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

Abstract

The induction of specific-locus mutations in the ad-3 region of Neurospora crassa after X-irradiation was studied in a two-component heterokaryon to determine: (1) the ratio of reparable ad-3 mutants (presumed gene/point mutations, designed ad-3 R) to irreparable ad-3 mutants (presumed multilocus deletions, designated ad-3 IR), and (2) the induction kinetics of each class (Webber and de Serres, 1965). More extensive genetic tests made subsequently (de Serres, 1989a) on the 832 X-ray-induced specific-locus mutations recovered in those experiments showed that unexpected high frequencies of reparable and irreparable ad-3 mutants are actually multiple-locus mutants that have additional, but separate, sites of recessive lethal ( RL CL) damage in the immediately adjacent genetic regions (designated ad-3 R + RL CL or ad-3 IR + RL CL). The frequencies of these X-ray-induced multiple-locus mutants in the ad-3 region are orders of magnitude higher than expected on the basis of target theory (where the frequency of the double mutant is expected to be the product of the frequencies of each single mutant) and classical models of chromosone structure during interphase (de Serres, 1989a). In the present paper, a random sample of 832 X-ray-induced ad-3 mutants of genotype ad-3A or ad-3B that are irreparable have been subjected to more extensive genetic fine-structure analysis. These experiments were designed to determine the extent of the functional inactivation in individual mutants in the ad-3 and immediately adjacent genetic regions in mutants classified as presumptive multilocus deletions or multiple-locus mutations. These experiments have shown that in Neurospora crassa most X-ray-induced irreparable mutants of genotype ad-3A or ad-3B map as a series of overlapping multilocus deletions. Among the 29 irreparable mutants of genotype ad-3A, there are 16 different subgroups of complementation patterns; and among the 63 irreparable mutants of genotype ad-3B, there are also 16 different subgroups. In addition, mutants classified as presumptive multiple-locus mutants result from a variety of separate, but closely linked, sites of genetic damage. Combining data from the present experiments with previously published data (de Serres, 1989a), the frequency of multiple-locus mutations of genotype ad-3 R + RL CL among X-ray-induced gene/point mutations ( ad3A R or ad-3B R) in the ad-3 region is 3.4% ( 22 644 ), and the frequency of multiple-locus mutations of genotype ad-3 IR + RL CL (or ad-3 IR + RL CL + RL CL) among X-ray-induced multilocus deletions ([ ad-3A ad-3B] IR, [ ad-3A ad-3B nic-2] IR) in the ad-3 region is 6.7% ( 13 195 ).