X-RAY-INDUCED SPECIFIC-LOCUS MUTATIONS IN THE AD-3 REGION OF 2-COMPONENT HETEROKARYONS OF NEUROSPORA-CRASSA .12. ANALYSIS OF MULTIPLE-LOCUSAD-3 MUTATIONS REVEALS A NONRANDOM DISTRIBUTION OF THE SEPARATE SITESOF RECESSIVE LETHAL DAMAGE THROUGHOUT THE GENOME

Previous studies on X-ray-induced adenine-3 mutations induced in heter okaryon 12 of Neurospora crassa showed that they consisted of gene/poi nt mutations, multilocus deletion mutations, and 3 different genotypic classes of multiple-locus mutations (designated [ad-3](IR) + RL(CL), ad-3(R) + RL(CL), and ad-3(R) + RL). In the present paper, multiple-lo cus mutations consisting of gene/point mutations at the ad-3A or the a d-3B locus with sites of recessive lethal damage closely linked to the ad-3 region (designated ad-3R + RL(CL)) Or with sites of recessive le thal damage elsewhere in the genome (designated ad-3(R) + RL) were ana lyzed to determine whether they resulted from mutations at the same si tes or different sites throughout the genome. It was assumed that if t he recessive lethal mutations in individual multiple-locus mutations s howed complementation on adenine-supplemented medium, they resulted fr om mutations at different sites. Multiple-locus mutations from both ma jor genotypic classes were combined, as forced heterokaryons, in all p ossible pairwise combinations and then were plated out on adenine-supp lemented medium. These studies indicated that 89.3% (50/56) of the rec essive lethal mutations in these 2 classes of multiple-locus mutations complement one another. Thus, they are presumed to have resulted pred ominantly from genetic damage at different sites throughout the genome . Within the group of 20 multiple-locus mutations that did not complem ent in various pairwise combinations, 90% (18/20) appear to map in a r egion, distal to the ad-3 region, defined by a series of overlapping m ultilocus deletion mutations in 6 mutations of genotype ad-3(R) + RL(C L). The other 10% (2/20) are located elsewhere on Linkage Group I or e lsewhere in the genome. The present data base on multiple-locus mutati ons is unique; such events either can not be detected, or can only be detected with difficulty, in other eukaryotic specific-locus assay sys tems such as mammalian cells in culture, Drosophila or mice. Our data on X-ray-induced ad-3 specific-locus mutations from the present and pr evious studies demonstrate the presence of additional sites of genetic damage, both closely linked with the ad-3 region or elsewhere in the genome, in ad-3 specific-locus mutations. Because the frequencies of e ach class of multiple-locus mutations is dose-dependent, they must be taken into account in genetic risk assessment exercises. Failure to ac knowledge the presence of such additional sites of genetic damage in t he utilization of specific-locus data could result in underestimation of the risk of human exposure to environmental mutagens.