X-ray-induced specific-locus mutations in the ad-3 region of two-component heterokaryons of Neurospora crassa. VIII. Dose-dependence of the overall spectrum.

There is considerable controversy in the literature concerning the nature of X-ray-induced specific-locus mutations in various experimental organisms. To investigate this problem in Neurospora crassa a series of experiments (Webber and de Serres, 1965) was performed to study the induction-kinetics of X-ray-induced mutation in the adenine-3 (ad-3) region of a two-component heterokaryon (H-12). Subsequent genetic analyses (de Serres, 1989a,b,c, 1990a), on a series of 832 mutants recovered in these experiments, have shown that 3 different classes of ad-3 mutants were recovered, namely gene/point mutations, multilocus deletions and multiple-site mutations. Complementation studies with a series of genetic markers that define 21 genetic loci in the ad-3 and immediately adjacent genetic regions have shown that ad-3 mutants classified as multilocus deletions result from the inactivation of a series of loci in the ad-3 and immediately adjacent regions of Linkage Group I, whereas multiple-locus mutations result from combinations of gene/point mutations and multilocus deletions. Analysis of the induction kinetics of these 3 different classes, after completion of the genetic characterization of all mutants (de Serres, 1990b) demonstrated that gene/point mutations increase linearly with X-ray dose, whereas multilocus deletions and multiple-site mutations increase as the square of X-ray dose. Further analysis of allelic complementation among the gene/point mutations at the ad-3B locus (de Serres, 1990c), demonstrated that the spectrum of complementation patterns was dose-dependent: complementing mutants with nonpolarized patterns decreased and noncomplementing mutations increased with increasing X-ray dose. There was little or no change with dose in the frequency of mutants with polarized patterns. In the present report, data from studies published previously have been utilized, along with additional data from the original X-ray experiments (12-5, 12-6, 12-7, and 12-10; see Webber and de Serres, 1965) to develop composite complementation maps of the X-ray-induced specific-locus mutations in the ad-3 and immediately adjacent regions as a function of X-ray dose. This analysis of the overall spectrum of X-ray-induced specific-locus mutations in the ad-3 region demonstrated marked dose-dependence and provides an explanation for the discrepancies in the literature with regard to specific-locus studies in different experimental organisms.