Meiotic chromosome segregation in triploid strains of Saccharomyces cerevisiae.

Meiosis in triploids results in four highly aneuploid gametes because six copies of each homolog must be segregated into four meiotic products. Using DNA microarrays and other physical approaches, we examined meiotic chromosome segregation in triploid strains of Saccharomyces cerevisiae. In most tetrads with four viable spores, two of the spores had two copies of a given homolog and two spores had only one copy. Chromosomes segregated randomly into viable spores without preferences for generating near haploid or near diploid spores. Using single-nucleotide polymorphisms, we showed that, in most tetrads, all three pairs of homologs recombined. Strains derived from some of the aneuploid spore colonies had very high frequencies of mitotic chromosome loss, resulting in genetically diverse populations of cells.

A fine-structure map of spontaneous mitotic crossovers in the yeast Saccharomyces cerevisiae.

Homologous recombination is an important mechanism for the repair of DNA damage in mitotically dividing cells. Mitotic crossovers between homologues with heterozygous alleles can produce two homozygous daughter cells (loss of heterozygosity), whereas crossovers between repeated genes on non-homologous chromosomes can result in translocations. Using a genetic system that allows selection of daughter cells that contain the reciprocal products of mitotic crossing over, we mapped crossovers and gene conversion events at a resolution of about 4 kb in a 120-kb region of chromosome V of Saccharomyces cerevisiae. The gene conversion tracts associated with mitotic crossovers are much longer (averaging about 12 kb) than the conversion tracts associated with meiotic recombination and are non-randomly distributed along the chromosome. In addition, about 40% of the conversion events have patterns of marker segregation that are most simply explained as reflecting the repair of a chromosome that was broken in G1 of the cell cycle.