Editing Tetrahymena

{{Short description|Genus of single-celled organisms}}
{{Split portions|date=October 2021|Tetrahymena thermophila|Tetrahymena pyriformis}}
{{Cleanup rewrite|date=October 2021}}
{{Automatic taxobox
| image             = Tetrahymena thermophila.png
| image_caption     = ''Tetrahymena thermophila''
| taxon             = Tetrahymena
| subdivision_ranks = [[Species]]
| subdivision       = 
}}

'''''Tetrahymena''''' is a genus of free-living [[ciliate]]s, examples of [[Unicellular organism|unicellular]] [[eukaryote]]s.<ref name=":1">{{Cite web|title=Tetrahymena - Encyclopedia of Life|url=https://eol.org/pages/61455|access-date=2021-10-16|website=eol.org}}</ref> The genus Tetrahymena is the most widely studied member of its [[phylum]].<ref name=":0">{{Cite book|url=https://books.google.com/books?id=67mUTAgUBFQC&dq=Tetrahymena+thermophila+closest+relative&pg=PA284|title=Tetrahymena Thermophila|date=2012-10-22|publisher=Academic Press|isbn=978-0-12-385968-6|language=en}}</ref>{{Rp|page=59}} It can produce, store and react with different types of hormones. ''Tetrahymena'' cells can recognize both related and hostile cells.<ref>{{cite journal |last1=Csaba |first1=György |title=Lectins and Tetrahymena – A review |journal=Acta Microbiologica et Immunologica Hungarica |date=September 2016 |volume=63 |issue=3 |pages=279–291 |doi=10.1556/030.63.2016.001 |pmid=27539329 |url=http://real.mtak.hu/41228/1/030.63.2016.001.pdf}}</ref>

They can also switch from [[commensalism|commensalistic]] to [[pathogenic]] modes of survival.{{Citation needed|date=October 2021}} They are common in freshwater lakes, ponds, and streams.<ref name=":0" />{{Rp|page=277}}

''Tetrahymena'' species used as [[model organisms]] in biomedical research are ''[[T. thermophila]]'' and ''[[T. pyriformis]]''.<ref>{{cite book | last = Elliott | first = Alfred M. | name-list-style = vanc | title = Biology of Tetrahymena | publisher = Dowen, Hutchinson and Ross Inc. | year = 1973 | isbn = 978-0-87933-013-2 }}</ref>{{Page needed|date=October 2021}}

== ''T. thermophila'':  a model organism in experimental biology ==
[[Image:Tetrachimena Beta Tubulin.png|thumb|200px|[[Tubulin|β-tubulin]] in ''Tetrahymena''.]]

As a ciliated [[protozoan]], ''Tetrahymena thermophila'' exhibits [[nuclear dimorphism]]: two types of cell [[Cell nucleus|nuclei]].  They have a bigger, [[Somatic cell|non-germline]] [[macronucleus]] and a small, [[germline]] [[micronucleus]] in each cell at the same time and these two carry out different functions with distinct cytological and biological properties. This unique versatility allows scientists to use ''Tetrahymena'' to identify several key factors regarding [[gene expression]] and genome integrity.  In addition, ''Tetrahymena'' possess hundreds of [[cilia]] and has complicated [[microtubule]] structures, making it an optimal model to illustrate the diversity and functions of microtubule arrays.

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|enzymatic]] activities and purification of [[Cell (biology)#Subcellular components|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 ([[mating type]]s) 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.<ref name="PLOS2013">{{cite journal | vauthors = Cervantes MD, Hamilton EP, Xiong J, Lawson MJ, Yuan D, Hadjithomas M, Miao W, Orias E | display-authors = 6 | title = Selecting one of several mating types through gene segment joining and deletion in Tetrahymena thermophila | journal = PLOS Biology | volume = 11 | issue = 3 | pages = e1001518 | year = 2013 | pmid = 23555191 | pmc = 3608545 | doi = 10.1371/journal.pbio.1001518 | doi-access = free }}</ref><ref>{{cite journal |last1=Quirk |first1=Trevor |title=How a microbe chooses among seven sexes |journal=Nature |date=27 March 2013 |pages=nature.2013.12684 |doi=10.1038/nature.2013.12684 |s2cid=179307103 }}</ref>

