Zygote heterogeneity and uniparental inheritance of mitochondrial genes in yeast

Cell biology of yeast zygotes, from genesis to budding

The zygote is the essential intermediate that allows interchange of nuclear, mitochondrial and cytosolic determinants between cells. Zygote formation in Saccharomyces cerevisiae is accomplished by mechanisms that are not characteristic of mitotic cells. These include shifting the axis of growth away from classical cortical landmarks, dramatically reorganizing the cell cortex, remodeling the cell wall in preparation for cell fusion, fusing with an adjacent partner, accomplishing nuclear fusion, orchestrating two steps of septin morphogenesis that account for a delay in fusion of mitochondria, and implementing new norms for bud site selection. This essay emphasizes the sequence of dependent relationships that account for this progression from cell encounters through zygote budding. It briefly summarizes classical studies of signal transduction and polarity specification and then focuses on downstream events.

Size and competitive mating success in the yeast Saccharomyces cerevisiae

In unicellular organisms like yeast, mating with the right partner is critical to future fitness because each individual can only mate once. Because cell size is important for viability, mating with a partner of the right size could be a significant advantage. To investigate this idea, we manipulated the size of unmated yeast cells and showed that their viability depended on environmental conditions; large cells do better on rich medium and small cells do better on poor medium. We also found that the fitness of offspring is determined by the size of their parents. Finally, we demonstrated that when a focal cell of one mating type was placed with a large and a small cell of the opposite mating type, it was more likely to mate with the cell that was closer to the optimum size for growth in a given environment. This pattern was not generated by differences in passive mating efficiency of large and small cells across environments but by competitive mating behavior, mate preference, or both. We conclude that the most likely mechanism underlying this interesting behavior is that yeast cells compete for mates by producing pheromone signals advertising their viability, and cells with the opportunity to choose prefer to mate with stronger signalers because such matings produce more viable offspring.

Septin-containing barriers control the differential inheritance of cytoplasmic elements

Fusion of haploid cells of Saccharomyces cerevisiae generates zygotes. We observe that the zygote midzone includes a septin annulus and differentially affects redistribution of supramolecular complexes and organelles. Redistribution across the midzone of supramolecular complexes (polysomes and Sup35p-GFP [PSI+]) is unexpectedly delayed relative to soluble proteins; however, in [psi-] × [PSI+] crosses, all buds eventually receive Sup35p-GFP [PSI+]. Encounter between parental mitochondria is further delayed until septins relocate to the bud site, where they are required for repolarization of the actin cytoskeleton. This delay allows rationalization of the longstanding observation that terminal zygotic buds preferentially inherit a single mitochondrial genotype. The rate of redistribution of complexes and organelles determines whether their inheritance will be uniform.

A mathematical model of extranuclear genes and the genetic variability maintained in a finite population

A mathematical theory of population genetics accounting for the genes transmitted through mitochondria or chloroplasts has been studied. In the model it is assumed that a population consists of Nm males and Nf females, the genetic contribution from a male is β and that from a female 1 – β, and each cell line contains n effective copies of a gene in its cytoplasm. Assuming selective neutrality and an infinite alleles model, it is shown that the sum (H) of squares of allelic frequencies within an individual and the corresponding sum (Q) for the entire population are, at equilibrium, given by

and

where ρ = 2β(1−β), Ne = {β2/Nm+(1−β)2 / Nf}−1, λ is the number of somatic cell divisions in one generation, and v is the mutation rate per cell division. If the genes are transmitted entirely through the female the formulae reduce to Ĥ ≃ 1/(1 + 2nv) and Q^ ≃ 1/{1 + (2Neλ+ 2n)υ}. Non-equilibrium behaviours of H and Q^ are also studied in the case of a panmictic population. These results are extended to geographically structured models, and applied to existing experimental data.

