An Analysis of Nuclear Differentiation in the Selfers of Tetrahymena

Allen, Sally Lyman; Nanney, D. L.

doi:10.1086/282022

Cited by 104 publications

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“…In the population of progeny, amoebae with one parental rDNA type predominated over those with the other from 4 (4) or to plasmid incompatibility in bacteria (16). However, using the equations developed for plasmids (16), we calculate that at least 4 months of growth would be required for about half of the nuclei to become fixed due to such fluctuation, a time that is too long to explain the inheritance observed.…”

Section: Discussionmentioning

confidence: 95%

Inheritance of extrachromosomal rDNA in Physarum polycephalum.

Ferris¹,

Vogt²,

Truitt³

1983

Mol. Cell. Biol.

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In the acellular slime mold Physarum polycephalum, the several hundred genes coding for rRNA are located on linear extrachromosomal DNA molecules of a discrete size, 60 kilobases. Each molecule contains two genes that are arranged in a palindromic fashion and separated by a central spacer region. We investigated how rDNA is inherited after meiosis. Two Physarum amoebal strains, each with an rDNA recognizable by its restriction endonuclease cleavage pattern, were mated, the resulting diploid plasmodium was induced to sporulate, and haploid progeny clones were isolated from the germinated spores. The type of rDNA in each was analyzed by blotting hybridization, with cloned rDNA sequences used as probes. This analysis showed that rDNA was inherited in an all-or-nothing fashion; that is, progeny clones contained one or the other parental rDNA type, but not both. However, the rDNA did not segregate in a simple Mendelian way; one rDNA type was inherited more frequently than the other. The same rDNA type was also in excess in the diploid plasmodium before meiosis, and the relative proportions of the two rDNAs changed after continued plasmodial growth. The proportion of the two rDNA types in the population of progeny clones reflected the proportion in the parent plasmodium before meoisis. The rDNAs in many of the progeny clones contained specific deletions of some of the inverted repeat sequences at the central palindromic symmetry axis. To explain the pattern of inheritance of Physarum rDNA, we postulate that a single copy of rDNA is inserted into each spore or is selectively replicated after meiosis.In some lower eucaryotic organisms, the multiple genes coding for rRNA are located on extrachromosomal DNA molecules. In the best studied of these systems, Tetrahymena species (ciliates) and Physarum polycephalum (an acellular slime mold), the extrachromosomal rDNAs are of a discrete size (about 20 and 60 kilobases [kb], respectively) and have a similar structure. The molecules are linear and are organized as palindromes, with two transcription units arranged in opposite orientations near the ends of the DNA, separated by a spacer region with a rotational symmetry axis in the middle (13, 21). In P. polycephalum rDNA this spacer is 20 kb in size and contains a complex series of directly repeated, inversely repeated, and unique sequences (8). Replication of P. polycephalum rDNA molecules (22) commences at one of four potential origins in the central spacer region and proceeds bidirectionally towards the ends of the linear molecules. In contrast to chromosomal DNA replication, rDNA replication takes place during most of the mitotic cycle, notjust in the S phase, and is unscheduled. That is, some rDNA molecules replicate once, some more than once, and some not at all, with the end result being that the total number of molecules after each mitosis is unchanged. The ends of the linear P. polycephalum rDNA molecules are heterogeneous in size, and near each end are located onenucleotide gaps on one strand at specific nucleotide seq...

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Section: Discussionmentioning

confidence: 95%

Inheritance of extrachromosomal rDNA in Physarum polycephalum.

Ferris¹,

Vogt²,

Truitt³

1983

Mol. Cell. Biol.

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“…In initially heterozygous MACs, the random distribution of allele copies generates, during subsequent MAC divisions, a genetic drift that results in MACs pure for one or the other allele. This phenomenon is known as macronuclear assortment and was discovered and analyzed by Allen and Nanney (1958). Heterozygotes propagated vegetatively for 300-500 fissions have high probability of having completed assortment at any given locus (Doerder et al 1975) and have been dubbed ''terminal assortants'' (Longcor et al 1996).…”

Section: Rationale For Mapping the Mac Genome By Assortmentmentioning

confidence: 99%

Mapping the Germ-Line and Somatic Genomes of a Ciliated Protozoan,Tetrahymena thermophila

Orias¹

1998

Genome Res.

