A nuclear mutant of Chlamydomonas that exhibits increased sensitivity to UV irradiation, reduced recombination of nuclear genes, and altered transmission of chloroplast genes

Meiotic progeny of Chlamydomonas reinhardtii normally receive chloroplast genomes only from the mt+ parent. However, exceptional zygotes, which transmit the chloroplast genomes of both parents or, more rarely, only those of the mt- parent, arise at a low frequency. Mutations at the mt(+)-linked mat-3 locus were found previously to elevate the transmission of chloroplast genomes from the mt- parent, resulting in a much higher than normal frequency of exceptional zygotes. In this paper we demonstrate that an ultraviolet-sensitive nuclear mutation mapping at the uvsE1 locus, which is unlinked to mating type, also promotes chloroplast genome transmission from the mt- parent. This mutant, which was previously shown to reduce recombination of nuclear genes in meiosis, acts synergistically with the mat-3-3 mutation to produce an extremely high frequency of exceptional zygotes. Through the use of restriction fragment length polymorphisms existing in the chloroplast genomes of C. reinhardtii and the interfertile strain C. smithii, we show that chloroplast DNA fragments from the mt- parent normally begin to disappear shortly after zygote formation. However, this process appears to be blocked totally in the absence of wild-type uvsE1 and mat-3 gene products. Our findings are consistent with the hypothesis that both gene products contribute to the mechanism responsible for uniparental inheritance of the chloroplast genome from the mt+ parent.     

Detection of chloroplast DNA by using fluorescent monoclonal anti-bromodeoxyuridine antibody and analysis of its fate during zygote formation in Chlamydomonas reinhardtii

Summary A monoclonal anti-bromodeoxyuridine antibody conjugated to fluorescein was used to detect the chloroplast nucleoids after specific incorporation of bromodeoxyuridine (BUdR) into the chloroplast DNA of Chlamydomonas reinhardtii. The incorporation of BUdR was enhanced by simultaneous treatment with fluorodeoxyuridine (FUdR). The method was applied to analyze the fate of chloroplast DNA in zygotes resulting from mating between BUdR-treated gametes (mt ⁺ or mt ^-) and untreated gametes of opposite mating-type. In crosses between wild-type strains, the nucleoids of mt ⁺ origin remained in the large majority of zygotes whereas those of mt ^- origin most often disappeared within the first hours following copulation. In crosses of the type mat-3 mt ⁺xwild-type mt ^- (the mat-3 mutation permits a high transmission of chloroplast genes from the mt ^- parent), the nucleoids of mt ^- origin were generally not eliminated which indicates that the mat-3 mutation prevents the selective destruction of paternal chloroplast DNA in the zygote.     

Transmission of chloroplast genes in crosses between Chlamydomonas reinhardtii diploids: Correlation with chloroplast nucleoid behavior in young zygotes

Summary In crosses between diploid strains of Chlamydomonas reinhardtii, the mode of chloroplast gene transmission was studied and correlated with the behavior of DAPI-stained chloroplast nucleoids in young tetraploid zygotes. Phenotypically plus (female) diploids with mating-type genotypes of mt ⁺/mt ⁺ or mt ⁺/0 (hemizygote) and phenotypically minus (male) diploids with genotypes of mt ⁺/mt ^– or mt ^–/mt ^– or 0/mt ^– were obtained by either crosses or polyethylene glycol induced somatic fusions between haploid strains. When crosses were made between mt ⁺/mt ⁺ or mt ⁺/0 and mt ^–/mt ^– or 0/mt ^–, the transmission of chloroplast genes occurred mostly from the female parent, indicating a typical maternal inheritance. In these cases the chloroplast nucleoids of one of the two parents disappeared within 2.5 h after mating. In contrast, when female diploids were crossed to mt ⁺/mt ^– male diploids, the chloroplast genes from both parents were transmitted to the progeny at equal frequency. The chloroplast nucleoids derived from the two parents were conserved for at least 2.5 h after mating. It is concluded that the chloroplast genes derived from the heterozygous male (mt ⁺/mt ^–) are protected from the degradation that takes place soon after zygote formation, by virtue of the presence of mt ⁺ gene although this allele is recessive with respect to the phenotypic expression of sexuality in diploids.     

