THE PARTICULATE NATURE OF NONCHROMOSOMAL GENES IN CHLAMYDOMONAS

Organellar inheritance in the green lineage: insights from Ostreococcus tauri

Along the green lineage (Chlorophyta and Streptophyta), mitochondria and chloroplast are mainly uniparentally transmitted and their evolution is thus clonal. The mode of organellar inheritance in their ancestor is less certain. The inability to make clear phylogenetic inference is partly due to a lack of information for deep branching organisms in this lineage. Here, we investigate organellar evolution in the early branching green alga Ostreococcus tauri using population genomics data from the complete mitochondrial and chloroplast genomes. The haplotype structure is consistent with clonal evolution in mitochondria, while we find evidence for recombination in the chloroplast genome. The number of recombination events in the genealogy of the chloroplast suggests that recombination, and thus biparental inheritance, is not rare. Consistent with the evidence of recombination, we find that the ratio of the number of nonsynonymous to the synonymous polymorphisms per site is lower in chloroplast than in the mitochondria genome. We also find evidence for the segregation of two selfish genetic elements in the chloroplast. These results shed light on the role of recombination and the evolutionary history of organellar inheritance in the green lineage.

Size of the Chloroplast Genome in Codium fragile

Chloroplasts isolated from the siphonous green alga Codium fragile yield circular DNA molecules averaging 27.3 micrometers in length and 56 x 10(6) daltons in molecular size. This chloroplast genome is 25 to 30 percent smaller than any reported. The small size of the Codium chloroplast genome may represent a primitive evolutionary condition in green plants.

NUCLEIC ACIDS OF CHLOROPLASTS AND MITOCHONDRIA IN SWISS CHARD

Nucleic acids in young leaves of Swiss chard have been studied by light and electron microscope techniques. Leaf DNA has also been characterized by density gradient centrifugation and shown to contain a minor band of higher guanine plus cytosine (GC) content, presumably attributable to chloroplasts. The chloroplasts were faintly stained by the Feulgen reaction; radioautography demonstrated the incorporation of tritiated thymidine in the cytoplasm and in some nuclei. The Feulgen stainability and most of the radioactivity were removable with DNase. Under the electron microscope, both mitochondria and chloroplasts were found to contain filamentous and particulate components within the matrix areas. The morphology of the filamentous component was dependent on the fixation, being partially clumped after OSO₄ or formalin, but finely filamentous after Kellenberger fixation. The filaments were stainable with uranyl acetate, and were extractable with DNase following formalin fixation under conditions in which nuclear DNA was also extracted. The particulate component, after formalin fixation and uranyl staining, was prominent in chloroplasts from young leaves, but was only sparsely distributed in mitochondria. The stainability was removed with ribonuclease. We have concluded that chloroplasts and mitochondria of Swiss chard possess a filamentous component that contains DNA, probably responsible for both cytoplasmic thymidine incorporation and the minor band in CsCl centrifugation. A particulate ribosome-like component that contains RNA is also present.

Chloroplast DNA polymorphisms in lodgepole and jack pines and their hybrids

Samples taken from throughout the ranges of distribution of lodgepole pine (Pinus contorta Dougl. ex. Loud.) and jack pine (Pinus banksiana Lamb.) were assayed for Sal I and Sst I chloroplast DNA restriction fragment variation. Although the chloroplast genome is often regarded as highly conserved, at least 2 distinct Sal I and 13 distinct Sst I restriction fragment banding patterns occur in these closely related species. None of the chloroplast DNA restriction fragment banding patterns observed in allopatric lodgepole pine was observed in allopatric populations of jack pine, and vice versa, even though the two species share an extensive zone of sympatry, and gene flow between the species has been reported for nuclear genes. However, several atypical Sst I restriction fragment banding patterns occur only in or near the zone of sympatry. Chloroplasts have been reported to be inherited maternally in the great majority of species studied; however, restriction fragment analyses indicated that chloroplasts are inherited paternally in controlled matings between lodgepole pine (female) and jack pine (male).

