The detection of monosomic colonies produced by mitotic chromosome non-disjunction in the yeast Saccharomyces cerevisiae.

A diploid yeast strain, D6 is described which monitors mitotic non-disjunction by the phenotypic expression of a set of coupled and recessive markers flanking the centromere of chromosome VII. These markers are not expressed in the heterozygous condition prevailing in D6. The left arm of chromosome VII carries a tightly centromere linked marker, leu1 (leucine requirement), distal to leu1 in this order: trp5 (tryptophan requirement), cyh2 (recessive resistance to cycloheximide) and met 13 (requirement for methionine). The right arm is marked with ade3 (simultaneous requirement for adenine and histidine). D6 is homozygous for ade2 and consequently, forms red rather than the normally white colonies. It shows no requirement for the above amino acids and it is sensitive to cycloheximide. Unmasking of all the markers on chromosome VII leads to colonies that are white because ade3 sets a block preceding the ade2 block (which causes the accumulation of a precursor of the red pigment), they require leucine, tryptophan and methionine, and grow on media with cycloheximide. Cells are plated on a cycloheximide medium where red and white colonies are formed. Colonies of spontaneous origin were tested. The majority of the white colonies expressed all the recessive markers whereas only few of the red colonies expressed all the markers on the left arm of chromosome VII. Basically expression of recessive markers on both sides of the centromere can be explained as a result of two coincident events of mitotic crossing over. However, the frequency of colonies expressing centromere linked leu1 was 14 times higher among the white types than the red ones. This suggested that the white, cycloheximide resistant, leucine requiring colonies arose by mitotic non-disjunction and not only by two coincident mitotic crossing over events. Presumptive spontaneous monosomic segregants were placed on sporulation medium. Only 8 out of 30 isolates sporulated, which showed that these eight segregants were diploid at the time of sporulation. They could have arisen by two coincident crossover events or through restoration of a normal disomic condition after non-disjunction had occurred. The genetic data thus leaves us with only its statistical argument in favour of non-disjunction. Further confirmation of monosomic nature of the white cycloheximide resistant colonies was provided by the estimates of their DNA contents. Compared to the stock wild type diploids the presumptive monosomics showed a reduction in DNA content. We have utilized D6 to investigate the possible induction of mitotic non-disjunction after treatment with gamma rays, heat shock at 52 degrees C and ultraviolet irradiation. In all cases white, cycloheximide resistant colonies were produced at levels significantly higher than that found in untreated cultures. In order to detect the production of monosomic cells, treated cultures were grown for 48 h in non-selective medium after exposure to allow for "expression" of the monosomic condition.