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Tetrahymena is a genus of free-living , examples of unicellular . The genus Tetrahymena is the most widely studied member of its phylum.
It can produce, store and react with different types of hormones. Tetrahymena cells can recognize both related and hostile cells.They can also switch from commensalism to pathogenic modes of survival. They are common in freshwater lakes, ponds, and streams.
Tetrahymena species used as model organisms in biomedical research are T. thermophila and T. pyriformis.
Because Tetrahymena can be grown in a large quantity in the laboratory with ease, it has been a great source for biochemical analysis for years, specifically for Enzyme activities and purification of sub-cellular components. In addition, with the advancement of genetic techniques it has become an excellent model to study the gene function in vivo. The recent sequencing of the macronucleus genome should ensure that Tetrahymena will be continuously used as a model system.
Tetrahymena thermophila exists in seven different sexes () that can reproduce in 21 different combinations, and a single tetrahymena cannot reproduce sexually with itself. Each organism "decides" which sex it will become during mating, through a stochastic process.
Studies on Tetrahymena have contributed to several scientific milestones including:
Typical of ciliates, T. thermophila differentiates its genome into two functionally distinct types of nuclei, each specifically used during the two different stages of the life cycle. The diploid germline micronucleus is transcriptionally silent and only plays a role during sexual life stages. The germline nucleus contains 5 pairs of chromosomes which encode the heritable information passed down from one sexual generation to the next. During sexual conjugation, haploid micronuclear meiotic products from both parental cells fuse, leading to the creation of a new micro- and macronucleus in progeny cells. Sexual conjugation occurs when cells starved for at least 2hrs in a nutrient-depleted media encounter a cell of complementary mating type. After a brief period of co-stimulation (~1hr), starved cells begin to pair at their anterior ends to form a specialized region of membrane called the conjugation junction. It is at this junctional zone that several hundred fusion pores form, allowing for the mutual exchange of protein, RNA and eventually a meiotic product of their micronucleus. This whole process takes about 12 hours at 30 °C, but even longer than this at cooler temperatures. The sequence of events during conjugation is outlined in the accompanying figure.
The larger polyploid macronucleus is transcriptionally active, meaning its genes are actively expressed, and so it controls somatic cell functions during vegetative growth. The polyploid nature of the macronucleus refers to the fact that it contains approximately 200–300 autonomously replicating linear DNA mini-chromosomes. These minichromosomes have their own telomeres and are derived via site-specific fragmentation of the five original micronuclear chromosomes during sexual development. In T. thermophila each of these minichromosomes encodes multiple genes and exists at a copy number of approximately 45-50 within the macronucleus. The exception to this is the minichromosome encoding the rDNA, which is massively upregulated, existing at a copy number of approximately 10,000 within the macronucleus. Because the macronucleus divides amitotically during binary fission, these minichromosomes are un-equally divided between the clonal daughter cells. Through natural or artificial selection, this method of DNA partitioning in the somatic genome can lead to clonal cell lines with different macronuclear phenotypes fixed for a particular trait, in a process called phenotypic assortment. In this way, the polyploid genome can fine-tune its adaptation to environmental conditions through gain of beneficial mutations on any given mini-chromosome whose replication is then selected for, or conversely, loss of a minichromosome which accrues a negative mutation. However, the macronucleus is only propagated from one cell to the next during the asexual, vegetative stage of the life cycle, and so it is never directly inherited by sexual progeny. Only beneficial mutations that occur in the germline micronucleus of T. thermophila are passed down between generations, but these mutations would never be selected for environmentally in the parental cells because they are not expressed.
A 2016 study found that cultured Tetrahymena have the capacity to 'learn' the shape and size of their swimming space. Cells confined in a droplet of a water for a short time were, upon release, found to repeat the circular swimming trajectories 'learned' in the droplet. The diameter and duration of these swimming paths reflected the size of the droplet and time allowed to adapt.
