Showing papers in "Current Genetics in 1980"

Studies on mutants deficient in the photosystem I reaction centers in Chlamydomonas reinhardtii.

Abstract: Genetic analysis of 25 nuclear mutants defective in the chlorophyll-protein complex CP1 was undertaken. The mutants belong to 13 complementation groups scattered throughout the nuclear genome. All these mutants lack the apoprotein of CP1 and, in addition, a specific set of six low molecular weight thylakoid polypeptides. System I particles obtained by treating WT thylakoid membranes with detergent specifically contain those polypeptides which the mutants lack. These observations suggest that a particular sub-structure of the thylakoid membrane associated with the photosystem I activity is missing from all 25 mutants studied, and that this general phenotype can result from mutation at any one of several unlinked Mendelian loci.

Loss of 2 um DNA from Saccharomyces cerevisiae transformed with the chimaeric plasmid pJDB219.

Abstract: Two plasmids containing Saccharomyces cerevisiae 2 µm DNA sequences and the S. cerevisiae LEU2 gene have been found to display incompatibility with 2 µm DNA; in the presence of the LEU2 plasmids, 2 µm DNA can be lost. The LEU2 plasmids can be lost spontaneously after (and before) 2 µm DNA loss has occurred, so that strains completely lacking 2 µm DNA sequences can be obtained routinely.

A novel mutation that affects utilization of galactose in Saccharomyces cerevisiae.

Abstract: A novel type of regulatory mutation for galactose metabolism in Saccharomyces cerevisiae is described. The mutation named gal11 was recessive, non-allelic to GAL4, GAL80, GAL2, or GAL3, and unlinked to the gene cluster of GAL1, GAL10, and GAL7. It caused a ‘coordinate’ reduction of galactokinase, galactose-1-P uridylyl transferase, and UDP-glucose 4-epimerase by a factor of more than 5, rendering the mutant cells galactose-nonfermenting. The effect of the mutation was manifested not only in cells grown on galactose but also in cells constitutively synthesizing the galactose-metabolizing enzymes.

Altered ribosomal protein L29 in a cycloheximide-resistant strain of Saccharomyces cerevisiae.

Abstract: A spontaneous high-level cycloheximide-resistant mutant of the yeast Saccharomyces cerevisiae (strain cy32) is found to have an altered protein of the large subunit (60S) of cytoplasmic ribosomes, namely protein L29. The resistance character segregates together with this biochemical defect and is semidominant in heterozygous diploids. Judged from in vitro susceptibility to inhibition by cycloheximide there are at least 50% resistant ribosomes present in such diploid strains. From these results it is concluded that cycloheximide resistance of mutant cy32 is caused by mutation of a single gene and that it is the structural gene for L29 which is affected.

Characterization of chloroplast DNA in Chlamydomonas eugametos and C. moewusii and its inheritance in hybrid progeny.

Abstract: The density, molecular weight, and cellular repetition of DNA molecules associated with the β-DNA satellite of the interfertile algae Chlamydomonas eugametos and C. moewusii are reported. The similarities between these values and those for the chloroplast DNA (cpDNA) in the related alga Chlamydomonas reinhardtii indicate that these satellites represent cpDNA. The buoyant densities of C. eugametos and C. moewusii cpDNAs are indistinguishable from one another, as are those of their respective nuclear DNAs. These densities differ slightly from the densities of the homologous components of C. reinhardtii whole cell DNA. All three species differ with respect to additional minor satellite DNAs and low molecular weight DNAs of unknown cellular location. Differences in the Aval and Smal restriction endonuclease fragmentation patterns of C. eugametos and C. moewusii cpDNAs were employed to study the inheritance of cpDNA in an F1 hybrid which had inherited a non-Mendelian streptomycin resistance marker (sr-2) from the C. eugametos mating-type plus (mt +) parent and in two homoplasmic mitotic segregants from a B 1 hybrid (F1 × C. moewusii) which had been initially heteroplasmic for the resistance marker. Although the cpDNA patterns in the F1 hybrid were similar to those of the C. eugametos ml 1 parent, important differences were noted which suggest that recombination between C. eugametos and C. moewusii cpDNA had occurred. Homoplasmic streptomycin resistant and sensitive mitotic segregants recovered from the B1 hybrid product reveal Aval restriction patterns similar to those of the respective resistant and sensitive parents. These data are consistent with the hypothesis that the sr-2 marker is located in cpDNA and that C. eugametos and C. moewusii cpDNA sequences can coexist in the same chloroplast and, at least sometimes, segregate without extensive recombination. The transmission of low molecular weight DNAs characteristic of C. moewusii but of unknown cellular origin shows no direct correlation with the transmission of the sr-2 marker.