Uniparental inheritance of mitochondrial genes in yeast: dependence on input bias of mitochondrial DNA and preliminary investigations of the mechanism

In Saccharomyces cerevisiae, previous studies on the inheritance of mitochondrial genes controlling antibiotic resistance have shown that some crosses produce a substantial number of uniparental zygotes, which transmit to their diploid progeny mitochondrial alleles from only one parent. In this paper, we show that uniparental zygotes are formed especially when one parent (majority parent) contributes substantially more mitochondrial DNA molecules to the zygote than does the other (minority) parent. Cellular contents of mitochondrial DNA (mtDNA) are increased in these experiments by treatment with cycloheximide, alpha-factor, or the uvsp5 nuclear mutation. In such a biased cross, some zygotes are uniparental for mitochondrial alleles from the majority parent, and the frequency of such zygotes increases with increasing bias. In two- and three-factor crosses the cap1, ery1, and oli1 loci behave coordinately, rather than independently; minority markers tend to be transmitted or lost as a unit, suggesting that the uniparental mechanism acts on entire mtDNA molecules rather than on individual loci. This rules out the possibility that uniparental inheritance can be explained by the conversion of minority markers to the majority alleles during recombination. Exceptions to the coordinate behavior of different loci can be explained by marker rescue via recombination. Uniparental inheritance is largely independent of the position of buds on the zygote. We conclude that it is due to the failure of minority markers to replicate in some zygotes, possibly involving the rapid enzymatic destruction of such markers. We have considered two general classes of mechanisms: (1) random selection of molecules for replication, as for example by competition for replicating sites on a membrane; and (2) differential marking of mtDNA molecules in the two parents, possibly by modification enzymes, followed by a mechanism that "counts" molecules and replicates only the majority type. These classes of models are distinguished genetically by the fact that the first predicts that the output frequency of a given allele among the progeny of a large number of zygotes will approximately equal the average input frequency of that allele, while the second class predicts that any input bias will be amplified in the output. The data suggest that bias amplification does occur. We hypothesize that maternal inheritance of mitochondrial or chloroplast genes in many organisms may depend upon a biased input of organelle DNA molecules, which usually favors the maternal parent, followed by failure of the minority (paternal) molecules to replicate in many or all zygotes.

Segregation and recombination of non-Mendellan genes in Chlamydomonas

Non-Mendelian genes in Chamydomonas reinhardtii are inherited in a uniparental (UP) fashion. Most zygotes and their progeny receive UP genes only from the mt(+) or maternal parent. However, a few exceptional zygotes are also found in which the mt(-) or paternal UP genome is transmitted. Most of the exceptional zygotes are biparental in that their progeny segregate UP genes transmitted by both parents. As a result, biparental zygotes have been extensively used to study the rules governing UP inheritance. The frequency of biparental zygotes can be greatly increased if the maternal parent is irradiated with ultraviolet light prior to mating. Based principally on studies with ultraviolet-induced biparental zygotes, Sager has argued that a vegetative cell contains two copies of the UP genome and that the progeny of a biparental zygote receive a copy derived from each parent. Results reported in this paper with spontaneous and ultraviolet-induced biparental zygotes do not support the two copy model, but argue for a mulitple copy model with most of the copies normally being transmitted by the maternal parent. A multiple copy model which accounts for both Sager's results and ours is presented.

Establishment and characterization of ethidium bromide resistance in simian virus 40-transformed hamster cells. Effects on mitochondria in vivo

This study describes the isolation and subsequent characterization of four mammalian cell lines resistant to ethidium bromide (EB). Treatment of the simian virus 40- (SV40) transformed hamster cell line F5-1 first led to the establishment of the F2 cell line, which is resistant to 2 microg EB/ml. At this concentration cytochromes c and b are present in almost normal or only slightly diminished amounts, whereas cytochromes a + a(3) show an obvious decrease. The mitochondria of the F2 cell show a normal ultrastructure, not distinct from the parental cell line F5-1, and contain closed circular DNA. The sensitive parental F5-1 cells, however, when exposed to the same dye concentration exhibit the typical EB-induced ultrastructural changes in the mitochondria, and no more component I mitochondrial DNA can be demonstrated. 1 yr after establishment we derived from the F2 cell three more cell lines, resistant against 4, 8, and 16 microg of EB/ml. These cell lines, termed F4, F8, and F16, respectively, also revealed relatively intact-appearing mitochondria, although distinguishable from F5-1 and F2 mitochondria by a more condensed or unorthodox cristae conformation. F4, F8, and F16 cell lines contained closed circular mitochondrial DNA in the same position as that of the parental F5-1 cells, when analyzed in an isopycnic CsCl-EB gradient. A small shoulder at the lower density side of the DNA I peaks was observed. The newly acquired drug resistance of the F cells is hereditarily transmitted to the progeny cells and retained even after a period of growth in EB-free medium.