Exposure of T. thermophila to UV light resulted in a greater than 100-fold increase in Rad51 gene expression. Treatment with the DNA alkylating agent methyl methanesulfonate also resulted in substantially elevated Rad 51 protein levels. These findings suggest that ciliates such as T. thermophila utilize a Rad51-dependent recombinational pathway to repair damaged DNA.
The Rad51 recombinase of T. thermophila is a homolog of the Escherichia coli RecA recombinase. In T. thermophila, Rad51 participates in homologous recombination during mitosis, meiosis and in the repair of double-strand breaks. During conjugation, Rad51 is necessary for completion of meiosis. Meiosis in T. thermophila appears to employ a Mus81-dependent pathway that does not use a synaptonemal complex and is considered secondary in most other model . This pathway includes the Mus81 resolvase and the Sgs1 helicase. The Sgs1 helicase appears to promote the non-crossover outcome of meiotic recombinational repair of DNA, a pathway that generates little genetic variation.
These morphological switches are triggered by an abundance of stomatin in the environment, a mixture of metabolic compounds released by competitor species, such as Paramecium, Colpidium, and other Tetrahymena. Specifically, chromatographic analysis has revealed that ferrous iron, hypoxanthine, and uracil are the chemicals in stomatin responsible for triggering the morphological change. Many researchers cite "starvation conditions" as inducing the transformation, as in nature, the compound inducers are in highest concentration after microstomal ciliates have grazed down bacterial populations, and ciliate populations are high. When the chemical inducers are in high concentration, T. vorax cells will transform at higher rates, allowing them to prey on their former trophic competitors.
The exact genetic, and structural mechanisms that underlie T. vorax transformation are unknown. However, some progress has been made in identifying candidate genes. Researchers from the University of Alabama have used cDNA subtraction to remove actively transcribed DNA from microstome and macrostome T. vorax cells, leaving only differentially transcribed cDNA molecules. While nine differentiation-specific genes were found, the most frequently expressed candidate gene was identified as a novel sequence, SUBII-TG.
The sequenced region of SUBII-TG was 912 bp long and consists of three largely identical 105 bp open-reading frames. A northern blot analysis revealed that low levels of transcription are detected in microstome cells, while high levels of transcription occur in macrostome cells. Furthermore, when the researchers limited SUBII-TG expression in the presence of stomatin (using antisense oligonucleotide methods), a 55% reduction in SUBII-TG mRNA correlated with a 51% decrease in transformation, supporting the notion that the gene is at least partially responsible for controlling the transformation in T. vorax. However, very little is known about the SUBII-TG gene. Researchers were only able to sequence a portion of the entire open-reading frame, and other candidate genes have not been investigated thoroughly. mRNA and amino acid sequencing indicate that ubiquitin may play a crucial role in allowing transformation to take place as well. However, no known genes in the ubiquitin family have been identified in T. vorax. Finally, the genetic mechanisms of the "tailed" microstome morph are completely unknown.
When researchers grew a sample of the T. thermophila population in normal growth medium (lacking Cd2+) for one month, the number of MTT1, MTT3, and CNBDP genes decreased to an average of three copies (135C). By seven months in normal growth medium, the T. thermophila cells were found reduced to just the wild type copy number (45C). When researchers returned cells from the same colony to Cd2+ medium, within a week MTT1, MTT3, and CNBDP genes increased to three copies once again (135C). Thus, the authors argue that chromosome amplification is an inducible and reversible mechanism in the Tetrahymena genetic response to metal stress.
Researchers also used gene-knockdown experiments, where the copy number of another metallothionein gene on a different chromosome, MTT5, was dramatically reduced. Within a week, the new strain was found to have developed four novel genes from at least one duplication of MTT1. However, chromosome duplication had not taken place, as indicated by the wild-type ploidy and the normal quantity of other genes on the same chromosomes. Rather, researchers believe that the duplication resulted from homologous recombination events, producing transcriptionally active, upregulated genes that carry repeated MTT1.
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