Disappearance of the heteroplasmic state for chloroplast markers in zygospores of Chlamydomonas reinhardtii

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

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.

Dispersive labelling of Chlamydomonas chloroplast DNA in (15)N- (14)N density transfer experiments

(15)N-(14)N density transfer experiments with synchronized vegetative cultures of Chlamydomonas reinhardtii revealed a dispersive labelling of chloroplast DNA (cpDNA) while the labelling of nuclear DNA was consistent with semiconservative replication. The dispersive labelling of cpDNA was progressive and extensive as after less than two net doublings of this DNA in (14)N-medium no significant amount of fully heavy, (15)N-strands could be detected in denatured cpDNA preparations; the average size of DNA in these preparations corresponded to 6% of the intact chloroplast genome or about 12 kbp. The density shifts of native cpDNA samples were found to be consistent with the net amounts of cpDNA synthesized. This observation indicates that essentially all (15)N atoms incorporated prior to the transfer were conserved and that metabolic turnover of cpDNA was probably absent. Our results are best explained by the exchange of homologous single-stranded segments between cpDNA molecules to form heteroduplex regions and by each DNA molecule undergoing several rounds of heteroduplex formation.

Perturbation of chloroplast gene transmission in diploid and triploid zygotes of Chlamydomonas reinhardi by 5-fluorodeoxyuridine

Haploid cells or diploid cells 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 crossed with haploid cells of opposite mating-type and carrying another chloroplast marker. Before mating, one or both of the parental strains were grown for 8 days on agar containing 1 mM 5-fluorodeoxyuridine (FUdR), which selectively reduces the amount of chloroplast DNA in Chlamydomonas. In all cases, the chloroplast allele of the treated parent was less frequently transmitted to the meiotic progeny of the zygote than in the corresponding control cross. The effect of FUdR was more pronounced on haploid cells than on diploid cells which initially contained a two-fold higher amount of chloroplast DNA.The results are discussed in relation to current models for uniparental inheritance of non-Mendelian genes.

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.

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.

Mating type linked mutations which disrupt the uniparental transmission of chloroplast genes in chlamydomonas

In Chlamydomonas reinhardtii, chloroplast genomes are normally transmitted by the mating type plus (mt+) parent and mitochondrial genomes by the mating type minus (mt-) parent. In this paper we describe three new nuclear mutations, designated mat-3-1 to -3, which are tightly linked to the mt+ allele and permit high transmission of chloroplast genomes from the mt- parent, but have no effect on transmission of mitochondrial genomes. We also show that mat-1, reported by others to be a nuclear mutation linked to mt- which promotes transmission of chloroplast genomes by the mt- parent, is probably a vegetative diploid since it contains both mt+ and mt- alleles. Vegetative diploids behave as if they are mt- with respect to mating, but possess a level of chloroplast gene transmission intermediate between that of haploid mt- and mt+ stocks.

Physical mapping of plastid DNA variation among eleven Nicotiana species

Plastid DNA of seven American and four Australian species of the genus Nicotiana was examined by restriction endonuclease analysis using the enzymes Sal I, Bgl I, Pst I, Kpn I, Xho I, Pvu II and Eco RI. These endonucleases collectively distinguish more than 120 sites on N. tabacum plastid DNA. The DNAs of all ten species exhibited restriction patterns distinguishable from those of N. tabacum for at least one of the enzymes used. All distinctive sites were physically mapped taking advantage of the restriction cleavage site map available for plastid DNA from Nicotiana tabacum (Seyer et al. 1981). This map was extended for the restriction endonucleases Pst I and Kpn I. In spite of variation in detail, the overall fragment order was found to be the same for plastid DNA from the eleven Nicotiana species. Most of the DNA changes resulted from small insertions/deletions and, possibly, inversions. They are located within seven regions scattered along the plastid chromosome. The divergence pattern of the Nicotiana plastid chromosomes was strikingly similar to that found in the genus Oenothera subsection Euoenothera (Gordon et al. 1982). The possible role of replication as a factor in the evolution of divergence patterns is discussed. The restriction patterns of plastid DNA from species within a continent resembled each other with one exception in each instance. The American species N. repanda showed patterns similar to those of most Australian species, and those of the Australian species N. debneyi resembled those of most American species.

The fate of chloroplast DNA during cell fusion, zygote maturation and zygote germination in Chlamydomonas reinhardi as revealed by DAPI staining

Chlamydomonas reinhardi, a haploid isogamous green alga, presents a classic case of uniparental inheritance of chloroplast genes. Since the molecular basis of this phenomenon is poorly understood, an examination of the cytology of the C. reinhardi plastid DNA was made in gametes, newly formed zygotes, maturing zygotes, and at zygote germination. The single plastid per cell of Chlamydomonas contains a small number of DNA aggregates ('nucleoids') which can be seen after staining with DNA-binding fluorochromes. In zygotes formed by pre-stained gametes, the fluorescing nucleoids disappear from the plastid of mating type minus (male) gamete plastids but not from the plastid of mating type plus (female) gamete plastids about 1 h after zygote formation. Subsequently, nucleoids aggregate slowly to a final average of two or three in the single plastid of the mature zygote. Quantitative microspectrofluorimetry indicates that gametes of both mating types have equal amounts of plastid DNA, and that zoospores arising from zygotes have 3.5 X as much as gametes. Assuming degradation of male plastid DNA, there must be a very major synthesis of plastid DNA between zygote formation and zoospore release when zygotes produce the typical 8-16 zoospores. That synthesis appears to occur at germination, where there is a massive increase in plastid DNA and nucleoid number beginning just prior to meiosis. The results support the theory that uniparental inheritance results from degradation of plastid DNA entering the zygote via the male gamete and suggest further studies, using mutants and altered conditions, which might explain how male plastid DNA sometimes survives.

The mechanism of the mixed inheritance of chloroplast genes in Pelargonium : Evidence from gene frequency distributions among the progeny of crosses

The distributions are given of gene frequencies among embryos after G X W and W X G plastid crosses within and between eight Pelargonium cultivars and some of their inbred or hybrid derivatives.Two distinct segregation patterns are recognized. Homozygous type I female parents (Pr1Pr1) have a high frequency of progeny with only maternal alleles, are intermediate for biparental and low for paternal offspring. Heterozygous type II female plants (Pr1Pr2) have an equally high frequency of maternal and paternal offspring and a generally low biparental frequency. These correspond to L-shaped and U-shaped gene frequency distributions respectively in which the only modes are at 0 per cent (maternal embryos) and 100 per cent (paternal embryos), with no mode corresponding to the population mean and no sign of a Gaussian distribution.The extremely variable plastid gene frequencies are strongly influenced by the maternal nuclear genotype and by the plastid genotype in which the wild-type allele is always more successful than the mutant in strict comparisons.The relative frequencies of maternal and paternal zygotes, and the mean gene frequency among all the zygotes in a cross, are explicable in terms of the input frequencies of genes from the two parents, their degree of mixing, and by some form of selective replication of plastids. This selection is controlled by nuclear and plastid genotypes which may act in the same direction, to increase the frequency of either the maternal or the paternal alleles, or in opposition. But selection alone is inadequate to explain the shapes of the gene frequency distributions. Instead, a model is proposed in which the segregation or replication of plastids appears to have a strong random element, which results in random drift of gene frequencies within a heteroplasmic zygote or embryo.