of specific-locus mutations in the ad-3 region have additional, but separate, sites of recessive lethal (RLCL) damage in the immediately adjacent genetic regions. The frequencies of these X-ray-induced multiple-locus mutants in the ad-3 region are orders of magnitude higher than that expected on the basis of target theory and classical models of chromosome structure during interphase (de Serres, 1989a). Genetic fine structure analyses, by means of homology tests with tester strains carrying genetic markers in the ad-3 and immediately adjacent regions, have been performed to map the presumed multiple-locus mutations. In a previous paper (de Serres, 1989c), X-ray-induced irreparable ad-3 mutants of the following genotypes and numbers (ad-3A or ad-3B were analyzed, and the high frequency of multiple-locus mutations was confirmed. In the present paper, X-ray-induced irreparable ad-3 mutants of the following genotypes and numbers (ad-3A ad-3B, ad-3A ad-3B nic-2, and ad-3B nic-2 have also been subjected to the same genetic fine structure analysis. These experiments, in the previous (de Serres, 1989c) and present papers, were designed to determine the extent of the functional inactivation in the ad-3 and immediately adjacent genetic regions in individual mutants classified as presumptive multilocus deletions or multiple-locus mutations.