Studies on ''Tetrahymena'' have contributed to several scientific milestones including:
# First cell which showed synchronized division, which led to the first insights into the existence of mechanisms which control the [[cell cycle]].<ref name="whitepaper">{{cite web | first = Eduardo | last = Orias | name-list-style = vanc | date = 10 February 2002 | url = http://www.lifesci.ucsb.edu/~genome/Tetrahymena/SeqInitiative/WhitePaper.htm | title = Sequencing the Tetrahymena thermophila Genome White Paper | work = National Human Genome Research Institute }}</ref>
# Identification and purification of the first [[cytoskeleton]] based [[motor protein]] such as ''[[dynein]]''.<ref name="whitepaper" />
# Aid in the discovery of ''[[lysosomes]]'' and ''[[peroxisomes]]''.<ref name="whitepaper" />
# Early molecular identification of somatic genome rearrangement.<ref name="whitepaper" />
# Discovery of the molecular structure of ''[[telomeres]]'', ''[[telomerase]]'' enzyme, the templating role of telomerase RNA and their roles in cellular senescence and chromosome healing (for which a Nobel Prize was won).<ref name="whitepaper" />
# Nobel Prize–winning co-discovery (1989, in Chemistry) of catalytic [[RNA]] (''[[ribozymes|ribozyme]]'').<ref name="whitepaper" /><ref>{{cite journal | vauthors = Kruger K, Grabowski PJ, Zaug AJ, Sands J, Gottschling DE, Cech TR | title = Self-splicing RNA: autoexcision and autocyclization of the ribosomal RNA intervening sequence of Tetrahymena | journal = Cell | volume = 31 | issue = 1 | pages = 147–57 | date = November 1982 | pmid = 6297745 | doi = 10.1016/0092-8674(82)90414-7 | s2cid = 14787080 }}</ref>
# Discovery of the function of [[histone]] [[acetylation]].<ref name="whitepaper" />
# Demonstration of the roles of [[posttranslational modification]] such as acetylation and glycylation on [[tubulins]] and discovery of the enzymes responsible for some of these modifications (glutamylation)
# Crystal structure of 40S ribosome in complex with its initiation factor eIF1
# First demonstration that two of the "universal" [[stop codon]]s, UAA and UAG, code for the amino acid [[glutamine]] in some eukaryotes, leaving UGA as the only termination codon in these organisms.<ref>{{cite journal | vauthors = Horowitz S, Gorovsky MA | title = An unusual genetic code in nuclear genes of Tetrahymena | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 82 | issue = 8 | pages = 2452–5 | date = April 1985 | pmid = 3921962 | pmc = 397576 | doi = 10.1073/pnas.82.8.2452 | bibcode = 1985PNAS...82.2452H | doi-access = free }}</ref>

[[File:Tetrahymena-research.png|350px|left|thumb|Main fields of biomedical research where ''Tetrahymena'' cells are used as models]]
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==Life cycle==
{{More citations needed|date=October 2021}}[[File:Tetrahymena conjugation.jpg|thumb|400px|'''''Tetrahymena'' conjugation.''' When nutrients are scarce, two individuals (A) pair with each other and begin sexual reproduction (conjugation).  (B) The diploid micronucleus in each individual undergoes meiosis to form four haploid nuclei, and three of these are degraded.  (C) The remaining haploid nucleus divides mitotically to form two pronuclei in each cell.  (D) One of the two pronuclei in each cell is exchanged with the mating partner, and fusion leads to the formation of the diploid [[zygote|zygotic]] nucleus. (E) The zygotic nucleus divides twice mitotically to form four nuclei. (F) Two nuclei become micronuclei, and the other two differentiate to become macronuclei; the original parental macronucleus is degraded. (G) Cell division occurs and the nuclei are distributed to the daughter cells so that each progeny receives one micronucleus and one macronucleus.]]

The life cycle of ''T. thermophila'' consists of an alternation between asexual and sexual stages.  In nutrient rich media during vegetative growth cells reproduce asexually by [[binary fission]].  This type of cell division occurs by a sequence of morphogenetic events that results in the development of duplicate sets of cell structures, one for each daughter cell.  Only during starvation conditions will cells commit to [[sexual conjugation]], pairing and fusing with a cell of opposite mating type.  Tetrahymena has seven mating types; each of which can mate with any of the other six without preference, but not its own.

