Mitotic chromosome loss in a disomic haploid of Saccharomyces cerevisiae

Experiments designed to characterize the incidence of mitotic chromosome loss in a yeast disomic haploid were performed; The selective methods employed utulize the non-mating property of strains disomic for linkage group III and heterozygous at the mating type locus. The principal findings are: (1) The grequency of spontaneous chromosome loss in the disome is of the order 10- minus 4 per cell; this value approximates the frequency in the same population of spontaneous mitotic exchange resulting in homozygosity at the mating type locus. (2) The recovered diploids are pure clones, and thus represent unique events in the disomic haploid. (3) Of the euploid chromosomes recovered after events leading to chromosome loss, approximately 90% retain the parental marker configuration expected from segregation alone; however, the remainder are recombinant for marker genes, and are the result of mitotic exchanges in the disome, especially in regions near the centromere. The recombinant proportion significantly exceeds that expected if chromosome loss and mitotic exchange in the disome were independent events. The data are consistent with a model proposing mitotic nondisjunction as the event responsible for chromosome loss in the disomic haploid.