Tetrahymena are free-living ciliate protozoa 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 they both 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:
- First cell which showed synchronized division, which led to the first insights into the existence of mechanisms which control the cell cycle.
- Identification and purification of the first cytoskeleton based motor protein such as dynein.
- Aid in the discovery of lysosomes and peroxisomes.
- Early molecular identification of somatic genome rearrangement.
- 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).
- Nobel Prize winning co-discovery (1989, in Chemistry) of catalytic ribonucleic acid (ribozyme).
- Discovery of the function of histone acetylation.
- 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
- Discovery of self-splicing RNA 
- Alfred M. Elliott (1973). "Biology of Tetrahymena". Dowen, Hutchinson and Ross Inc. ISBN 0-87933-013-9.
- Selecting One of Several Mating Types through Gene Segment Joining and Deletion in Tetrahymena thermophila Cervantes MD, Hamilton EP, Xiong J, Lawson MJ, Yuan D, et al. (2013) Selecting One of Several Mating Types through Gene Segment Joining and Deletion in Tetrahymena thermophila. PLoS Biol 11(3): e1001518. doi:10.1371/journal.pbio.1001518
- Tetrahymena Genome Sequencing White Paper
- Kruger K, Grabowski PJ, Zaug AJ, Sands J, Gottschling DE, Cech TR (November 1982). "Self-splicing RNA: autoexcision and autocyclization of the ribosomal RNA intervening sequence of Tetrahymena". Cell 31 (1): 147–57. doi:10.1016/0092-8674(82)90414-7. PMID 6297745.
1. Methods in Cell Biology Volume 62: Tetrahymena thermophila, Edited by David J. Asai and James D. Forney. (2000). By Academic Press ISBN 0-12-544164-9
2. Collins, K. and Gorovsky, M.A. (2005). Tetrahymena thermophila Curr Biol. 15: R317-8.
- Tetrahymena Stock Center at Cornell University
- ASSET: Advancing Secondary Science Education thru Tetrahymena
- Tetrahymena Genome Database
- Tetrahymena thermophila Genome Project at The Institute for Genomic Research
- Tetrahymena thermophila Genome Sequence Synopsis
- Tetrahymena thermophila genome paper
- Tetrahymena experiments on Journal of Visualized Experiments (JoVE) website
- Microbial Digital Specimen Archives: Tetrahymena image gallery