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.
[[File:Mating Tetrahymena.jpg|thumbnail|Two ''Tetrahymena'' cells of complementary mating types pair to exchange nuclei during sexual conjugation.]]
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&nbsp;°C, but even longer than this at cooler temperatures.  The sequence of events during conjugation is outlined in the accompanying figure.<ref>{{cite journal | last1 = Elliott | first1 = AM | last2 = Hayes | first2 = RE | year = 1953 | title = Mating Types in ''Tetrahymena''| journal = Biological Bulletin | volume = 105 | issue = 2 | pages = 269–284 | doi=10.2307/1538642| jstor = 1538642 | url = https://www.biodiversitylibrary.org/part/7815 }}</ref>

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.<ref name="pmid8078435">{{cite journal | vauthors = Prescott DM | title = The DNA of ciliated protozoa | journal = Microbiological Reviews | volume = 58 | issue = 2 | pages = 233–67 | date = June 1994 | pmid = 8078435 | pmc = 372963 | doi = 10.1128/MMBR.58.2.233-267.1994 }}</ref>

== Behavior ==
Free swimming cells of ''Tetrahymena'' are attracted to certain chemicals by [[chemokinesis]]. The major chemo-attractants are peptides and/or proteins.<ref>{{cite journal | vauthors = Leick V, Hellung-Larsen P | title = Chemosensory behaviour of Tetrahymena | journal = BioEssays | volume = 14 | issue = 1 | pages = 61–6 | date = January 1992 | pmid = 1546982 | doi = 10.1002/bies.950140113 }}</ref>

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.<ref>{{cite journal | vauthors = Kunita I, Yamaguchi T, Tero A, Akiyama M, Kuroda S, Nakagaki T | title = A ciliate memorizes the geometry of a swimming arena | journal = Journal of the Royal Society, Interface | volume = 13 | issue = 118 | pages = 20160155 | date = May 2016 | pmid = 27226383 | pmc = 4892268 | doi = 10.1098/rsif.2016.0155 }}</ref>

==DNA repair==

It is common among protists that the sexual cycle is inducible by stressful conditions such as starvation.<ref>Bernstein H, Bernstein C, Michod RE. Sex in microbial pathogens. Infect Genet Evol. 2018 Jan;57:8-25. doi: 10.1016/j.meegid.2017.10.024. Epub 2017 Oct 27. PMID 29111273</ref>  Such conditions often cause DNA damage.  A central feature of meiosis is homologous recombination between non-sister chromosomes.  In ''T. thermophila'' this process of meiotic recombination may be beneficial for repairing DNA damages caused by starvation.

Exposure of ''T. thermophila'' to UV light resulted in a greater than 100-fold increase in ''[[RAD51|Rad51]]'' gene expression.<ref name="pmid9628914">{{cite journal | vauthors = Campbell C, Romero DP | title = Identification and characterization of the RAD51 gene from the ciliate Tetrahymena thermophila | journal = Nucleic Acids Research | volume = 26 | issue = 13 | pages = 3165–72 | date = July 1998 | pmid = 9628914 | pmc = 147671 | doi = 10.1093/nar/26.13.3165 }}</ref>  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|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.<ref>{{cite journal | vauthors = Marsh TC, Cole ES, Stuart KR, Campbell C, Romero DP | title = RAD51 is required for propagation of the germinal nucleus in Tetrahymena thermophila | journal = Genetics | volume = 154 | issue = 4 | pages = 1587–96 | date = April 2000 | doi = 10.1093/genetics/154.4.1587 | pmid = 10747055 | pmc = 1461009 }}</ref>  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 [[eukaryote]]s.<ref>{{cite journal | vauthors = Chi J, Mahé F, Loidl J, Logsdon J, Dunthorn M | title = Meiosis gene inventory of four ciliates reveals the prevalence of a synaptonemal complex-independent crossover pathway | journal = Molecular Biology and Evolution | volume = 31 | issue = 3 | pages = 660–72 | date = March 2014 | pmid = 24336924 | doi = 10.1093/molbev/mst258 | doi-access = free }}</ref>  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,<ref name="pmid23935123">{{cite journal | vauthors = Lukaszewicz A, Howard-Till RA, Loidl J | title = Mus81 nuclease and Sgs1 helicase are essential for meiotic recombination in a protist lacking a synaptonemal complex | journal = Nucleic Acids Research | volume = 41 | issue = 20 | pages = 9296–309 | date = November 2013 | pmid = 23935123 | pmc = 3814389 | doi = 10.1093/nar/gkt703 }}</ref> a pathway that generates little genetic variation.

