Replication of chromosomal and cytoplasmic DNA during mitosis and meiosis in the eucaryote Chlamydomonas reinhardi

Flow Cytometric Methods for Indirect Analysis and Quantification of Gametogenesis in Chlamydomonas reinhardtii (Chlorophyceae)

Induction of sexual reproduction in the facultatively sexual Chlamydomonas reinhardtii is cued by depletion of nitrogen. We explore the capacity for indirect monitoring of population variation in the gametogenic process using flow cytometry. We describe a high-throughput method capable of identifying fluorescence, ploidy and scatter profiles that track vegetative cells entering and undergoing gametogenesis. We demonstrate for the first time, that very early and late growth phases reduce the capacity to distinguish putative gametes from vegetative cells based on scatter and fluorescence profiles, and that early/mid-logarithmic cultures show the optimal distinction between vegetative cells and gamete scatter profiles. We argue that early/mid logarithmic cultures are valuable in such high throughput comparative approaches when investigating optimisation or quantification of gametogenesis based on scatter and fluorescence profiles. This approach provides new insights into the impact of culture conditions on gametogenesis, while documenting novel scatter and fluorescence profile shifts which typify the process. This method has potential applications to; enabling quick high-throughput monitoring, uses in increasing efficiency in the quantification of gametogenesis, as a method of comparing the switch between vegetative and gametic states across treatments, and as criteria for enrichment of gametic phenotypes in cell sorting assays.

Transmission of chloroplast alleles in somatic fusion products obtained from vegetative cells and/or 'Gametes' of Chlamydomonas reinhardi

Somatic fusion between cells of Chlamydomonas containing complementing cell wall and auxotrophic mutations and bearing chloroplast markers for resistance to antibiotics (streptomycin or spectinomycin) have been performed to analyze the mode of chloroplast gene transmission in the fusion products. Prototrophic colonies developed from mitotic divisions of 'diploid' fusion products were isolated on minimal medium and analyzed for their resistance to antibiotics. Fusion was performed between vegetative or nitrogen-starved cells (non-flagellated gametes) of the same or of opposite mating type. In all cases, about one third of the fusion products (mt (+)/mt (+), 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 exclusively (uniparental, or UP fusion products). The results indicate that the preferential elimination of paternal chloroplast alleles (i.e. maternal inheritance) observed after sexual fusion does not occur following artificially induced cell fusion, and that heterozygosity at the mt locus is not sufficient to ensure a directionality in uniparental chloroplast gene transmission.When somatic fusions were made between vegetative cells and nitrogen-starved cells, preferential transmission of the chloroplast alleles of the vegetative parent was observed, independently of the mating type of the parent. The data can be interpreted in terms of differences in the input frequencies of parental chloroplast genomes at the time of cell fusion. The possible importance of flagellar contact between opposite mating types in determining patterns of chloroplast gene transmission is also discussed.

Cytological detection of the basis of uniparental inheritance of plastid DNA in Chlamydomonas moewusii

By using the fluorochrome 4'-6-diamidino-2-phenylindole (DAPI) to stain DNA one can follow the pattern of events affecting plastid DNA which occur in the formation and maturation of individual zygotes of the green flagellate Chlamydomonas moewusii. This species, like C. reinhardi, expresses uniparental inheritance of plastid DNA characters among zygote progeny, and is particularly favorable for cytological observation because the locale of the contribution of each gamete can still be recognized in mature zygotes. Gametes contribute equal numbers of DNA nucleoids, and amounts of plastid DNA (as measured by DAPI-DNA micro spectrofluorometry), to the zygote at fusion. Starting at nine hours, coincident with the further fusion of cell contents, plastid DNA disappears from the plastid contributed by one gamete. Further slow coalescence of nucleoids leads to a final nucleoid number per zygote approximately 1/3 of the sum of the 2 gametes.The DNA loss from one gamete plastid may require plastid contact to be initiated. Both light and nutrient availability affect the final number and distribution of plastid DNA nucleoids in the mature zygote. These observations are related to known genetic and biochemical data on uniparental inheritance of plastid characters.

Transmission of chloroplast genes in triploid and tetraploid zygospores of Chlamydomonas reinhardtii: Roles of mating-type gene dosage and gametic chloroplast DNA content

Diploid clones homozygous (mt(+)/mt(+) or mt(-)/mt(-)) or heterozygous (mt(+)/mt(-); phenotypically mt(-)) for the mating-type locus and homoplasmic for a chloroplast marker conferring resistance to an antibiotic were isolated by artificially induced cell fusion or sexual mating. These diploids were crossed with haploid or diploid strains of opposite mating type and carrying another chloroplast marker. The transmission of the chloroplast genes was analyzed in the triploid and tetraploid zygospores in comparison with diploid zygospores used as controls. The transmission was almost exclusively maternal (mt(+)) (>94%) in the crosses mt(+) x mt(-), mt(+)/mt(+) x mt(-), and mt(+)/mt(+) x mt(-)/mt(-). The transmission was preferentially maternal (>76%) in the crosses mt(+) x mt(-)/mt(-) whereas in the crosses mt(+) x mt(+)/mt(-), <50% of the zygospores transmitted the chloroplast allele of maternal (mt(+)) origin. The zygospores produced in crosses mt(+)/mt(+) x mt(+)/mt(-) transmitted the alleles from both parents in >60% of cases. The results show that (i) the presence of one mt(+) allele in the mt(+)/mt(-) (phenotypically mt(-)) diploid gametes and (ii) the higher amount of chloroplast DNA molecules (input) present in the diploid gametes versus the haploid ones favor the transmission of the chloroplast allele contributed by these gametes. Moreover, because the zygospores issued from crosses mt(+)/mt(+) x mt(-) and mt(+) x mt(+)/mt(-) were genotypically identical mt(+)/mt(+)/mt(-)) but behaved very differently in their chloroplast gene transmission, it was concluded that the molecular events leading to preferential elimination of paternal DNA copies must occur before the fusion of nuclei or chloroplasts in the newly formed zygotes.

