The Model Tetrahymenidans

Ciliates have long been one of the most (if not the most) confidently recognised groups of unicellular eukaryotes owing to their distinctive array of features, in particular locomotion by means of more or less dense tracts of small cilia that often run the length of the organism. And of all ciliates, perhaps none have been more extensively studied than species of the genus Tetrahymena such as T. thermophila. Being easily cultured in the laboratory, Tetrahymena species have become model organisms for the study of a great many genetic and cellular systems such as cell division and gene function. At least two Nobel prizes have been awarded for work based on Tetrahymena that established the functions of telomeres and ribozymes. But Tetrahymena is just one genus of larger group of ciliates, the Tetrahymenida.

Tetrahymena thermophila, from Robinson 2006.

In general, tetrahymenidans are more or less 'typical'-looking ciliates with an ovoid body form and a well-developed 'mouth' at one end. The name Tetrahymena, meaning 'four membranes', refers to the presence of four membrane-like structures inside the oral cavity, a larger, ciliated undulating membrane on the left and three membranelles (formed from polykinetids, complex arrays of cilia and associated basal bodies and fibrils). Most tetrahymenidans possess some variation of this arrangement with the exception of Curimostoma, a genus of parasites of freshwater flatworms and molluscs that lack oral structures (Lynn & Small 2002). Life cycles may contain a number of morphologically differentiated stages. A more mobile theront stage will seek out food sources then transform into a feeding trophont. Mature trophonts may divide asexually or reproduce through conjugation. Cellular multiplication often involves successive divisions so a single parent cell may give rise to four daughter cells. In a number of species, resistant resting cysts may form under adverse conditions.

Glaucoma scintillans, another well-studied tetrahymenidan, copyright Proyecto Agua.

Tetrahymenidans are also ecologically diverse, occupying a range of freshwater habitats. They may be free-living, feeding on bacteria, or they may be parasitic or histophagous, feeding on the tissues of invertebrates. Some species may switch between one or the other depending on circumstances. A few Tetrahymena species have even been cultured in the laboratory axenically: that is, absorbing nutrients directly from a culture broth without requiring a bacterial food supply. Recently, the first confirmed case of a tetrahymenidan containing endosymbiotic algae was described by Pitsch et al. (2016). The species Tetrahymena utriculariae inhabits the bladders of the carnivorous bladderwort Utricularia reflexa. Endosymbiotic green algae provide it with oxygen, allowing the ciliate to survive within the anoxic environment of the bladders.

REFERENCES

Lynn, D. H., & E. B. Small. 2002. Phylum Ciliophora Doflein, 1901. In: Lee, J. J., G. F. Leedale & P. Bradbury (eds) An Illustrated Guide to the Protozoa: Organisms traditionally referred to as Protozoa, or newly discovered groups 2^nd ed. vol. 1 pp. 371–656. Society of Protozoologists: Lawrence (Kansas).

Pitsch, G., L. Adamec, S. Dirren, F. Nitsche, K. Šimek, D. Sirová & T. Posch. 2016. The green Tetrahymena utriculariae n. sp. (Ciliophora, Oligohymenophorea) with its endosymbiotic algae (Micractinium sp.), living in traps of a carnivorous aquatic plant. Journal of Eukaryotic Microbiology 64: 322–335.