Mutagenesis in Yeasts

Summary

Among lower eukaryotes, the genetics of the yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe are the most extensively studied. They are highly suitable for use in assaying genetic effects of environmental chemicals, as their genic and chromosomal structures show strong similarities to those of higher organisms. S. cerevisiae strains are suitable for routine use only for the measurement of genetic endpoints that are detectable in haploids. The advantages of yeasts in mutagenicity studies are:the variety of genetic end-points that may be assayed (gene-mutation; mitotic recombination and gene-conversion; aneuploidy); the low cost and the limited requirements of technical expertise and laboratory facilities; and the existence of extensive background information.

Both forward and reverse mutations can be detected. The former type can be analyzed in S. pombe strain ade 6-60, rad 10-198, h-, whereas for reverse mutations, the XV185-14 strain of S. cerevisiae, which contains a variety of different molecular revertible sites in 5 genes, is used. In both strains base-pair substitution and insertion-deletion mutations are inducible and can be easily scored. D7 strain of S. cerevisia can detect reverse-mutations induced in a diploid structure. D4 and JD1 diploid strains of S. cerevisiae are usually utilized for mitotic gene-conversion induction, while d7 diploid strain is used to detect mitotic gene-conversion and mitotic crossing-overs. D6 diploid strain S. cerevisiae had been used for induction of chromosome aneuploidy, a genetic end-point of great relevance for the identification of environmental mutangens.

Several in vitro and in vivo methodologies have been developed for the two species of yeast. The former may include the use of exogenous as well as endogenous metabolic activation systems. Host mediated asasy techniques have been shown to be useful for testing the in vivo formation of mutagens from unreactive precursors.

The evaluation made by the GENE-TOX Program indicated that up to 1981 some 521 chemicals were tested on yeasts, of which 257 gave a positive response. Since that time, about 200 more chemicals have been tested.