Early vegetative segregation of mitochondrial genes in Saccharomyces cerevisiae

Abstract

The vegetative segregation of seven mitochondrial gene loci was studied in yeast. At various times after mating antibiotic resistant and sensitive strains, samples of the diploid progeny were examined to determine the segregation rates of the alleles at each locus in three- and four-factor crosses. The rate of segregation was approximately the same for the cap1, ery1, oli1, oli2, and par1 loci, which are scattered over about two-thirds of the mitochondrial DNA molecule. Differences in segregation rates were found but showed no consistent relationship to the map positions of the loci. This is in contrast to the segregation of chloroplast genes in Chlamydomonas, where loci segregate at rates proportional to their distance from an “attachment point” which appears to govern the partitioning of chloroplast DNA molecules between daughter chloroplasts when the chloroplast divides. Our data are compatible with a model in which the mitochondrial DNA molecules in a cell occur in a small number of groups corresponding to individual nucleoids or mitochondria. Most or all of the molecules in a group carry the same allele at any given locus. These genetically homogeneous groups of molecules may thus be the units of segregation, and may be partitioned randomly between mother cell and bud at each division.