2 μm plasmid copy number in different yeast strains and repartition of endogenous and 2 μm chimeric plasmids in transformed strains.

Abstract: By using the renaturation kinetics technique we tried to get informations about the maintenance of the 2 μm plasmid in yeast cells. For this purpose we determined the 2 μm plasmid copy number: in various yeast strains, in a special set of mutants, in cells treated with ethidium bromide and cycloheximide and in different yeast strains obtained by transformation with 2 μm chimeric plasmids.

Expression of the split gene COB in yeast mtDNA : Mutational arrest in the pathway of transcript splicing.

Abstract: Electrophoretic separation of mitochondrial RNA followed by hybridization with restriction fragments of mtDNA has been used to identify transcripts of the split gen COB which codes for apocytochrome b.

A model for the rapid vegetative segregation of multiple chloroplast genomes in Chlamydomonas: Assumptions and predictions of the model.

Abstract: Physical evidence indicates that the chloroplast DNA of Chlamydomonas reinhardtii is composed of approximately 75 copies of a small unique sequence. Genetic analysis of zygotes biparental for chloroplast genes shows rapid vegetative segregation of parental chloroplast alleles. Zygote clones composed entirely of homoplasmic progeny cells predominate within 10-20 post-mating generations. A model is proposed here which reconciles the high multiplicity of chloroplast genes with their rapid vegetative segregation rates. Clustering of genomes into a small number of discrete areas (nucleoids) within the chloroplast reduces the effective number of segregating units. A non-random distribution of nucleoids to daughter cells, dictated solely by the spatial arrangement of parental nucleoids with respect to the plane of chloroplast division, further increases the rate of segregation from heteroplasmic cells. Recombination between parental chloroplast genomes is viewed as an indication of nucleoid fusion, and can account for differences in the patterns and rates of segregation at different gene loci. Within such fused nucleoids, clustering of parental genomes and a non-random distribution, again based solely on physical positioning of the genomes, to daughter nucleoids, could act to promote rapid genetic purification of heteroplasmic nucleoids. The effects of biased parental nucleoid ratios, and of potentially unequal nucleoid distributions to daughter chloroplasts are also discussed with respect to observed rates and patterns of chloroplast gene segregation.

Transformation to senescence with plasmid like DNA in the ascomycete Podospora anserina.

Abstract: In the ascomycete Podospora anserina senescence through strain aging is under nucleo-cytoplasmic control and inducible in juvenile mycelia by an ‘infective principle’ transferred after cytoplasmic contact via anastomoses. A specific DNA called plasmid-like (pl) DNA, present exclusively in aging mycelia, was found to be identical with this ‘infective principle’, since it was possible to transform juvenile protoplasts to senescence by using purified p1DNA. Therefore a specific function may be attributed to this ccc DNA. Its direct involvement in a genetically programed senescence is confirmed and its development as a vector for transfer of genetic information in eukaryotes can be undertaken.

Assembly of the mitochondrial membrane system. Organization of yeast mitochondrial DNA in the Oli1 region.

Abstract: The mitochondrial DNA (mtDNA) of a cytoplasmic petite mutant (DS401) of Saccharomyces cerevisiae genetically marked for the ATPase proteolipid, serine tRNA and varl genes has been characterized by restriction endonuclease analysis and DNA sequencing. The DS401 mtDNA segment is 5.3 kb long spanning the region between 79.1 and 86.8 units of the wild type genome. Most of the DS401 mtDNA consists of A+T rich sequences. In addition, however, there are ten short sequences with a high content of G+C and two sequences that have been identified as the ATPase proteolipid and the serine tRNA genes. The two genes map at 81 and 83 units and are transcribed from the same DNA strand. Even though there are other possible coding sequences in the DNA segment, none are sufficiently long to code for a gene product of the size of the varl protein. Based on the relative organization of the G+C rich clusters and genes, a model has been proposed for the processing of mitochondria) RNA. This model postulates the existence of mitochondrial double strand specific RNases that cleave the RNA at the G+C clusters.

Constitutive mutants for orotidine 5 phosphate decarboxylase and dihydroorotic acid dehydrogenase in Saccharomyces cerevisiae.