Heteroplasmy and organelle gene dynamics

This study assesses factors that influence the rates of change of organelle gene diversity and the maintenance of heteroplasmy. Losses of organelle gene diversity within individuals via vegetative segregation during ontogeny are paramount to resultant spatial and temporal patterns. Steady-state losses of organelle variation from the zygote to the gametes are determined by the effective number of organelles, which will be approximately equal to the number of intracellular organelles if random segregation prevails. Both rapid increases in organelle number after zygote formation and reductions at germ lines will reduce variation within individuals. Terminal reductions in organelles must be to very low copy numbers (<5) for substantial losses in variation to occur rapidly. Nonrandom clonal expansion and vegetative segregation during gametogenesis may be effective in reducing genetic variation in gametes. If organelles are uniparentally inherited, the asymptotic expectations for effective numbers of gametes and spatial differentiation will be identical for homoplasmic and heteroplasmic conditions. The rate of attainment of asymptote for heteroplasmic organelles, however, is governed by the rate of loss of variation during ontogeny. With sex-biased dispersal, the effective number of gametes is maximized when the proportional contributions of the sex having the higher dispersal rate are low.

Intervacuole exchange in the yeast zygote: a new pathway in organelle communication

A new pathway of vesicle traffic between organelles has been identified. The vacuoles (lysosomes) of Saccharomyces cerevisiae zygotes rapidly exchange their contents at a specific point in the cell cycle. With the use of fluorescence microscopy, "tracks" were observed that connect the original parental vacuoles to the newly forming bud vacuoles. These observations suggest that vacuole-derived vesicles rapidly move along the tracks in both directions, equilibrating vacuole contents. This rapid vesicle movement may be responsible for vacuole formation in newly developing cells.

Kinetic and segregational analysis of mitochondrial DNA recombination in yeast

A pair of yeast strains of opposite mating type was constructed to contain polymorphisms at three loci on the mitochondrial genome--the 21 S rRNA gene, var1, and cob--such that parental and recombinant forms of these genes could be easily detected by Southern blot analysis. These polymorphisms were used to measure in a single cross gene conversions at the 21 S rRNA and var1 loci and a reciprocal recombination at cob. For all three loci, recombination initiates at about the same time, 4 to 6 h after mixing cells, and increases with similar kinetics over a 24-h period. The segregation of parental and recombinant forms of these genes was then followed by pedigree analysis. The results, which show a high variance in the distribution of parental and recombinant forms of all three genes in cells derived from both the first bud and the mother zygote, are consistent with the segregation of a small number of mitochondrial DNA molecules from the zygote to diploid buds. Based on these results and previous experiments of this type, a limited "zone of mixing" of parental mitochondrial DNA molecules probably exists in the zygote. The extent of sampling from this zone, together with the intrinsic properties of the recombination events themselves, is likely to determine the observed pattern of recombination of mitochondrial DNA sequences at the population level.

The non-reciprocality of organelle gene recombination in Chlamydomonas reinhardtii and Saccharomyces cerevisiae: some new observations and a restatement of some old problems

Organelle recombinant genotype frequencies, derived from analysis of individual mitotic zygote clones of Chlamydomonas reinhardtii and Saccharomyces cerevisiae, were subjected to two types of statistical tests in an attempt to detect the occurrence of reciprocal recombination: (i) calculation of correlation coefficients for the frequencies of two recombinant genotypes (reciprocal or non-reciprocal pairs) within individual zygote clones, and (ii) application of the chi-square test for independence to the frequencies of zygotes yielding one or the other, neither, or both of a given recombinant pair. Applying test (i), the strongest correlations are found for non-reciprocal rather than reciprocal pairs. When the data are analyzed by method (ii), some reciprocal as well as non-reciprocal pairs appear to be produced concurrently in zygote clones. However, such deviations from independence are greatest for non-reciprocal pairs. These tests yield comparable results for yeast mitochondrial and Chlamydomonas chloroplast gene recombination, and provide no convincing evidence for reciprocal genetic exchange. Explanations for the observed lack of reciprocality are discussed with reference both to our present understanding of the molecular events responsible for genetic recombination, and to the problems which may be unique to the analysis of organelle gene recombination.

