Partial pedigree analysis of the segregation of yeast mitochondrial genes during vegetative reproduction

Persistent heteroplasmic cells for mitochondrial genes in Saccharomyces cerevisiae

Genes in mitochondria and chloroplasts segregate rapidly during vegetative reproduction. Models to explain this vegetative segregation invoke either random segregation of organelle DNA molecules, or nonrandom segregation with random recombination events. All such models are basically stochastic. To look at vegetative segregation we took heteroplasmic (HET) cells containing mitochondrial mutations at the cap1, eryl and olil loci from several crosses. HETs were repeatedly selected and subcloned. Even after three to five successive subclonings (approximately 60-100 generations) some cells remained heteroplasmic. This confirms and extends previous observations of persistent HETs by Rank and Bech-Hansen (1972) and Forster and Kleese (1975), and by Bolen et al. (1980) for chloroplast genes in Chlamydomonas.

Transmission of yeast mitochondrial loci to progeny is reduced when nearby intergenic regions containing ori sequences are deleted

Mitochondrial DNAs (mtDNA) from four stable revertant strains generated from high frequency petite forming strains of Saccharomyces cerevisiae have been shown to contain deletions which have eliminated intergenic sequences encompassing ori1, ori2 and ori7. The deleted sequences are dispensable for expression of the respiratory phenotype and mutant strains exhibit the same relative amount of mtDNA per cell as the wild-type (wt) parental strain. These deletion mutants were also used to study the influence of particular intergenic sequences on the transmission of closely linked mitochondrial loci. When the mutant strains were crossed with the parental wt strains, there was a strong bias towards the transmission into the progeny of mitochondrial genomes lacking the intergenic deletions. The deficiency in the transmission of the mutant regions was not a simple function of deletion length and varied between different loci. In crosses between mutant strains which had non-overlapping deletions, wt mtDNA molecules were formed by recombination. The wt recombinants were present at high frequencies among the progeny of such crosses, but recombinants containing both deletions were not detected at all. The results indicate that mitochondrial genomes can be selectively transmitted to progeny and that two particular intergenic regions positively influence transmission. Within these regions other sequences in addition to ori/rep affect transmission.

A direct study of the relative synthesis of petite and grande mitochondrial DNA in zygotes from crosses involving suppressive petite mutants of Saccharomyces cerevisiae

Work in recent years has produced indirect evidence to support the view that the phenomenon of suppressiveness in yeast is the result of the ability of the petite mtDNA to out-replicate the wild-type genome. We have developed a method, based on fluorography of gels containing restriction fragments of radioactively labelled zygotic mtDNA, by which it has been possible to follow directly the incorporation of label into the two mtDNA species and hence their relative synthesis. Four petite isolates of 70%, 43%, 23% and 12% suppressiveness were tested by this method in crosses with a grande strain. Only the mtDNA from the 70% suppressive petite showed a replicative advantage over the grande mtDNA. The mtDNA from the 43% and 23% suppressive actually appeared to undergo, if anything, less replication in the zygote than the grande mtDNA. It is concluded that while some petites may exhibit suppressiveness as a result of enhanced replicative efficiency of their mtDNA, this cannot be the explanation for all suppressive petite strains.

The origin of mutant cells: mechanisms by which Saccharomyces cerevisiae produces cells homoplasmic for new mitochondrial mutations

Haploid yeast cells have about 50 copies of the mitochondrial genome, and a mutational event is unlikely to affect more than one of these at a time. This raises the question of how such cells, or their progeny, become fixed (homoplasmic) for the mutant alele. We have tested the roles of six hypothetical mechanisms in producing erythromycin-resistant mutant cells: (i) random partitioning of mitochondrial genomes at cell division; (ii) intracellular selection for mtDNA molecules of one genotype; (iii) intracellular random drift of mitochondrial allele frequencies; (iv) intercellular selection for cells of a particular mitochondrial genotype; (v) induction of mitochondrial gene mutations by the antibiotic used to select mutants; and (vi) reduction in the number of mitochondrial genomes per cell by the antibiotic. Our experiments indicate that intracellular selection plays the major role in producing erythromycin-resistant mutant cells in the presence of the antibiotic. In the absence of the antibiotic, the combined effects of random drift and random partitioning are most important in determining the fate of new mutations, most of which are lost rather than fixed. Our experiments provide no evidence for mutation induction or ploidy reduction by erythromycin.

The partitioning of cytoplasmic organelles at cell division

When an organism has only one or two mitochondria or chloroplasts per cell, it is probable that their partitioning is always stringently controlled so that each daughter cell always receives half the organelles in the parent cell. When there are more copies of an organelle, the available data suggest that partitioning is stochastic but far from random, with a strong tendency toward equality. The molecular mechanisms that promote equal partitioning are not known in any case, but the great variety of organelle behavior suggests that many different mechanisms are involved in different organisms. As Wilson (1925) pointed out, the precision of partitioning of cytoplasmic organelles rarely if ever equals that of mitosis, but it is still an expression of selection for mechanisms that will ensure the hereditary continuity of the organelles. How cells compensate for unequal partitioning by controlling organelle replication is known for only one case. But when one considers that Tetrahymena and Paramecium use different methods to compensate for unequal partitioning of macronuclear DNA, it would not be surprising if organisms use a variety of different compensating replication modes for organelles as well. What is surprising is that so little attention has been paid to these problems. Nothing could be simpler than counting organelles in dividing cells, but this has been done on a large scale in only two systems. Quantitative techniques in cell biology have been developed to the point where such studies could be done even on cells that have too many organelles for direct counting. Molecular mechanisms of partitioning have scarcely been touched on. Much more has been done on the role of the cytoskeleton in determining cell shape, and some observations have been made on its role in positioning organelles in interphase cells, but these kinds of studies have not been extended to dividing cells. Some experiments and observations have been made on the role of microtubules and microfilaments in moving cytoplasmic organelles around the cell during interphase, but again nothing has been done on their possible role in partitioning organelles at cytokinesis. The major lesson of this article is how little has been done, and how much can be done. The partitioning of cytoplasmic organelles at cell division is a wide-open field for future research, and one of great importance for both genetics and cell biology.