AbstractThe use of the yeastSaccharomyces cerevisiaein genetic toxicology testing is discussed. The biology of this unicellular eukaryote allows the study of genetic effects in haploid and diploid cells, and in mitotic as well as meiotic cultures. It is at present a prime organism for the study of eukaryotic molecular genetics and cell biology. The genetic system has been thoroughly analysed, and many different types of genetic end points can be studied by using genetically well‐defined but simple tests. Induction of point mutation has been investigated in a number of strains, both in forward and selective reverse mutation assays. One of the most frequently studied genetic effects has been the induction of mitotic recombination. This test has shown a good response to many mutagens and carcinogens, and must be considered as a repair test since mitotic recombination is inducible by damage inflicted upon the genetic nuclear material. There are basically two types of mitotic recombination. At the intragenic level, recombination between different mutational alterations within the same gene can conveniently be studied by using selective techniques; it is due principally to the occurrence of non‐reciprocal recombination between the two mutation sites and is now called mitotic gene conversion. Another type of mitotic recombination is the classical type of reciprocal crossing‐over. Even though testing by using mitotic crossing‐over and associated processes (cumulatively called mitotic segregation) has been based on non‐selective techniques, there is a considerable volume of testing data. Tests for chromosomal malsegregation leading to aneuploidy are now available, but the number of agents tested is still small. Various important and critical aspects of using different yeast systems are discussed. Metabolic activation in yeast is provided by endogeneous P450 systems, but exogenous activation of various kinds, including that available in host mediated assays, has successfully
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