Development of a specific-locus assay in the ad-3 region of two-component heterokaryons of Neurospora: a review.

In recognition of the need for a more comprehensive data base for genetic risk assessment of human exposure to mutagenic agents in the environment, a model system was developed for specific-locus studies in Neurospora crassa. This lower eukaryotic organism permits the utilization of microbial techniques for recovery of large numbers of specific-locus mutations at two closely linked loci as well as their subsequent genetic analysis. In particular, this assay makes possible exploratory experiments with different environmental mutagens to obtain data on a wide variety of experimental conditions. Such data make it possible to study induction kinetics and mutational spectra in a manner that is not as yet feasible in higher eukaryotic organisms. The adenine-3 (ad-3) specific-locus assay was modeled after the 2-gene, morphological specific-locus assay in the dilute-short-ear region of the mouse, and it also detects forward-mutations at two closely linked loci, namely, ad-3A and ad-3B. Because ad-3 mutations are recovered by a direct method, based on the accumulation of a reddish-purple pigment in the vacuoles of the mycelium rather than their requirement for adenine, this system is both a morphological and biochemical specific-locus assay. The use of the ad-3 assay system in experiments with different environmental mutagens has provided precise dose-response curves not only for inactivation, but also the overall induction of ad-3 mutations. Genetic characterization of these ad-3 mutations by a series of 3 rapid and simple genetic tests permits the identification of 18 subclasses of gene/point mutations, and 12 subclasses of multilocus deletion mutations. These subclasses also include 3 different classes of multiple-locus mutations with separate sites of recessive lethal damage either in the immediately adjacent regions or elsewhere in the genome. In summary, this specific-locus assay provides a capability that is unique among eukaryotic organisms for the recovery and analysis of genetic damage at 2 closely linked loci.