Distribution of mitochondrially inherited drug-resistance genes to tetrads from young zygotes in yeast

Pairs of strains of opposite mating type were isolated from a strain of Saccharomyces cerevisiae. From these isogenic strains, mitochondrially inherited resistant mutants to antimycin A and erythromycin were isolated. By using the two resistance genes as mitochondrial markers, it was proposed that the distribution of the mitochondrial genomes from zygotes to tetrads seemed not to be random but the genomes from either a or alpha parent would be selected with approximately equal frequencies after zygote formation and subsequently distributed uniparentally to meiotic products.

Extrachromosomal inheritance in Schizosaccharomyces pombe. VII. Studies by zygote clone analysis on transmission, segregation, recombination, and uniparental inheritance of mitochondrial markers conferring resistance to antimycin, chloramphenicol, and erythromycin

Crosses involving mitochondrial markers conferring resistance to antimycin (anar, AR), chloramphenicol (capr, CR), and erythromycin (eryr, ER) in cis- and trans-configuration were studied by zygote clone analysis. Mutant anar-8, from which all other drug--resistant isolates were derived, exhibits a highly biased transmission (6.8% anar) in an analysis of 100 individual zygote clones. Important results of zygote clone analyses were:--Zygote clones may contain one, two, three, or four mitochondrial genotypes.--The proportion of the two parental and the two recombinant genotypes in individual zygote clones can vary almost over the entire range of percentages.--Proportions of the two corresponding recombinant types in individual clones are usually unequal.--Transmission rates of markers are higher in trans- than in cis-crosses, indicating additivity of bias by two mutated alleles in coupling.--Transmission rates are different for the three markers both in cis- and trans-crosses, being lowest for CR and highest for ER.--Up to more than 80% uniform clones, expressing only one genotype, can be produced in cis- and trans-crosses. In cis-crosses always the double-sensitive parental type becomes uniform, in trans-crosses this may be the case for parental and/or recombinant genotypes. A tentative map is presented using data from cis- and trans-crosses, including a correction by omission of uniform clones. Phenomena of transmission, segregation, and formation of uniform clones are discussed with special regard to the difference brought about by fission versus budding. A comparison with relevant data from Saccharomyces cerevisiae and other organisms is presented.

Mitotic segregation of cytoplasmic determinants for chloramphenicol resistance in mammalian cells II: Fusions with human cell lines

Cytoplasmically inherited chloramphenicol (CAP) resistance in human cells has been used to study the interaction between sensitive and resistant mitochondria. Cybrids between two HeLa cells were stable for resistance, grew rapidly and cloned well in CAP, and were O2 tolerant. HeLa-HeLa hybrids were also stable up to 70 doublings in the absence of CAP. Cybrids between HeLa and WI-L2 cells were unstable for resistance for up to 40 doublings, grew slowly and cloned poorly in CAP, and were O2 sensitive (S phase). The growth rate then increased and the cells became stable for resistance, cloned well, and were not O2 sensitive (F phase). Doubling time for S but not F phase cells was proportional to CAP concentration, indicating that both kinds of mitochondria were present and functioning. The instability of CAP resistance in many interstrain but not in intrastrain mouse and human cybrids and hybrids is interpreted in relation to lower eukaryotes.

Maternal inheritance of extranuclear mitochondrial markers in Aspergillus nidulans

Maternal inheritance of extranuclear mitochondrial genes has been demonstrated inAspergillus nidulansusing the ‘blue’ ascospore colour mutants in combination with heterokaryon incompatible strains. It appears that heterokaryosis is not a prerequisite of sexual outcrossing, and that recombination of extranuclear mitochondrial markers does not occur in the sexual stage of the cell cycle.