''Tetrahymena thermophila'' is a species of

Ciliophora The ciliates are a group of alveolates characterized by the presence of hair-like organelles called cilia, which are identical in structure to eukaryotic flagella, but are in general shorter and present in much larger numbers, with a different ...

in the family Tetrahymenidae. It is a free living protozoa and occurs in fresh water. There is little information on the ecology and natural history of this species, but it is the most widely known and widely studied species in the genus ''

Tetrahymena ''Tetrahymena'', a unicellular eukaryote, is a genus of free-living ciliates. 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 recog ...

''. The species has been used as a model organism for molecular and

cellular biology Cell biology (also cellular biology or cytology) is a branch of biology that studies the structure, function, and behavior of cells. All living organisms are made of cells. A cell is the basic unit of life that is responsible for the living and ...

. It has also helped in the discovery of new

gene In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...

s as well as help to understand the mechanisms of certain genes functions. Studies on this species have contributed to major discoveries in biology. For example, the MAT locus found in this species has provided a foundation for the evolution of

mating system A mating system is a way in which a group is structured in relation to sexual behaviour. The precise meaning depends upon the context. With respect to animals, the term describes which males and females mate under which circumstances. Recognised ...

s. The species was first considered to be ''Tetrahymena pyriformis''. '' T. malaccensis'' is the closest relative to''T. thermophila''.

Characteristics

It is about 50 μm long. One famous trait this species is known for is that has 7 different mating types, unlike most eukaryotic organisms, who usually only have 2.

Taxonomy

''T. thermophila'' along with other ''

'' species were originally lumped together as a single species called ''T. pyriformis''. With ''T. thermophila'' first being called T. pyriformis variety 1 and then T. pyriformis syngen 1. It was later renamed to ''T. thermophila'' in 1974.

Genetics

''Tetrahymena thermophila'' has a large genome with about 200 million nucleotides and 27 thousand genes in its nuclear genome. It also exhibits

nuclear dimorphism Nuclear dimorphism is a term referred to the special characteristic of having two different kinds of nuclei in a cell. There are many differences between the types of nuclei. This feature is observed in protozoan ciliates, like ''Tetrahymena'', an ...

: two types of cell nuclei. They have a bigger, non-germline macronucleus and a small, germline

micronucleus Micronucleus is the name given to the small nucleus that forms whenever a chromosome or a fragment of a chromosome is not incorporated into one of the daughter nuclei during cell division. It usually is a sign of genotoxic events and chromosomal i ...

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.

DNA repair

When ''T. thermophila'' is exposed to UV light it results in a greater than 100-fold increase in '' Rad51'' gene expression. Treatment with the DNA alkylating agent

methyl methanesulfonate Methyl methanesulfonate (MMS), also known as methyl mesylate, is an alkylating agent and a carcinogen. It is also a suspected reproductive toxicant, and may also be a skin/sense organ toxicant. It is used in cancer treatment.Rad51 recombinase of ''T. thermophila'' is a homolog of the ''Escherichia coli''

RecA RecA is a 38 kilodalton protein essential for the repair and maintenance of DNA. A RecA structural and functional homolog has been found in every species in which one has been seriously sought and serves as an archetype for this class of homolog ...

recombinase. In ''T. thermophila'', Rad51 participates in

homologous recombination Homologous recombination is a type of genetic recombination in which genetic information is exchanged between two similar or identical molecules of double-stranded or single-stranded nucleic acids (usually DNA as in cellular organisms but may ...

during mitosis,

meiosis Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately r ...

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 eukaryotes. 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.

Reproduction

The species reproduces by asexual reproduction with binary fission and conjugation. In nature the species is an outbreeder. ''Tetrahymena thermophila'' has 7

mating type Mating types are the microorganism equivalent to sexes in multicellular lifeforms and are thought to be the ancestor to distinct sexes. They also occur in macro-organisms such as fungi. Definition Mating types are the microorganism equivalent to ...

s determined by a single

locus Locus (plural loci) is Latin for "place". It may refer to: Entertainment * Locus (comics), a Marvel Comics mutant villainess, a member of the Mutant Liberation Front * ''Locus'' (magazine), science fiction and fantasy magazine ** ''Locus Award' ...

with various

allele An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution. ::"The chro ...

s. The mating types are named I, II, III, IV, V, VI and VII. The mating types 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.

Occurrence

The species lives in freshwater. It usually lives in streams, ponds, and lakes. The

phylogeography Phylogeography is the study of the historical processes that may be responsible for the past to present geographic distributions of genealogical lineages. This is accomplished by considering the geographic distribution of individuals in light of ge ...

of the species is fairly unexplored, it has been observed along the eastern coast of the

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territori ...

. But it has not been observed in other continents with it currently only being reported in North America. However it is said to have an occurrence across the world.

History

Since the 1930s it has been known that the species has 7

s. Immobilized antigens were found in this species were first explored by workers in Ray Owen’s lab. In 1953, the MAT locus in this species was first described by David L. Nanney. In 1982, the group 1 intron was discovered located in the rRNA transcript of this species by

Thomas Cech Thomas Robert Cech (born December 8, 1947) is an American chemist who shared the 1989 Nobel Prize in Chemistry with Sidney Altman, for their discovery of the catalytic properties of RNA. Cech discovered that RNA could itself cut strands of RNA, ...

and his coworks. This was considered the first

ribozyme Ribozymes (ribonucleic acid enzymes) are RNA molecules that have the ability to catalyze specific biochemical reactions, including RNA splicing in gene expression, similar to the action of protein enzymes. The 1982 discovery of ribozymes demons ...

that can self-splice from a primary transcript without the help of proteins. Cech also later on discovered enzymatic RNA in this species.

Telomerase Telomerase, also called terminal transferase, is a ribonucleoprotein that adds a species-dependent telomere repeat sequence to the 3' end of telomeres. A telomere is a region of repetitive sequences at each end of the chromosomes of most euka ...

and

telomere A telomere (; ) is a region of repetitive nucleotide sequences associated with specialized proteins at the ends of linear chromosomes. Although there are different architectures, telomeres, in a broad sense, are a widespread genetic feature mos ...

s were first discovered in this species as well by

Elizabeth Blackburn Elizabeth Helen Blackburn, (born 26 November 1948) is an Australian-American Nobel laureate who is the former president of the Salk Institute for Biological Studies. Previously she was a biological researcher at the University of California, ...

and Carol Greider. The first report of HAT activity was reported in this species in the year 1995. As well as the first type A HAT was discovered in this species.

References

{{Taxonbar, from=Q4050875 Oligohymenophorea