Aging and long-term temporal regulation in ciliated protozoa. A critical review

Nanney, D. L.

doi:10.1016/0047-6374(74)90008-6

Cited by 69 publications

(26 citation statements)

References 44 publications

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“…caudatum, P. multimicronucleatum, P. jenningsi, E. patella, E. crassus, E. ninuta, E. woodruffi, Oxytricha bifaria, Stylonychia mytilus, Tokophrya lemnarum, T. rostrata, species of the T. pyriformis complex (general reviews by Sonneborn, 1957;Siegel, 1967;1. R. Preer, 1968;Bleyman, 1971;Grell, 1973;Nanney, 1974Nanney, , 1977Nanney, , 1980Nanney, , 1986Nanney and McCoy, 1976;Smith-Sonneborn, 1981;Takagi, 1988).…”

Section: The Life Cycle Of Ciliatesmentioning

confidence: 97%

“…Amicronucleate strains of T. pyriformis are not special, however, in their capacity for indefinite somatic multiplication. Also, micronucleate strains of T. thermophila established directly from nature or generated from crosses in the laboratory show an indefinite maturity period following a sexual immaturity lasting about 50 fissions (for review, see Nanney, 1974). Nevertheless, notwithstanding an ability to continue growth and mating indefinitely, the capacity of these micronucleate strains to participate successfully in conjugation comes to an end: the ability to produce viable progeny in crosses drops gradually to zero over a few years of growth in mass culture.…”

Section: The Life Cycle Of Ciliatesmentioning

confidence: 98%

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Sex in Ciliates

Dini

Nyberg

1993

Advances in Microbial Ecology

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Section: The Life Cycle Of Ciliatesmentioning

confidence: 97%

Section: The Life Cycle Of Ciliatesmentioning

confidence: 98%

Sex in Ciliates

Dini

Nyberg

1993

Advances in Microbial Ecology

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“…However, low and asynchronous mating of these TWI1 somatic knockout cells prevented us from determining whether the small RNAs were absent or just greatly reduced in the absence of TWI1. Although the cause of this low and asynchronous mating was not clear, we suspected that senescence (Nanney 1974) or cellular damage occurred during the long period (∼40 d = 300∼400 generations) of culture and treatment with high concentration of the drug (up to 50 mg/mL of paromomycin sulfate) required for phenotypic assortment to produce somatic knockouts. To avoid these problems, we used a different strategy to make TWI1 knockout cells.…”

Section: Expression Of Small Rna Is Greatly Reduced But Not Eliminatementioning

confidence: 99%

Conjugation-specific small RNAs in Tetrahymena have predicted properties of scan (scn) RNAs involved in genome rearrangement

Mochizuki¹,

Gorovsky²

2004

Genes Dev.

156

171

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We proposed a scan-RNA model for genome rearrangement based on finding small RNAs that hybridized preferentially to micronuclear-specific sequences and on the properties of Twi1p, a PPD protein required for both sequence elimination and small RNA accumulation in Tetrahymena. Here we show that Twi1p interacts with the small RNAs in both the old and the developing macronucleus, and is required for their stability. We show that the specificity of the small RNAs for micronuclearlimited sequences increases during conjugation. These results indicate that the small RNAs observed in conjugating cells have the properties predicted for scan RNAs. Like most ciliated protozoans, Tetrahymena thermophila (referred to as Tetrahymena below) have two structurally and functionally different nuclei in a single cell (see Karrer 2000). The diploid, germ-line micronucleus and the polyploid, somatic macronucleus are derived from the same zygotic nucleus formed by fertilization of two micronucleus-derived, haploid, meiotic nuclei during the sexual process of conjugation. Concomitant with formation of a new macronucleus during conjugation, the old macronucleus is destroyed. Most, if not all, transcription required for vegetative cell growth occurs in the macronucleus.Tetrahymena undergoes extensive programmed genome rearrangement during conjugation (see Yao et al. 2002) resulting in elimination of ∼15% of the genome. About 6000 internal eliminated sequences (IESs), varying from 0.5 to >20 kb in length, are eliminated, and flanking, macronucleus-destined sequences are ligated during macronuclear development in Tetrahymena. The precise ends of IESs can occur reproducibly at a specific site or at a limited number of alternative sites. Because no obvious consensus sequence had been found in and around IESs, it was not clear how IESs were precisely recognized until recently. We and others showed that an RNAi-related mechanism was involved in the IES elimination in Tetrahymena (Mochizuki et al. 2002;Yao et al. 2003). A PPD (PAZ-Piwi Domain) protein, Twi1p, was required for IES elimination, and for accumulation of the siRNAlike small RNAs homologous to micronuclear-specific (largely IES) sequences. Twi1p was detected only during conjugation and accumulated first in the cytoplasm, then in parental (old) macronuclei and then relocalized to developing (new) macronuclei.Based on the results above, on the presence of micronuclear transcripts from both strands of the IESs in conjugating cells (Chalker and Yao 2001), and on the demonstration that sequences in the old macronucleus could epigenetically affect IES elimination (Chalker and Yao 1996), we proposed a model to explain how IESs, lacking any consensus sequences, are recognized during macronuclear development (Mochizuki et al. 2002;Mochizuki and Gorovsky 2004). First, the micronuclear genome is transcribed bidirectionally to make double-stranded (ds) RNAs that are processed to small RNAs by a Dicer-related RNase. We named these hypothetical small RNAs scan (scn) RNAs. We proposed that the scnRN...