== Phenotypic and genotypic plasticity ==
Many species of ''Tetrahymena'' are known to display unique response mechanisms to stress and various environmental pressures. The unique genomic architecture of the ciliates (presence of a MIC, high ploidy, large number of chromosomes, etc.) allows for differential gene expression, as well as increased genomic flexibility. The following is a non-exhaustive list of examples of phenotypic and genotypic plasticity in the Tetrahymena genus.

=== Inducible trophic polymorphisms ===
''T. vorax'' is known for its inducible trophic polymorphisms, an ecologically offensive tactic that allows it to change its feeding strategy and diet by altering its morphology.<ref>{{cite journal |last1=Banerji |first1=Aabir |last2=Morin |first2=Peter J. |title=Trait-mediated apparent competition in an intraguild predator-prey system |journal=Oikos |date=May 2014 |volume=123 |issue=5 |pages=567–574 |doi=10.1111/j.1600-0706.2013.00937.x |bibcode=2014Oikos.123..567B }}</ref> Normally, ''T. vorax'' is a bacterivorous microstome around 60 μm in length. However, it has the ability to switch into a carnivorous macrostome around 200 μm in length that can feed on larger competitors. If ''T. vorax'' cells are too nutrient starved to undertake transformation, they have also been recorded as transforming into a third "tailed"-microstome morph, thought to be a defense mechanism in response to cannibalistic pressure. While ''T. vorax'' is the most well studied ''Tetrahymena'' that exhibits inducible trophic polymorphisms, many lesser known species are able to undertake transformation as well, including ''T. paulina'' and ''T. paravorax''.<ref>{{cite book |doi=10.1016/s0074-7696(01)12006-1 |chapter=Phenotype Switching in Polymorphic Tetrahymena: A Single-Cell Jekyll and Hyde |title=A Survey of Cell Biology |series=International Review of Cytology |year=2002 |last1=Ryals |first1=Phillip E. |last2=Smith-Somerville |first2=Harriett E. |last3=Buhse |first3=Howard E. |volume=212 |pages=209–238 |pmid=11804037 |isbn=978-0-12-364616-3 }}</ref> However, only ''T. vorax'' has been recorded as having both a macrostome and tailed-microstome form.

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.<ref>{{Cite journal |last1=Smith-Somerville |first1=Harriett E. |last2=Hardman |first2=John K. |last3=Timkovich |first3=Russell |last4=Ray |first4=William J. |last5=Rose |first5=Karen E. |last6=Ryals |first6=Phillip E. |last7=Gibbons |first7=Sandra H. |last8=Buhse |first8=Howard E. |date=2000-06-20 |title=A complex of iron and nucleic acid catabolites is a signal that triggers differentiation in a freshwater protozoan |journal=Proceedings of the National Academy of Sciences |volume=97 |issue=13 |pages=7325–7330 |doi=10.1073/pnas.97.13.7325 |pmc=16544 |pmid=10860998 |bibcode=2000PNAS...97.7325S |doi-access=free }}</ref> 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.<ref>{{Cite journal |last1=Green |first1=M. M. |last2=LeBoeuf |first2=R. D. |last3=Churchill |first3=P. F. |date=2000 |title=Biological and molecular characterization of cellular differentiation in Tetrahymena vorax: a potential biocontrol protozoan |journal=Journal of Basic Microbiology |volume=40 |issue=5–6 |pages=351–361 |doi=10.1002/1521-4028(200012)40:5/6<351::aid-jobm351>3.0.co;2-q |pmid=11199495 |s2cid=21981461 }}</ref> 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''.<ref>{{cite thesis |id={{ProQuest|304234889}} |last1=Martin |first1=Teresa Dianne |date=1996 |title=Analysis of ubiquitin and differential gene expression during differentiation in Tetrahymena vorax }}</ref> Finally, the genetic mechanisms of the "tailed" microstome morph are completely unknown.

