Genetic analysis of unequal transmission of the mitochondrial markers in Saccharomyces cerevisiae

Elimination of mitochondrial elements and improved viability in hybrid cells

Experiments were carried out to determine whether the mitochondria-specific dye rhodamine-6G (R6G) can affect transmission of cytoplasmic determinants in mammalian cells. When one parental cell type was treated with R6G prior to fusion with an untreated partner, the subsequent hybridization frequencies in both intra- and interspecific crosses were not adversely affected, even though R6G was extremely toxic to the parental cells. In addition, cells lethally treated with R6G could be rescued by fusion with cytoplasm alone from untreated cells. When chloramphenicol (CAP) resistant cells were used as the R6G-treated parent, the expression of CAP resistance in hybrids and cybrids was greatly reduced. Thus R6G can be used to control the input of cytoplasmic determinants into fused cells. In the interspecific (Chinese hamster x mouse) crosses, it was also seen that the majority of hybrids which had not been R6G pretreated grew poorly or degenerated after a short time. In contrast, nearly all hybrids in crosses where the hamster parent was R6G pretreated grew vigorously. The concomitant elimination of inviability and loss of mitochondrial determinants in R6G-pretreated hybrids suggests that interactions involving mitochondrial gene products or components can influence growth characteristics in interspecific somatic cell hybrids.

Suppression of mitochondrially-determined resistance to chloramphenicol and paromomycin by nuclear genes in Saccharomyces cerevisiae

Phenotypic "revertants" of a drug resistant strain of Saccharomyces cerevisiae were induced by mutgenesis with manganese. Several of these drug sensitive mutants have been shown to result from mutations in the nuclear genome that cause phenotypic modification (suppression) of the mitochondrially-determined drug resistant genotype. Four mutants carrying a single recessive nuclear gene capable of modifying mitochondrial chloramphenicol resistance are described; these may be assigned to three complementation groups. Chloramphenicol resistant mutants mapping at five separate mitochondrial loci are described. At least two of the nuclear genes cause modification of mitochondrial chloramphenicol resistance determined by mutations at three of these loci, but the other two loci are apparently non-suppressible by these nuclear alleles. This indicates that these modifiers do not act by causing a general decrease in cellular or mitochondrial permeability to the drug. A single dominant nuclear modifier of mitochondrial paromomycin resistance has been identified. It is non-allelic to and does not interact with the genes modifying mitochondrial chloramphenicol resistance.

Mitotic segregation of cytoplasmic determinants for chloramphenicol resistance in mammalian cells. I: Fusion with mouse cell lines

The segregation of cytoplasmically inherited chloramphenicol (CAP) resistance in mouse cells was investigated in fusions between CAP-resistant cells or cytoplasts (enucleated cells) and CAP-sensitive cells of varying tissue origin. All hybrids formed in cell-cell fusions were initially CAP-resistant, indicating that CAP resistance is dominant. Hybrids from fusions of cells of the same tissue origin (homologous) were stably CAP-resistant, whereas the hybrid population from fusions of different origins (heterologous) showed a rapid diminution of average CAP resistance. Individual hybrid clones from these heterologous fusions also showed an overall loss of CAP resistance, and a wide variation in CAP resistance which is consistent with a large number of genetic determinants (possibly mitochondrial DNA molecules) contributing to the CAP phenotype. Similar results were obtained from cytoplast-cell fusions, so the observed CAP segregation is not the result of nuclear-nuclear interactions. This segregation of CAP resistance constitutes a second criterion of cytoplasmic inheritance in mammalian cells.

Effects of elevation of strain-ploidy on transmission and recombination of mitochondrial drug resistance genes in Saccharomyces cerevisiae

In order to study the effects of strainploidy on the transmission and recombination of the mitochondrial genes C, E and O conferring the resistance to chloramphenicol, erythromycin and oligomycin, respectively, haploids were crossed to diploids and the results of genetic analysis were compared with those from haploid X haploid crosses. All haploid X haploid crosses showed an increased transmission of diploid derived alleles, relative to haploid derived ones, but the pattern of increase differed between homosexual and heterosexual crosses. In omega-haploid X omega-diploid homosexual crosses, the increase was of roughly equal magnitude at the C, E and O LOCI: there was a polar co-transmission of the diploid derived alleles. In omega plus haploid by omega-diploid heterosexual crosses, on the contrary, a differential increase was observed at the different loci, the magnitude being the smallest at the C locus and the largest at the O locus. As a result, there was a preferential transmission in favor of the haploid derived C alleles and of the diploid derived O alleles. A near equal transmission from both parents was observed for the E alleles. A decrease and an increase in the recombination frequency were noticed in the above haploid by diploid homosexual and heterosexual crosses, respectively. The above phenomena were ascribed to different dosages of mitochrondrial genomes from parents. Experimental data were well accorded with the theoretical expectation which were obtained on the assumptions that diploids contain twice as many mitochondrial genomes as haploids, and that random pairings and recombination would occur among mitochrondrial genomes from parents. The elevation of strain-ploidy did not affect the recombination polarity which is under the control of the omega gene. It was theoretically predicted that a preferential transmission in favor of diploid derived alleles at all the C, E and O loci would be seen in omega-haploid x omega plus diploid heterosexual crosses as well as in omega plus haploid x omega plus diploid homosexual crosses, but that the magnitude of the polar transmission would vary depending upon the loci in the former crosses, while it would be the same at all the loci in the latter ones. The recombination frequency was predicted to decrease in both of these crosses.

Pattern of somatic segregation of the cytoplasmic drug-resistance factors in yeast

The pattern of somatic segregation of the cytoplasmic factors confering resistances to chloramphenicol, erythromycin and oligomyin in S. cerevisiae was studied. The fractions of the zygotes heterozygous for the chloramphenicol-resistance factor and for the erythromycin-resistance factor decreased exponentially with generation number of zygotes. The rate of the segregation was highest for the chloramphenicol-resistance factor and lowest for the oligomycin-resistance factor. The segregation rate as well as the transmission polarity of the chloramphenicol-resistance factor varied with different carbon sources with which the parental haploids were grown prior to mating.