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Ciliates are among the very few eukaryotes in which the powers of molecular biology, conventional genetics, and microbial methodology can be synergistically combined. Because ciliates also are distant relatives of vertebrates, fungi, and plants, the sequencing of a ciliate genome will be of import to our understanding of eukaryotic biology. Tetrahymena thermophila is the only ciliate in which a systematic genetic mapping of DNA polymorphisms has begun. Tetrahymena has many biological features that make it a specially or uniquely useful experimental system for fundamental research in cell and molecular biology and for biotechnological applications. A key factor in the usefulness of Tetrahymena is the speed, facility, and versatility with which it can be cultivated under a wide range of nutrient conditions, temperature, and scale. This article describes the progress made in genetically and physically mapping the genomes of T. thermophila: the micronuclear (germ-line) genome, which is not transcriptionally expressed, and the macronuclear (somatic) fragmented genome, which is actively expressed and determines the cell's phenotype. Introduction to the Ciliated ProtozoaTetrahymena is a member of the Ciliated Protozoa, a monophyletic group of unicellular eukaryotes, whose biology was reviewed recently in Hausmann and Bradbury (1996). The ciliates diverged earlier than plants and fungi in the evolutionary line leading to the vertebrates (see Pace 1997). Yet the ciliates possess a typical eukaryote life cycle, with conventional meiosis and biparental fertilization through the union of haploid gametes. While retaining unicellularity, ciliates exhibit evolutionary advances reminiscent of features of multicellular eukaryotes. For example, ciliates possess vegetative growth restricted to the diploid phase of their life cycle; extensive compounding of cellular structure that has led to the evolution of macroscopically observable unicells; elaborate structural differentiations accompanied by complex morphogenetic mechanisms; internal fertilization through direct exchange of gamete nuclei; and nuclear dimorphism, that is, the possession of differentiated germ-line and somatic nuclei, described below.The possession of two related but functionally differentiated genomes within one cell is a diagnostic ciliate feature. The germ-line genome is carried in the diploid micronucleus (MIC), whereas the somatic genome is contained in the polyploid macronucleus (MAC). MIC and MAC differentiate from mitotic descendants of the fertilization nucleus formed during conjugation, the sexual stage of the life cycle (Fig. 1). MAC differentiation is accompanied by chromosome fragmentation and thousands of other DNA rearrangements (Prescott 1994;Coyne et al. 1996). These rearrangements, which are developmentally programmed and site-specific, reconfigure the genome extensively. Nuclear differentiation results in a MIC that is not transcriptionally expressed and an actively expressed MAC that determines the cell's phenotype. The nuclear dualism of the...

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“…This situation was clarified by the observation that exconjugant heterozygotes are sensitive to dglc immediately after conjugation, at which time the polyploid (45N) macronucleus responsible for phenotypic expression still contains both resistant and sensitive alleles. If, however, exconjugant clones are allowed to undergo several fissions in nonselective medium (the routine procedure employed in genetic analyses to generate sufficient numbers of cells for replication to several media), the random distribution of macronuclear gene copies (1,17) produces some cells with enough copies of the resistant allele to allow them to grow when subsequently exposed to dglc. Although this random segregation allows some cells to become homogeneous for any allele in the macronucleus, they all remain gIcA heterozygotes in the diploid micronuclear germline.…”

Section: Resultsmentioning

confidence: 99%

Selection and characterization of a glucokinase-deficient mutant of Tetrahymena thermophila.

Roberts¹,

Lavine²,

Morse³

1982

Mol. Cell. Biol.

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We have isolated a mutant of Tetrahymena thermophila that is resistant to inhibition of growth by the glucose analog 2-deoxyglucose. The mutant exhibits a deficiency in a cytoplasmic glucokinase. This enzymatic defect and the attendant inability to convert 2-deoxyglucose to toxic phosphorylated derivatives is apparently the sole basis for the mutant phenotype since transport of glucose and 2-deoxyglucose is unimpaired; there is no elevation of glucose-6-phosphatase activity, which could decrease the level of toxic 2-deoxyglucose metabolites. Genetic analyses have shown that the mutant allele is recessive and inherited as a single Mendelian mutation. The glucokinase-deficient strain described here is useful for the selection of other mutants in this organism and for the investigation of various cellular processes initiated or modulated by glucose and its analogs. We have exploited the molecular defect in this strain to investigate the initial steps in the cyclic AMP-mediated repression of galactokinase gene expression which is caused by glucose.During the course of our initial studies on the regulation of galactose metabolism in Tetrahymena (20), we found that high concentrations of the sugar analogs 2-deoxygalactose and 2-deoxyglucose (dglc) inhibited growth of wild-type cells in galactose-supplemented minimal medium. Since the elucidation of the molecular mechanisms responsible for the expression of the genes controlling galactose metabolism would be aided by the study of appropriate mutants, we isolated a series of strains resistant to the inhibitory effects of these compounds.Several 2-deoxygalactose-resistant mutants were found to be deficient in galactokinase (ATP:D-galactose-1-phosphotransferase, EC 2.7.1.6), presumably due to lesions in the galactokinase structural gene (21). This represented one of the mutant classes to be expected on the basis of the metabolism necessary for the toxicity of this analog (26) and the results of similar selections in bacterial (2) and mammalian cells (27).We then examined dglc-resistant strains in the hope of identifying mutants with specific defects in the metabolism of glucose. In analogy to the selection of the galactokinase-deficient mutants, a major mutant class was expected to be deficient in glucose-phosphorylating ability; such strains would aid in investigating the repression of the galactokinase gene by glucose.In this paper we describe the isolation and the biochemical and genetic characterization of a mutant resistant to dglc by virtue of a deficiency in the soluble glucokinase (ATP:D-glucose-6-phosphotransferase, EC 2.7.1.2) required for conversion of dglc to toxic metabolites. In a forthcoming communication

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An Analysis of Nuclear Differentiation in the Selfers of Tetrahymena

Cited by 104 publications

References 0 publications

Inheritance of extrachromosomal rDNA in Physarum polycephalum.

Inheritance of extrachromosomal rDNA in Physarum polycephalum.

Mapping the Germ-Line and Somatic Genomes of a Ciliated Protozoan,Tetrahymena thermophila

Selection and characterization of a glucokinase-deficient mutant of Tetrahymena thermophila.

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