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“…While a cell reproduces asexually, germline DNA is not transcribed, and therefore mutations in that genome will have no phenotypic consequences. Indeed, cells that have lost their germline genomes are capable of normal asexual reproduction (Nanney 1974;Allen et al 1984). Natural selection will "see" new germline mutations only after conjugation when the cell generates a new somatic genome from its germline genome.…”

mentioning

confidence: 99%

Accumulation of Spontaneous Mutations in the CiliateTetrahymena thermophila

2013

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Knowledge of the rate and fitness effects of mutations is essential for understanding the process of evolution. Mutations are inherently difficult to study because they are rare and are frequently eliminated by natural selection. In the ciliate Tetrahymena thermophila, mutations can accumulate in the germline genome without being exposed to selection. We have conducted a mutation accumulation (MA) experiment in this species. Assuming that all mutations are deleterious and have the same effect, we estimate that the deleterious mutation rate per haploid germline genome per generation is U = 0.0047 (95% credible interval: 0.0015, 0.0125), and that germline mutations decrease fitness by s = 11% when expressed in a homozygous state (95% CI: 4.4%, 27%). We also estimate that deleterious mutations are partially recessive on average (h = 0.26; 95% CI: -0.022, 0.62) and that the rate of lethal mutations is ,10% of the deleterious mutation rate. Comparisons between the observed evolutionary responses in the germline and somatic genomes and the results from individual-based simulations of MA suggest that the two genomes have similar mutational parameters. These are the first estimates of the deleterious mutation rate and fitness effects from the eukaryotic supergroup Chromalveolata and are within the range of those of other eukaryotes. MUTATIONS are the ultimate source of variation responsible for evolutionary change. Thus, knowledge of the rate and fitness consequences of spontaneous mutations is essential to understanding evolution (Charlesworth 1996). These parameters have been estimated in a handful of species and have provided many important insights into the evolutionary process (reviewed in Lynch et al. 1999;Halligan and Keightley 2009;Lynch 2010). For example, the observation that the deleterious mutation rate is less than one per genome per generation in several species suggests that the mutational deterministic hypothesis is insufficient to explain the widespread maintenance of sexual reproduction (Kondrashov 1988; Halligan and Keightley 2009). However, among eukaryotes, these mutational parameters have been estimated only in Opisthokonta (animals, fungi, and relatives) and Archaeplastida (red and green algae, land plants, and relatives), leaving the majority of eukaryotic diversity unexamined.Mutation accumulation (MA) is the best experimental tool with which to study mutation rates and fitness effects. Typically, MA experiments consist of allowing spontaneous mutations to arise and fix in parallel, replicate populations over many generations. Small populations are used to reduce the effectiveness of natural selection in determining the fate of new mutations. The quality of the estimates of mutational parameters derived from MA experiments is constrained by the biology of the organism used. Organisms with long-generation times experience a slow rate of MA [e.g., one experiment on Arabidopsis thaliana (Shaw et al. 2000), which has a generation time of 10 weeks, lasted only 17 generations], leading to imp...

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Aging and long-term temporal regulation in ciliated protozoa. A critical review

Cited by 69 publications

References 44 publications

Sex in Ciliates

Sex in Ciliates

Conjugation-specific small RNAs in Tetrahymena have predicted properties of scan (scn) RNAs involved in genome rearrangement

Accumulation of Spontaneous Mutations in the CiliateTetrahymena thermophila

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