=== Metal resistance, gene and genome amplification ===
Other related species exhibit their own unique responses to various stressors. In ''T. thermophila'', chromosome amplification and gene expansion are inducible responses to common organometallic pollutants such as cadmium, copper, and lead.<ref>{{Cite journal |last1=de Francisco |first1=Patricia |last2=Martín-González |first2=Ana |last3=Turkewitz |first3=Aaron P. |last4=Gutiérrez |first4=Juan Carlos |date=July 2018 |title=Genome plasticity in response to stress in Tetrahymena thermophila : selective and reversible chromosome amplification and paralogous expansion of metallothionein genes |journal=Environmental Microbiology |volume=20 |issue=7 |pages=2410–2421 |doi=10.1111/1462-2920.14251 |pmc=6117198 |pmid=29687579 |bibcode=2018EnvMi..20.2410D }}</ref> Strains of ''T. thermophila'' that were exposed to large quantities of Cd<sup>2+</sup> over time were found to have a 5-fold increase of ''MTT1'', and ''MTT3'' (metallothionein genes that code for Cadmium and Lead binding proteins) as well as ''CNBDP'', an unrelated gene that lies just upstream of ''MTT1'' on the same chromosome. The fact that a non-metallothionein gene on the same locus as ''MTT1'' and ''MTT3'' increased copy number indicates that the entire chromosome had been amplified, as opposed to just specific genes. ''Tetrahymena'' species are 45-ploid for their macronucleus, meaning that the wild type of ''T. thermophila'' normally contains 45 copies of each chromosome. While the actual number of unique chromosomes are unknown, the number is thought to be around 187 in the MAC, and 5 in the MIC.<ref>{{cite journal |last1=Yao |first1=Meng-Chao |last2=Chao |first2=Ju-Lan |last3=Cheng |first3=Chao-Yin |title=Programmed Genome Rearrangements in Tetrahymena |journal=Microbiology Spectrum |date=21 November 2014 |volume=2 |issue=6 |pages=2.6.31 |doi=10.1128/microbiolspec.MDNA3-0012-2014 |pmid=26104448 }}</ref> Thus, the Ca<sup>2+</sup> adapted strain contained 225 copies of the specific chromosome in question. This resulted in a nearly 28-fold increase in detected expression levels of ''MTT1'', and slightly less in ''MTT3''.

When researchers grew a sample of the ''T. thermophila'' population in normal growth medium (lacking Cd<sup>2+</sup>) 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 Cd<sup>2+</sup> 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''.

=== Enhanced motility and dispersal ===
''T. thermophila'' also undergoes phenotypic changes when faced with limited resource availability. Cells are capable of changing their shape and size, along with behavioral swimming strategies in response to starvation.<ref>{{cite journal |last1=Junker |first1=Anthony D. |last2=Jacob |first2=Staffan |last3=Philippe |first3=Hervé |last4=Legrand |first4=Delphine |last5=Pearson |first5=Chad G. |title=Plastic cell morphology changes during dispersal |journal=iScience |date=August 2021 |volume=24 |issue=8 |pages=102915 |doi=10.1016/j.isci.2021.102915 |pmc=8367785 |pmid=34430806 |bibcode=2021iSci...24j2915J }}</ref> The more motile cells that change in response to starvation are known as dispersers, or disperser cells. While rates and levels of phenotypic change differ between strains, disperser cells form in nearly all strains of ''T. thermophila'' when faced with starvation. Dispersers, and non-dispersing cells both become dramatically thinner and smaller, increasing the basal body and cilia density, allowing them to swim between two and three times faster than normal cells.<ref>{{cite journal |last1=Fjerdingstad |first1=Else J |last2=Schtickzelle |first2=Nicolas |last3=Manhes |first3=Pauline |last4=Gutierrez |first4=Arnaud |last5=Clobert |first5=Jean |title=Evolution of dispersal and life history strategies – Tetrahymena ciliates |journal=BMC Evolutionary Biology |date=2007 |volume=7 |issue=1 |pages=133 |doi=10.1186/1471-2148-7-133 |pmc=1997130 |pmid=17683620 |doi-access=free |bibcode=2007BMCEE...7..133F }}</ref> Some strains of ''T. thermophila'' have also been found to develop a single, non-beating, enlarged cilia that assists the cell in steering or directing movement. While the behavior has been shown to correlate with faster dispersal and form as a reversible trait in ''Tetrahymena'' cells, little is known about the genetic or cellular mechanisms that allow for its development. Furthermore, other studies show that when genetically variable populations of ''T. thermophila'' were starved, dispersal cells actually increased in cell length, despite still becoming thinner.<ref>{{cite journal |last1=Jacob |first1=Staffan |last2=Laurent |first2=Estelle |last3=Morel-Journel |first3=Thibaut |last4=Schtickzelle |first4=Nicolas |title=Fragmentation and the context-dependence of dispersal syndromes: matrix harshness modifies resident-disperser phenotypic differences in microcosms |journal=Oikos |date=February 2020 |volume=129 |issue=2 |pages=158–169 |doi=10.1111/oik.06857 |bibcode=2020Oikos.129..158J |s2cid=208584362 }}</ref> More research is needed to determine the genetic mechanisms that underlie disperser formation.

