Genomic stability of Saccharomyces cerevisiae baker's yeasts

The objective of this study has been to gather data on genomic stability of baker's yeast strains during long-term mitotic growth under restrictive co nditions so that comparisons could be made to other studies indicating geno mic instability during meiosis. The work describes the analysis of mitotic stability of the nuclear and mitochondrial genomes in the baker's yeast str ain V1 during incubation in continuous culture for 190 generations (300 day s). The cells were cultured in complete medium containing 2% glucose and 8 to 12% ethanol, as a mutagenic agent specific for mtDNA. The high concentra tion of ethanol severely limited the growth rate of the cells. DNA samples were monitored for chromosomal pattern, polymorphisms in selected nuclear g enes (SUC2, MALIT, ADH1) and mobile genetic elements (Ty1 and Y'), and for RFLPs in mtDNA. The results show that both the nuclear and mitochondrial ge nomes of grande cells were very stable. However, the frequency of petite mu tants in the population varied dramatically during the course of the experi ment, reaching as high as 87% petite during the first 27 days of the experi ment and declining to 5.8% petite at the end. This decline can be attribute d to selection against petite mutants in media containing high concentratio ns of ethanol. Moreover, when samples and the parental strain were compared at the end of the experiment, no change could be observed in parameters su ch as their growth rate in different media, capacity to leave doughs, viabi lity in ethanol or frequency of petite mutants. Results therefore indicated that the majority of the cells in the population were very similar to the parental throughout the experiments, with no apparent molecular or phenotyp ical changes.