Tetrahymena

Tetrahymena, a unicellular eukaryote, is a genus of free-living ciliates that can also switch from commensalistic to pathogenic modes of survival. They are common in freshwater ponds. "Tetrahymena" species used as model organisms in biomedical research are "T. thermophila" and "T. pyriformis".
"T. thermophila": a model organism in experimental biology.
As a ciliated protozoan, Tetrahymena thermophila exhibits nuclear dimorphism: two types of cell nuclei. They have a bigger, 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 enzymatic 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 7 different sexes (mating types) 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: 
Life cycle.
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.
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.
Behavior.
Free swimming cells of "Tetrahymena" are attracted to certain chemicals by chemokinesis. The major chemo-attractants are peptides and/or proteins. 
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.
DNA repair.
It is common among protists that the sexual cycle is inducible by stressful conditions such as starvation. 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" gene expression. Treatment with the DNA alkylating agent methyl methane sulfonate 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 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.
Uses in education.
The ASSET program seeks to use this organism to help teach biological concepts and methods to secondary students. An outreach program, Advancing Secondary Science Education thru Tetrahymena, has been established at Cornell University.