== Species in genus ==
Species in this genus include.<ref name=":1" />
*''[[Tetrahymena americanis]]''
*''[[Tetrahymena asiatica]]''
*''[[Tetrahymena australis]]''
*''[[Tetrahymena bergeri]]''
*''[[Tetrahymena borealis]]''
*''[[Tetrahymena canadensis]]''
*''[[Tetrahymena capricornis]]''
*''[[Tetrahymena caudata]]''
*''[[Tetrahymena chironomi]]''
*''[[Tetrahymena corlissi]]''
*''[[Tetrahymena cosmopolitanis]]''
*''[[Tetrahymena dimorpha]]''
*''[[Tetrahymena edaphoni]]''
*''[[Tetrahymena elliotti]]''
*''[[Tetrahymena empidokyrea]]''
*''[[Tetrahymena farahensis]]''
*''[[Tetrahymena farleyi]]''
*''[[Tetrahymena furgasoni]]''
*''[[Tetrahymena glochidiophila]]''
*''[[Tetrahymena hegewischi]]''
*''[[Tetrahymena hyperangularis]]''
*''[[Tetrahymena leucophrys]]''
*''[[Tetrahymena limacis]]''
*''[[Tetrahymena lwoffi]]''
*''[[Tetrahymena malaccensis]]''
*''[[Tetrahymena mimbres]]''
*''[[Tetrahymena mobilis]]''
*''[[Tetrahymena nanneyi]]''
*''[[Tetrahymena nipissingi]]''
*''[[Tetrahymena paravorax]]''
*''[[Tetrahymena patula]]''
*''[[Tetrahymena pigmentosa]]''
*''[[Tetrahymena pyriformis]]''
*''[[Tetrahymena rostrata]]''
*''[[Tetrahymena rotunda]]''
*''[[Tetrahymena setifera]]''
*''[[Tetrahymena setigera]]''
*''[[Tetrahymena setosa]]''
*''[[Tetrahymena shanghaiensis]]''
*''[[Tetrahymena sialidos]]''
*''[[Tetrahymena silvana]]''
*''[[Tetrahymena skappus]]''
*''[[Tetrahymena sonneborni]]''
*''[[Tetrahymena stegomyiae]]''
*''[[Tetrahymena thermophila]]''
*''[[Tetrahymena tropicalis]]''
*''[[Tetrahymena vorax]]''

==In education==
[[Cornell University]] offers a [[National Institutes of Health]] (NIH) funded program through the Science Education Partnership Award (SEPA) Program called [[Advancing Secondary Science Education thru Tetrahymena]] (ASSET).<ref>{{cite news |title=Cornell develops educational toolkit for testing e-cigarettes |url=https://www.vet.cornell.edu/news/20200109/cornell-develops-educational-toolkit-testing-e-cigarettes |work=Cornell University College of Veterinary Medicine |date=9 January 2020 }}</ref> The group develops stand-alone labs or lessons using Tetrahymena as training modules that teachers can use in classes.

