Development ofa specific-locus assay in thead-3 region of two-component heterokaryons ofneurospora: A review

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Abstract
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.

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