Molecular basis of maternal inheritance

The mechanism of preferential transmission (i.e., maternal inheritance) of cytoplasmic genes was investigated with chloroplast DNA of Chlamydomonas as a model system. The behavior of nuclear and chloroplast DNAs were compared in the sexual cycle; DNAs from male and female parents were distinguished by labeling with (14)N- or (15)NH(4)Cl and then by making the crosses: (14)N (female) x (15)N (male) and the reciprocal. Chloroplast DNAs from the two parents followed different paths in the zygote, but nuclear DNAs showed no differences. Chloroplast DNA from the female parent persists in the zygote, but undergoes a density shift of 0.003-0.005 g/cm(3) to a lighter buoyant density, whereas that from the male disappears soon after zygote formation. The possibility is discussed that a modification-restriction system may be involved.

A genetic map of non-Mandelian genes in Chlamydomonas

A group of eight non-Mendelian genes have been shown by recombination analysis to be linked into a linear structure or chromosome. Similar genetic maps of gene order and relative distances between genes have been constructed by two methods, one based on additivity of recombination frequencies, the other on frequency of reciprocal recombination with a postulated attachment point. The data indicate that the progeny are diploid for this linkage group, and that the strands are distributed in a precisely oriented manner at mitosis. Evidence is discussed in support of the view that this linkage group is located in chloroplast DNA.

Recombination of nonchromosomal mutations: a 3-point cross in the green alga Chlamydomonas reinhardi

The heterothallic, unicellular green alga Chlamydomonas reinhardi possesses both chromosomal and nonchromosomal systems of inheritance. Mutations belonging to the latter system are capable of recombination. The experiments reported show that double and triple recombinants can be obtained from crosses employing three phenotypically distinct mutations and that certain pairs of mutants recombine more freely than others.

Genetic studies in Eudorina

A formal genetic analysis of the heterothallic, colonial green algaEudorina eleganshas been described. Wild-type strains were found to be sensitive to different drugs when grown on minimal agar containing very low concentrations of these drugs. Mutant strains resistant to high concentrations of drugs have been isolated. These aremsr-500(resistant to 500 μg/ml of DL-methionine-DL-sulfoximine),ery-r-100(resistant to 100 μg/ml of erythromycin) andsr-100(resistant to 100 μg/ml of streptomycin). The wild-type phenotypes sensitive to these drugs have been designated asmss-500,ery-s-100andss-100respectively. Thesr-100also showed cross-resistance to other antibiotics belonging to the streptomycin group.

On genetic analysis, themsr-500andery-s-100were found to be inherited in a Mendelian way. These alleles are not linked to each other or to the mating type locus. The inheritance of mating type was found to be due to a single gene difference.

The inheritance ofss-100/sr-100was found to be non-chromosomal and was characteristically uniparental, always transmitted through the female parent. The evidence for the non-chromosomal gene (NC genes) controllingsr-100/ss-100phenotypes in this organism has been derived from the exceptional zygotes in which the male parent apparently transmits streptomycin resistance to the progeny. Although ultraviolet or gamma-radiation resulted in normal survival curves of the exposed cells, no mutant deficient in any nutritional requirement was isolated.

Biogenesis of chloroplast membranes. I. Plastid dedifferentiation in a dark-grown algal mutant (Chlamydomonas reinhardi)

This paper describes the morphology and photosynthetic activity of a mutant of Chlamydomonas reinhardi (y-1) which is unable to synthesize chlorophyll in the dark. When grown heterotrophically in the light, the mutant is indistinguishable from the wild type Chlamydomonas. When grown in the dark, chlorophyll is diluted through cell division and the photosynthetic activity (oxygen evolution, Hill reaction, and photoreduction of NADP) decays at a rate equal to or faster than that of chlorophyll dilution. However, soluble enzymes associated with the photosynthetic process (alkaline FDPase, NADP-linked G-3-P dehydrogenase, RuDP carboxylase), as well as cytochrome f and ferredoxin, continue to be present in relatively high concentrations. The enzymes involved in the synthesis of the characteristic lipids of the chloroplast (including mono- and digalactoside glycerides, phosphatidyl glycerol, and sulfolipid) are still detectable in dark-grown cells. Such cells accumulate large amounts of starch granules in their plastids. On onset of illumination, dark-grown cells synthesize chlorophyll rapidly, utilizing their starch reserve in the process. At the morphological level, it was observed that during growth in the dark the chloroplast lamellar system is gradually disorganized and drastically decreased in extent, while other subchloroplast components are either unaffected (pyrenoid and its tubular system, matrix) or much less affected (eyespot, ribosomes). It is concluded that the dark-grown mutant possesses a partially differentiated plastid and the enzymic apparatus necessary for the synthesis of the chloroplast membranes (discs). The advantage provided by such a system for the study of the biogenesis of the chloroplast photosynthetic membranes is discussed.