==References==
{{Reflist}}

== Further reading ==
{{Refbegin}}
* {{cite book | series = Methods in Cell Biology | volume = 62 | title = Tetrahymena thermophila | editor-first1 = David J. | editor-last1 = Asai | editor-first2 = James D. | editor-last2 = Forney | name-list-style = vanc | date = 2000 | publisher = Academic Press | isbn = 978-0-12-544164-3 | url-access = registration | url = https://archive.org/details/tetrahymenatherm62acad }}
* {{cite journal | vauthors = Collins K, Gorovsky MA | title = Tetrahymena thermophila | journal = Current Biology | volume = 15 | issue = 9 | pages = R317-8 | date = May 2005 | pmid = 15886083 | doi = 10.1016/j.cub.2005.04.039 | s2cid = 11738486 | doi-access = free | bibcode = 2005CBio...15.R317C }}
* {{cite journal | vauthors = Eisen JA, Coyne RS, Wu M, Wu D, Thiagarajan M, Wortman JR, Badger JH, Ren Q, Amedeo P, Jones KM, Tallon LJ, Delcher AL, Salzberg SL, Silva JC, Haas BJ, Majoros WH, Farzad M, Carlton JM, Smith RK, Garg J, Pearlman RE, Karrer KM, Sun L, Manning G, Elde NC, Turkewitz AP, Asai DJ, Wilkes DE, Wang Y, Cai H, Collins K, Stewart BA, Lee SR, Wilamowska K, Weinberg Z, Ruzzo WL, Wloga D, Gaertig J, Frankel J, Tsao CC, Gorovsky MA, Keeling PJ, Waller RF, Patron NJ, Cherry JM, Stover NA, Krieger CJ, del Toro C, Ryder HF, Williamson SC, Barbeau RA, Hamilton EP, Orias E | display-authors = 6 | title = Macronuclear genome sequence of the ciliate Tetrahymena thermophila, a model eukaryote | journal = PLOS Biology | volume = 4 | issue = 9 | pages = e286 | date = September 2006 | pmid = 16933976 | pmc = 1557398 | doi = 10.1371/journal.pbio.0040286 | doi-access = free }}
{{Refend}}

== External links ==
*[https://tetrahymena.vet.cornell.edu/ ''Tetrahymena'' Stock Center at Cornell University]
*[https://tetrahymenaasset.vet.cornell.edu/ ASSET: Advancing Secondary Science Education thru ''Tetrahymena'']
*[http://www.ciliate.org ''Tetrahymena'' Genome Database]
*[http://www.life.illinois.edu/nanney/ Biogeography and Biodiversity of ''Tetrahymena'']
*[http://www.tigr.org/tdb/e2k1/ttg/index.shtml ''Tetrahymena thermophila'' Genome Project] at [[The Institute for Genomic Research]]
*[http://biology.plosjournals.org/perlserv/?request=get-document&doi=10.1371/journal.pbio.0040304 ''Tetrahymena thermophila'' Genome Sequence Synopsis]
*[http://biology.plosjournals.org/perlserv/?request=get-document&doi=10.1371/journal.pbio.0040286 ''Tetrahymena thermophila'' genome paper]
*[http://www.jove.com/search?q=tetrahymena ''Tetrahymena'' experiments] on [[Journal of Visualized Experiments]] (JoVE) website
*Microbial Digital Specimen Archives: [https://archive.today/20030316203538/http://mtlab.biol.tsukuba.ac.jp/WWW/PDB/Images/Ciliophora/Tetrahymena/ ''Tetrahymena'' image gallery]
*[http://video.scientificamerican.com/services/player/bcpid942857632001?bckey=AQ~~,AAAAAFNl7zk~,OmXvgxJOvrEe6iL4yPGYhfN9p4d-ZfPq&bctid=4515147199001 All Creatures Great and Small: Elizabeth Blackburn]

{{Model Organisms}}
{{Alveolata}}
{{Taxonbar|from=Q1198419}}
{{Authority control}}

[[Category:Ciliate genera]]
[[Category:Oligohymenophorea]]
[[Category:Model organisms]]