Abstract: A mutant yeast constitutive for synthesis of both dihydroorotic acid dehydrogenase and orotidine 5′ phosphate decarboxylase has been isolated. This phenotype is due to a single gene unlinked to any of the five genes of the pyrimidine pathway. This gene, called ppr1, induces the unlinked genes ural and ura3. Non-chromosomal cloned ura3 is also under the control of ppr1.

The Mating System of the Cellular Slime Mould Dictyostelium discoideum

Abstract: The mating reaction (macrocyst formation) and vegetative compatibility (which is believed to be associated with only the mating type locus in D. discoideum) of asexual, bisexual and homothallic strains of Dictyostelium discoideum were examined. Three asexual strains were vegetatively compatible with a matA tester strain and vegetatively incompatible with a mata tester strain, so we propose that these asexual strains are in fact strains of matA mating type with defective mating capacity. Two bisexual and two homothallic strains were vegetatively incompatible with both matA and mata tester strains, indicating that they either express both mating alleles or that they have a third mating type allele. On the basis of observations on the relative mating capacity with matA and mata tester strains, we propose that bisexual strains are closely related to homothallic strains and are not strains carrying a third mating type allele. Hence we suggest that D. discoideum has a one locus, two allele mating system. The homothallic strains AC4 and ZA3A may express both established mating type alleles (matA and mata). They are, however, normal haploid strains with seven chromosomes. The hypothesis that the homothallic P. pallidum strain PP28S is a diploid, was rejected.

High frequency recombination and the expression of genes cloned on chimeric yeast plasmids: Identification of a fragment of 2-μm circle essential for transformation.

Abstract: We have carried out experiments aimed at explaining the observed variations in transformation frequencies when Saccharomyces cerevisiae or Saccharomyces carlbergensis are transformed with chimeric plasmids that contain one of 4 possible EcoRI fragments of the yeast 2-μm circle. These plasmids fall into 2 classes when used to transform 2 different yeast his3 auxotrophs, one (strain LL20) harbours indigenous 2-μm circle, and the other (strain YF233) is devoid of this plasmid. Hybrid plasmids containing either the 2.4 mega-dalton (mD) R-form EcoRI fragment (pYF88) or the l.4 mD L-form EcoRI fragment (pYF177) of 2-μm circle transform either of the two hosts at a high frequency (50,000 colonies per Mg in LL20 and 10,000 colonies per μg in YF233). Hybrid plasmids containing the 1.5 mD R-form EcoRI fragment (pYF87) or the 2.5 mD L-form EcoRI fragment (pYF178) of the 2-μm circle transform LL20 at a reduced frequency (6,000-16,000 colonies per μg) and YF233 at extremely low frequencies (1-5 colonies per μg). All plasmids retrieved from strain YF233 that had been transformed with pYF88 or pYF177 were identical to the original transforming plasmid. Of the plasmids retrieved from strain LL20 that had been transformed with pYF87 and pYF178, approximately half had acquired an extra copy of the 2-μm circle. Of the plasmids retrieved from strain LL20 that had been transformed with pYF88 and pYF177, an average of only approximately 13% had acquired an extra copy of 2-μm circle. Taken together, these observations indicate that the transformation of yeast by a plasmid lacking the ability to replicate (pYF87 and pYF1780) occurs by the recombinational acquisition of 1 copy of the host 2-μm circle, which serves to supply the incoming plasmid with missing essential sequences. A comparison of 2-μm circle DNA fragments carried by pYF88 and pYF177 indicates that the region of 2-μm circle required for high frequency transformation is a 1.2 mD segment that is common to the 2.4 mD R-form and 1.4 ml) L-form EcoRI fragments. This region extends from the EcoRI cut site adjacent to the PstI site, through to the end of the inverted repeat. However, the inverted repeat sequence alone is not sufficient to bestow high frequency transformation of yeast.

Mitotic versus meiotic recombination in Saccharomyces cerevisiae.

Abstract: As part of a comparative analysis of spontaneous mitotic and meiotic recombination we have compared the mitotic and meiotic maps of the wild type and yeast hybrids homozygous for reml-l, a mitosis-specific hyper-rec mutation (Golin and Esposito, 1977; Golin, 1979). In wild type yeast strains recombination in centromere proximal intervals occurs relatively more frequently in mitosis than in meiosis. In reml-1/rem1-1 hybrids the distribution of mitotic exchange events is more similar to the distribution observed in meiosis.