Chloroplast DNA methylation and inheritance in Chlamydomonas

When Chlamydomonas reinhardtii cells mate, a zygotic maturation program is activated, part of which leads to destruction of chloroplast DNA (cpDNA) from the mating type minus (mt-) parent, and, therefore, to uniparental inheritance of mating type plus (mt+) cpDNA. A long-standing model that explains the selective destruction of mt(-) cpDNA in zygotes invokes a methylation-restriction system. We tested this model by using the potent methylation inhibitor 5-aza-2'-deoxycytidine (5adc) to hypomethylate parental cpDNA and found that the pattern of cpDNA inheritance is altered by 5adc in a manner that is consistent with the model. Surprisingly, however, hypomethylated mt+ cpDNA is not destroyed in zygotes as the methylation-restriction model predicts it should be. Destruction of mt- cpDNA is also unaffected when the parental mt+ cpDNA is hypomethylated. Instead, loss of methylation affects the relative rates of replication of residual mt- cpDNA and mt+ cpDNA in germinating zygotes. The mode of action for 5adc on cpDNA replication in germinating zygotes may be via hypomethylation of mt+ cpDNA, but is also consistent with its action as a DNA-damaging agent. Interestingly, 5adc causes reduced cpDNA replication only in germinating zygotes, not in vegetatively grown cells, indicating that cpDNA replication is qualitatively different in these two stages of the life cycle. Our results demonstrate that methylation is not necessary for protection of the mt+ cpDNA in early zygotes and uncover a novel stage of the Chlamydomonas life cycle when replication of cpDNA is highly susceptible to perturbation. Our data support a model in which differential cpDNA replication in germinating zygotes is used as a mechanism to selectively amplify intact and properly methylated cpDNA molecules.

Evidence for chromosome endoreduplication in Eudorina californica, a colonial alga

Several nuclear events seen during the cleavage period in Eudorina suggest that chromosome endoreduplication, proportional to the number of cells to be produced, may occur in the gonidia prior to cleavage. Presumably the DNA concentration is reduced to the haploid level during rapid, successive divisions of the cleavage period. To test this hypothesis, I determined DNA content of gonidia as they grew from 4 mum to 38 mum in diameter between cleavage periods. During growth from 4 mum to 8 mum in diameter, the DNA concentration remained at the haploid level of 0.17 pg/cell. As gonidia in 64 cell colonies continued to grow from 8 mum to 33 mum in diameter, their DNA concentrations increased 60-fold. Analysis of the Eudorina DNA by equilibrium centrifugation in CsCl showed only 2 bands with buoyant densities of 1.721 g/cm3 and 1.699 g/cm3, presumed to be nuclear and chloroplast, respectively, on the basis of labelling with 3H-thymidine and 3H-adenine. The 8:2 ratio of the two bands did not change with increase in cell size and no other bands were detected, suggesting that both nuclear and chloroplast DNAs were synthesised proportionately prior to the cleavage period.

Effect of growth rate on the macromolecular composition of Prototheca zopfii, a colorless alga which divides by multiple fission

Prototheca zopfii, a eukaryote that divides by multiple fission, was investigated to determine how growth rate controls daughter cell number. The macromolecular composition, cell size, and number of nuclei per cell were determined in cultures during balanced growth in various media. Cellular mass, ribonucleic acid (RNA), deoxyribonucleic acid (DNA), carbohydrate, and nuclear number increased as positive linear functions of growth rate, whereas nuclear ploidy remained constant with a value of 0.098 pg of DNA/nucleus. The ratios of RNA to protein, protein to mass, and carbohydrate to mass were unaffected by growth rate, whereas the ratios of DNA to protein and RNA to DNA could be expressed as curvilinear functions of growth rate, the former negative and the latter positive. The dependency of normalized gene dosage (DNA/protein) on growth rate appeared as a distinguishing feature of multiple fission. Determination of the normalized rates of protein and RNA synthesis revealed that both increase linearly with growth rate. It is concluded that Prototheca zopfii may exist in a number of physiological states which are characterized by a unique size and macromolecular composition and which are dictated by growth rate.

Recombination during blocked chromosome replication in temperature-sensitive strains ofUstilago maydis

tsd1–1 strains ofUstilago maydisare blocked in DNA synthesis under restrictive conditions. On recovery from the block, both intergenic and allelic recombination are enhanced, and aneuploids are generated. UV is synergistic in the induction of recombination. Direct assays of allelic recombination have shown that gene conversion occurs during the block in DNA synthesis.

Mosaic mutants: absence in a eucaryotic organism

Exposure of procaryotic and eucaryotic cells to mutagenic agents generally gives both complete mutants and mosaic mutants. Irradiation of the eucaryotic multicellular alga Ulva mutabilis with ultraviolet light has given exclusively complete mutants.