About biased and non-biased transmission of chloroplast genes following artificial fusion of gametes in Chlamydomonas reinhardtii

Summary Artificial polyethyleneglycol induced fusions of gametes of opposite mating-types carrying chloroplast markers give rise to fusion products transmitting either both markers or the marker from the mt ⁺ or from the mt ^– parent exclusively. The frequencies of the three classes of products were approximately equal in our experiments (Matagne 1981). Similar experiments performed by Matsuda et al. (1983) gave different results, namely a preferential transmission of chloroplast gene from the mt ⁺ parent, very similar to that observed in vegetative zygotes obtained in sexual crosses. Results described here show that in experimental conditions used by Matsuda et al., sexual copulation does occur, leading to formation of zygotes which were misinterpreted as artificial fusion products and gave a biased transmission of chloroplast genes.     

Chloroplast Gene Transmission in Chlamydomonas somatic fusion products

Summary In sexual crosses between the two mating types of Chlamydomonas reinhardtii the progeny normally only receive chloroplast genes from the mt ⁺ parent. This pattern of inheritance is not seen following the rare mating events leading to the formation of vegetative diploids, or when the cells are fused using polyethylene glycol (PEG). Vegetative diploids usually carry chloroplast genes from both parents, and most of those that are uniparental carry the maternal parent's genes. In a PEG fusion the products are evenly divided among those uniparental for each of the parental chloroplast markers and those that are biparental. However, fusion in a sexual cross is usually preceded by the flagellaragglutination of the two mating types. When cells undergoing PEG fusion have been pretreated with isolated gametic flagella the transmission pattern in changed. If two mt ⁺ cells were fused, and one of them had been pretreated with isolated mt ^– flagella, then the majority of the progeny carried chloroplast genes only from the treated parent. This suggests that the signal for the induction of the events leading to uniparental maternal inheritance is the agglutination between mt ⁺ and mt ^– flagella that precedes cell fusion, and that the subsequent fate of chloroplast genes is controlled primarily by the mt ⁺ parent.     

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

Summary 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.     

Chloroplast gene inheritance studied by somatic fusion in Chlamydomonas reinhardtii

Summary 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.     

A pair of invertedly repeated genes in Chlamydomonas reinhardtii encodes a zygote-specific protein whose expression is UV-sensitive

Uniparental inheritance of the chloroplast genome has been observed in a wide variety of green plants. In Chlamydomonas this phenomenon, which can be selectively inhibited by UV irradiation of mt ⁺ gametes, has been shown cytologically to be due to the preferential degradation of mt ⁻-derived chloroplast nucleoids in young zygotes. The zygote-specific pair of zys1 genes, zys1A and zys1B, is expressed earliest among five genes isolated from a “10-min” zygote library. We report here that the ZYS1 protein, which is encoded by the invertedly duplicated zys1 gene, accumulates in zygotes and is localized in nuclei. In addition, when mt ⁺ gametes (but not mt ⁻ gametes) are UV-irradiated before mating, only very limited accumulation of ZYS1 protein can be detected in the resulting zygotes. Received: 29 July 1998 / 30 April 1999     

The fate of mitochondrial DNAs of mt + and mt − origin in gametes and zygotes of Chlamydomonas

Summary In order to study the mechanism responsible for the uniparental transmission of the mitochondrial genome in crosses between Chlamydomonas reinhardtii and C. smithii, we have analyzed the fate of mitochondrial DNA during gametogenesis, zygospore differentiation and sporulation by hybridization experiments. Both mt ⁺ and mt ^– gametes contain the same amount of mitochondrial DNA and the two parental genomes persist for several days in the zygotes. The DNA of mt ⁺ origin is slowly eliminated during the period of zygote maturation. Light is required for total elimination of mt ⁺ mitochondrial DNA in the zygospores. Using appropriate restriction enzymes, we have been unable to detect methylation of the mitochondrial DNA during gametogenesis or zygospore formation. The possibility that the mt ⁺ mitochondria themselves are specifically eliminated in the course of zygote maturation is discussed.     