Cloned ß-galactosidase gene of Escherichia coli is expressed in the yeast Saccharomyces cerevisiae

Abstract: The expression of the lacZ gene of E. coli in S. cerevisiae has been studied. The enzymatic activity coded by the lacZ gene in E. coli, β-galactosidase, is detectable in yeast cells harboring a chimeric plasmid carrying the gene. On the basis of size and immunological criteria, no difference was detected between the Coli-in-yeast β-galactosidase and the E. coli enzyme.

Mapping of mitochondrial DNA from Torulopsis glabrata : Location of ribosomal and transfer RNA genes.

Abstract: Large and small rRNAs have been isolated from mitochondria of the yeast Torulopsis glabrata and have been shown to have lengths of 2,700 bases and 1,400 bases respectively. Construction of a restriction endonuclease site map of mitochondria) DNA has enabled us to position these rRNAs by hybridization of labelled RNA to DNA fragments transferred to nitrocellulose. The large and small mt rRNA genes are separated by a minimum of 1,820 by and a maximum of 2,765 by on the 18,870 by mitochondria1 genome. tRNA genes map within this separating sequence but they are also located distal to both rRNA genes. The implication of these results to the structural relationships of mitochondrial DNAs from yeasts is discussed.

The origin of a centromere effect on mitotic recombination : A study in the fission yeast Schizosaccharomyces pombe.

Abstract: Spontaneous meiotic and mitotic rates of recombination were measured for 50 intragenic intervals in 9 genes of Schizosaccharomyces pombe. A much smaller mitotic/meiotic recombination ratio is observed for genes unlinked to the centromere (average ratio: 0.005) than for genes close to the centromere (up to 0.17). The high ratio observed for the latter genes is due to a high rate of mitotic recombination rather than to a centromeric inhibition of meiotic recombination, since there was no meiotic inhibition in the genes considered. As already reported for one pair of genes in Saccharomyces cerevisiae (Henaut et Luzzati, 1972), cells that have recombined at one locus have a meiotic rate of coincident recombination at an unlinked locus, even if the latter is not on the same chromosome. This coincidence is much lower when intragenic recombinants are selected in the centromere-linked gene lys1. In modification of an earlier hypothesis (Hurst and Fogel, 1964), we propose that: (a) pairing between homologous chromosomes is the major rate-limiting factor in mitotic recombination; (b) mitotic pairing of homologous chromosomes is frequent, possibly occuring in all vegetative cells, but is usually restricted to the centromere region; and (c) pairing along the whole genome is restricted to a small subpopulation among the mitotically dividing cells.

Gene conversion in nonsense suppressors of Schizosaccharomyces pombe : II. Specific marker effects.

Abstract: Gene conversion and postmeiotic segregation patterns have been analysed at 14 mutant sites of sup3, sup8 and sup9 including 5 alleles with a strong marker effect on recombination frequencies in two-factor crosses. The total frequency of gene conversion and postmeiotic segregation tetrads is fairly constant within each gene, but may vary from one gene to another. About 97% of the conversion events are coconversions spanning the whole sup gene. Postmeiotic segregations are usually quite rare. None of the marker-effect alleles has an increased rate of hybrid DNA formation at the allele considered, as judged from the frequency of gene conversion and postmeiotic segregation in one-factor crosses. At least two of them, sup3-e and sup9-e, are associated with a high frequency of postmeiotic segregation indicating a poor repair of the corresponding base-pair mismatches. This is also observed in a two-factor cross and can account for the marker effect on recombination frequencies. The properties of a third marker effect allele, sup3-e,r10, are best explained by a higher probability of single site conversions as opposed to coconversions in two-factor crosses involving the mutant site r10.

Uniparental inheritance of chloroplast DNA sequences in interspecific hybrids of Chlamydomonas.

Abstract: The meiotic transmission of chloroplast DNA (cpDNA) was studied in crosses between two species of Chlamydomonas (C. moewusii and C. eugametos) which have substantial differences in cpDNA restriction patterns. The results provide a direct demonstration that cpDNA can be inherited in a uniparental pattern, paralleling the transmission of a uniparentally inherited antibiotic resistance marker. Thus, cpDNA could carry the uniparental genes of these species, but other extrachromosomal DNAs are not excluded as possible carriers. For example, C. moewusii was found to contain a set of low molecular weight (LMW) DNA species which cannot be detected in C. eugametos. These LMW DNA species are also transmitted uniparentally in the tetrads studied. Uniparentai transmission may not be an exclusive property of cpDNA in Chlamydomonas species.

Supersuppressive "petite" mutants of yeast.

