X-Ray-induced Specific Locus Mutations in the ad-3 Region of Two-Component Heterokaryons of Neurospora crassa. III. Genetic Fine Structure Analysis of the ad-3 and Immediately Adjacent Genetic Regions by Means of Complementation Tests

Abstract

Genetic fine structure analysis of the ad-3 and immediately adjacent genetic regions was made by means of complementation tests on all possible pairwise combinations of 50 X-ray-induced irreparable adenine-3 mutants (designated ad-3IR). All mutants were induced in either heterokaryon 11 or heterokaryon 12 of Neurospora crassa, 2-component heterokaryons heterozygous for mutants at the 3 closely linked loci ad-3A and ad-3B and nic-2 (nicotinamide-requiring) located about 5.0 map units distal to ad-3B. The complementation tests involved mutants of the following genotypes: 15 ad-3A, 27 ad-3B, 7 ad-3A ad-3B nic-2 and 1 ad-3B nic-2. To facilitate mapping, 5 additional strains (each consisting of a gene/point mutation at the ad-3A or ad-3B locus and a separate site of closely linked recessive lethal damage in the immediately adjacent regions [designated ad-3R + RLCL]) were also included. The data from these complementation tests showed that the majority (46/50) of X-ray-induced irreparable ad-3 mutants mapped as a series of overlapping multilocus deletions that extend both proximally and distally into the immediately adjacent genetic regions, as well as into the 'X' region (a region of unknown, but essential function) between ad-3A and ad-3B. The remaining mutants (4/50) were found to result from a series of closely linked, but separate, mutations (designated multilocus mutations) of the type ad-3IR + RLCL, different from those found in previous studies (de Serres, 1968; de Serres and Brockman, 1968). The data from the present complementation tests have expanded the process of genetic fine structure mapping of the ad-3 and immediately adjacent regions (de Serres, 1968) and defined the presence of the following 11 genetic loci: (a) 4 loci (with either known [i.e. col-1t] or unknown [i.e. unknA]) function proximal to ad-3A: unknA, unknB, col-1t, and col-2t, (b) 4 loci in the 'X' region: unknC, unknD, unknE, and unknF, (c) 2 loci distal to ad-3B: unknG, col-3t, and (d) 1 locus distal to nic-2: unknH.