Chloroplast gene transmission in Chlamydomonas reinhardtii. A model for its control by the mating-type locus

Summary In sexual crosses of Chlamydomonas reinhardtii, genes residing in the chloroplast (cp) are most often transmitted from the mating-type plus (mt ⁺) parent only. Galloway and Goodenough (1985) proposed a model in which the mt locus (linkage group VI) is a complex region containing several genes involved in the control of both gametic differentiation and cp inheritance. The mt ⁺ locus contains: the sfu locus necessary for sexual fusion between gametes; the upp locus (uniparental plus) which controls cp gene inheritance and also perhaps zygote maturation; and the sad locus which functions in sexual adhesion. The mt ^– locus also contains a sad locus as well as a regulatory element (mid) necessary for the minus dominance in mt ⁺/mt ^– diploid gametes. This model has been extended to include new genetic functions linked to the mt ⁺ or mt ^– locus. In this new system, there is a group of genes (maps for mating-type plus structure), present in both plus and minus strains, controlling some mt ⁺ phenotypical traits as well as the synthesis of an activator of the cp DNA nuclease; two genes contained in the mt ^– locus — one (cge for cp genomic elimination) coding for a nuclease (in an inactive form) located in the chloroplast and another (mid) coding for a repressor of maps; one gene (upp) contained in the mt ⁺ locus, coding for a substance preventing the synthesis or the activity of the nuclease and perhaps also acting on the maps product. The model can be applied to explain the complex dominance/ recessivity relationships observed between the mt ⁺ and mt ^– alleles in heterozygous diploid gametes, the basis for mitotic vs meiotic zygote formation and the differences in transmission observed for these two physiologically distinct zygote classes. Application of the model for prediction of mutant phenotypes and the design of future experiments is also described.     

Transmission and recombination of chloroplast genes in asexual crosses of Chlamydomonas reinhardii

Summary Chlamydomonas reinhardii diploids homozygous for the plus mating-type (mt ⁺) allele were constructed via polyethylene glycol (PEG)-induced cell fusion to investigate the transmission of chloroplast genes.We used two methods to determine whether the fusion products (PEG diploids) had inherited chloroplast markers uniparentally or biparentally. One method (multiple clone analysis) was found to markedly improve the detection of biparental transmission. With this method the frequency of biparental PEG-induced diploids was comparable to that seen in sexual diploids. Multiple clone analysis also demonstrated that fusion products which showed biparental inheritance were an extremely heterogeneous group.In some crosses, pre-treatment of one parent with isolated flagella from cells of the opposite mating type was used. This pre-treatment has been reported to promote chloroplast gene transmission from the mt ⁺ parent in diploids produced by PEG fusion (Adams 1982). In our crosses flagellar pretreatment had no significant effect on chloroplast gene transmission or recombination frequences. A significant bias was found for the date a cross was performed, but neither parent preferentially transmitted its alleles in all crosses. This indicates the necessity of comparing data from treated and control crosses done on the same day. We conclude from our data that separate hypotheses are not required to explain biparental inheritance in sexual versus PEG-induced diploids. The absence of biased parental allelic ratios with PEG-induced biparental diploid clones underscores their usefulness for the study of biparental gene transmission.Supported by National Institutes of Health Fellowship F32 GM085 34     

Cytological demonstration of chloroplast DNA behavior during gametogenesis and zygote formation in Chlamydomonas reinhardtii