Abstract: A class of suppressive “petite” mutants of S. cerevisiae, called here supersuppressive, is characterized by a) the fact that their unmodified mitochondrial genomes are the only ones found in the progeny of crosses with wild-type cells; b) very short repeat units (400–900 base pairs) in their mitochondrial genomes. The repeat units of the three supersuppressive “petites” investigated here share a common 83 nucleotide sequence, which seems to correspond to an initiation site of DNA replication; the multiple copies of this site in the mitochondrial genomes of supersuppressive “petites” might explain why these genomes can compete out those of wild-type cells.

Control of recombination within and between DNA plasmids of Saccharomyces cerevisiae.

Abstract: [2 μm+ and [2μm°] yeast were transformed to stable leucine prototrophy with the hybrid yeast — E. coli plasmid, pJDB219. This plasmid contains the entire sequence of the endogenous 2 μm yeast DNA plasmid in addition to the yeast nuclear LEU2+ gene and the Co1E1 derivative, pMB9. In the [2 μm+] transformants, a new wholly yeast LEU2+ plasmid, pYX, was generated, probably by a recombination event between pJDB219 and 2 μm DNA. The plamid, pYX, in the absence of 2 μm DNA, was found to exist in equimolar amounts of two forms, A and B, which probably arise by intramolecular recombination across the inverted repeat sequences of the 2 μm DNA portion of the plasmid. pJDB219 was found to require the presence of 2 μm DNA to undergo this intramolecular recombination. The results suggest that 2, μm DNA and pYX code for a gene product required in this recombination event which pJDB219 cannot produce.

Physical mapping of genetic determinants on yeast mitochondrial DNA affecting the apparent size of the Var 1 polypeptide.

Abstract: The var1 locus on yeast mitochondria) DNA is defined by a set of alleles that affect the apparent size (Mr 40,000–44,000) of a mitochondria) translation product (var1), a protein of the mitochondria) 38S ribosomal subunit. Using petite deletion and restriction endonuclease site mapping, we have defined the physical location of all var1 alleles to a specific restriction fragment of roughly 2.1 kbp located between the antibiotic resistance loci ery1 and olil. Surprisingly, from base sequence studies of this region by Tzagoloff et al. (1980), it appears that the DNA fragment we have mapped contains little or none of the structural gene for the var1 protein since the fragment is composed primarily of long stretches of dA + dT interspersed with short clusters high in dG + dC. Nevertheless, by a type of complementation test termed zygotic gene rescue (Strausberg and Butow, 1977), we show that with petites retaining that restriction fragment and short flanking sequences, var1 polypeptide characteristic of the strain from which the petite was derived, is expressed in zygotes formed between the petite and a wild-type tester. Thus the ability of the var1 determinant to act in trans suggests that control of expression of different var1 species involves intermolecular interactions, perhaps at the level of RNA splicing. Our results are discussed in terms of several possible models for the organization and control of the var1 structural gene.

The effect of hydroxyurea on the mechanism of DNA synthesis in the yeast Saccharomyces cerevisiae.

Abstract: Newly synthesised DNA molecules the same size as replicons (7 million-60 million daltons) accumulate in yeast cells treated with hydroxyurea. During prolonged incubation in low concentrations of the drug, there is a large accumulation of these molecules without any corresponding increase in their molecular weight. On release from the inhibtion the molecules are converted to large molecular weight DNA. These observations are consistent with an inhibition by hydroxyurea of the joining of completed replicons. In addition, newly synthesised DNA molecules the size of yeast Okazaki fragments also accumulate in cells treated with hydroxyurea.

Chloroplast gene inheritance studied by somatic fusion in Chlamydomonas reinhardtii.

Abstract: Somatic fusion between strains of Chlamydomonas containing complementing cell-wall and auxotrophic mutations, having the same mating-type (mt) and bearing chloroplast markers, have been performed to study the mode of chloroplast gene inheritance in the fusion products. About one third of the fusion products (mt +/mt + or mt −/mt −) transmitted chloroplast markers from both parents (= biparental fusion products). The rest of the population was equally distributed between fusion products transmitting the chloroplast marker of one parent or the other (uniparental fusion products) exclusively. Incubation of the fusion products in the dark for 48 hours, immediately after the fusion, decreases the frequency of biparental fusion products. The results indicate that the general process of elimination of chloroplast alleles is independent of the presence of both mt + and mt − alleles in the cell. In contrast, directional elimination (i.e. preferential elimination of paternal chloroplast alleles) does appear to depend upon heterozygosity at the mt locus. These results are discussed in relation to the models which have been proposed to explain the maternal inheritance of chloroplast genes in Chlamydomonas.

Recombinational analysis of OXI1 mutants and preliminary analysis of their translation products in S. cerevisiae

Abstract: Recombinational analysis of oxil mutants was performed using a and a mutant strains with the same mitochondrial and nuclear backgrounds, derived from strain 777-3A.

Restoration to the parental genotype of mismatches formed in recombinant DNA heteroduplex

Abstract: Crosses were set up, using the b2 locus of Ascobolus immersus, to detect and measure the restoration of the parental genotype of a chromatid from hetero-duplex. From the results it can be concluded that: (1) restoration occurs, and its frequency is comparable to the frequency of conversion; (2) the relative frequency of conversion and restoration on the two homologous chromatids is that expected if the disparity in conversion to mutant and to wild-type of the marker is caused by the decision whether to excise the mutant or wild-type chain. This decision is the same on both chromatids. However, when all data are pooled, a slight, but significant, excess of conversion over restoration is found. The cause of this in discussed; (3) the absence of an excess of restoration over conversion is taken to imply that, in this system, the length of heteroduplex formed in an event is continuous; (4) these findings support the interpretation of tetrads where a crossover is separated from conversion, that they arise by independent correction giving both conversion and restoration in a heteroduplex tract continuous with the crossover.

Behavior of chloroplast genes during the early zygotic divisions of Chlamydomonas reinhardtii.

Abstract: Chloroplast mutations in the green alga Chlamydomonas reinhardtii exhibit a predominantly maternal pattern of inheritance and this pattern can be perturbed by UV irradiation of the maternal gametes prior to mating In a series of crosses over a range of UV doses, the transmission, segregation, and recombination of mutations at three closely linked chloroplast loci have been examined by pedigree analysis of products arising from the first three post-zygotic divisions Stocks used in these crosses were constructed to permit identification of the nuclear products of each of the two meiotic divisions and the first post-meiotic mitotic division

Relationship between asymmetrical and symmetrical hybrid DNA formation during meiotic recombination.

Abstract: The cross b2-17-A4 x b2+ involving 2 mutant sites located one at each end of the b2 locus in Ascobolus immersus was analysed in order to look for aberrant segregations at one site among asci with aberrant 4:4 segregation at the other site. Most double site conversion asci with an aberrant 4:4 at one site showed a 5:3 segregation at the other site. Almost always, the aberrant 4:4 involved the low conversion end site and the 5:3 involved the high conversion end site. It is concluded that, in each individual meiosis, symmetrical hybrid DNA is often, if not always, physically associated with asymmetrical hybrid DNA. The asymmetrical hybrid DNA portion lies near the high conversion end and the symmetrical hybrid DNA portion near the low one.

Changes in chloroplast genome composition and recombination during the maturation of zygospores of Chlamydomonas reinhardtii.

Abstract: In crosses of the unicellular green alga Chlamydomonas reinhardtii, the chloroplast genes are normally transmitted exclusively by the maternal parent to zygospore progeny. However, transmission of the paternal chloroplast alleles can be increased markedly by certain pretreatments of the maternal parent prior to mating. As zygospores age prior to induction of meiosis, they display decreased biparental transmission of chloroplast alleles and increased transmission of chloroplast alleles from only the maternal or paternal parent. In this report, chloroplast genome composition of biparental zygospores is shown to change in several ways during zygospore maturation. Allelic ratios of chloroplast genes within biparental zygospore clones become maternally or paternally skewed as the zygospores age, cotransmission of chloroplast alleles is reduced, and recombination increases, resulting in an expansion of genetic map distances between chloroplast markers used in this cross. The recovery of unequal frequencies of zygospore progeny expressing reciprocal recombinant genotypes confirms and extends other reports of the predominance of nonreciprocal recombination in organelle genetic systems.

Protoplast fusion in a petite-negative yeast, Kluyveromyces lactis.

Abstract: The technique of protoplast fusion has been applied to the problem of unstable diploidy in the yeast Kluyveromyces lactis. By protoplast fusion between heterothallic strains of like mating-type, sporulation-deficient hybrids can be obtained. Biochemical, cytological, and genetical characterisation of these hybrids suggests that the majority of fusion products are diploid. Sporulating hybrids can be constructed by protoplast fusion between homothallic strains. Tetrad analysis of these hybrids demonstrates conclusively the diploid nature of fusion products.