Summary We used the flourescent dye DAPI to visualize nucleoids of chloroplast DNA and follow their behavior through sexual reproduction by counting nucleoids in fixed cells at various stages. Nucleoid number varied greatly among cells at each stage. The mean number of nucleoids per cell was similar in mt ⁺ and mt ^– vegetative cells, and declined similarly during gametogenesis. Longer periods of nitrogen starvation reduced the mean nucleoid number further. Mean nucleoid number declined again in mating pairs, and continued to drop in zygotes up to the latest stage that can be examined (24-h zygotes). The oldest zygotes had means of about 2 to 3 nucleoids in different experiments, significantly fewer than in the mt ⁺ gametes (usually 4 to 5). The quantitative data on nucleoid number, mating efficiency, and germination efficiency allowed us to show that the decrease in nucleoid number is not limited to gametes that do not mate, or to zygotes that do not germinate. These data are consistant with earlier biochemical studies showing loss of chloroplast DNA during gametogenesis in both mating types, and with the degradation of paternal chloroplast DNA detected biochemically and (in non-quantitative studies) by DAPI staining. There may also be some fusion of nucleoids, although if it occurs it is not complete by 24 h of zygote maturation.     

Loss of mt +-derived zygotic chloropiast DNA is associated with a lethal allele in Chlamydomonas monoica

Summary The Chlamydomonas monoica mutant allele mtl-1, is associated with the formation of nonviable zygospores following self-mating of the mutant strain. Furthermore, mtl-1 heterozygote populations show a 50% reduction in germination frequency and no transmission of a chloroplast antibiotic resistance marker carried by the mtl-1 parent. To determine whether the effects on zygospore viability and chloroplast gene transmission resulted from the direct involvement of the mtl-1 locus in the control of mt ⁺-directed uniparental inheritance of chloroplast DNA (cpDNA), we have used the DNA-specific fluorochrome DAPI to follow the fate of cpDNA during the maturation of zygotes. Throughout the first few hours after the initial fusion of gametes, the young zygotes show DAPI-fluorescent nucleoids distributed symmetrically around the region of nuclear fusion, and presumably located within both of the parental chloroplasts. Wild-type and mtl-1 mutant zygotes show similar early staining patterns. As the zygotes age, the staining patterns become asymmetric for the wildtype population, with all of the visible cytoplasmic nucleoids restricted to one side of the zygote. In contrast, mtl-1 homozygotes appear to lose cytoplasmic nucleoids from both sides of the zygote simultaneously and within 24 h are apparently devoid of cpDNA. By introducing a mutation which arrests cell fusion (and prevents plastid fusion), we can show that (1) the asymmetric nucleoid distribution in wildtype zygotes results from the loss of nucleoids from one gamete in each mating event, and (2) the additional loss of cpDNA in mtl-1 homozygotes does not require contact between parental plastids (thus the nuclease responsible for cpDNA degradation is not sequestered within the chloroplast of one gamete). We propose that the mtl-1 mutant strain is defective for a process which normally protects cpDNA of mt ⁺ origin.     

Evidence for persistence of chloroplast markers in the heteroplasmic state in Chlamydomonas reinhardtii

Summary In the green alga Chlamydomonas reinhardtii, reciprocal crosses between strains carrying non-allelic chloroplast mutations to streptomycin dependence (sd-u) produce streptomycin sensitive (sd-u ⁺) recombinant progeny. Transfer of these sd-u ⁺progeny to streptomycin-containing medium results in a much higher frequency of recovery of streptomycin dependent isolates than expected by mutation. Failure to recover the more commonly encountered class of streptomycin resistant mutants also suggests that mutation is not responsible for appearance of the new dependent isolates. Backcrosses of these new sd-u isolates to strains carrying the original sd-u mutations demonstrate their allelism with the sd-u mutation contributed by the mt ⁺parent. Earlier work by Schimmer and Arnold (1969, 1970a-d) indicated that newly isolated sensitive revertants of the streptomycin dependent mutant sd-u-3-18 also yielded high frequencies of sd-u cells but these were never analyzed genetically. We have now obtained new sd-u. isolates from streptomycin sensitive revertants of sd-u-318 and shown them all to be allelic with the original sd-u3-18 mutation. Thus hidden sd-u alleles can coexist with sd-u ⁺alleles in heteroplasmic cells. These heteroplasmic cells are streptomycin sensitive in phenotype and may arise in crosses or from new mutation.     

Mutations in four chloroplast loci of Chlamydomonas reinhadtii affecting the photosystem I reaction centers

Summary In this work seven chloroplast mutations conferring a deficiency in photosystem I reaction centers have been mapped at four chloroplast loci in Chlamydomonas reinhardtii. Recombination frequencies were estimated from diploid progeny of vegetative zygotes. These four loci were scattered throughout the chloroplast genome. The three mutations at locus I were found to be tightly linked to a mutation in the rbcL gene coding for the large subunit of ribulose-1,5-bisphosphate carboxylase (Dron et al. 1983). As the psaA2 gene coding for one apoprotein of the chlorophyll-complex CPI, identified by its homology with the corresponding maize gene (Fish et al. 1985), has been found close to the rbcL gene (Dron et al. 1982), the psaA2 gene could be at locus 1.     

Transmission,recombination and conversion of mitochondrial markers in relation to the mobility of a group I intron in Chlamydomonas

Mitochondrial DNA transmission has been analyzed in diploids produced from sexual crosses or artificial fusions between Chlamydomonas strains which differ by several genetic markers: a group I intron (Cs cob.1 or intron), three restriction sites (Nh, Nc and H markers) located 0.5–5 kb from the insertion site of the intron, and a MUD2 point mutation (27 bp from the insertion site) conferring resistance to myxothiazol. Recombination between mitochondrial markers is a general property of all crosses and fusions analyzed. In crosses between two intron-containing (⁺) strains or two intron-less (^–) strains, the transmission is preferentially paternal (mt ^-), with a preoponderance depending on the nature of the parental genomes. In crosses between (⁺) and (^–) strains, the conversion of intron-less molecules into intron⁺ is frequent when the (⁺) parent is maternal (mt ⁺) and nearly absolute when the (⁺) parent is paternal (mt ^-). In 94% of cases, the conversion is accompanied by the co-conversion of the MUD2 marker. In both crosses and artificial fusions, the conversion of (^–) into (⁺) also influences the transmission of the more distant Nh, Nc and H markers. It is hypothesized that the more frequent transmission of the genome containing the intron results from the elimination of (^–) molecules, as a result of a double-strand cut which is induced by an endonuclease encoded by the intron.     

Optional elements in the chloroplast DNAs of Chlamydomonas eugametos and C. moewusii: unidirectional gene conversion and co-conversion of adjacent markers in high-viability crossses

Unlike most polymorphic markers in the Chlamydomonas eugametos and Chlamydomonas moewusii chloroplast DNAs (cpDNAs), the C. moewusii 6- and 21-kb extra sequences and the C. eugametos-specific CeLSU ⋅ 5 intron are transmitted to all of the few viable progeny in reciprocal crosses between the two green algae. To determine whether this unidirectional transmission pattern is due to gene conversion or to selection for F1 hybrid survival, we followed the inheritance of the parental alleles at the loci featuring these three deletions/additions and at several other polymorphic cpDNA loci in zygospore clones derived from high-viability crosses. The great majority of the zygospore clones examined inherited exclusively the long alleles from the mt ^– parent at the loci containing the three optional cpDNA elements, but as expected, they preferentially inherited the markers from the mt ⁺ parent at most other loci. Our results therefore indicate that all three optional cpDNA sequences propagate themselves very efficiently by gene conversion in crosses between strains differing by the presence of these elements. The co-conversion tracts associated with these sequences are longer (>3 kb) than those previously reported for mobile elements spreading by gene conversion. Our results also revealed that less efficient gene conversion events occurred at two other cpDNA loci. Received: 12 February / 14 May 1996     

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

Summary 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.     

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

Summary The density, molecular weight, and cellular repetition of DNA molecules associated with the -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.     

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

Summary 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.