Paramecium: a ciliate model

Scientific work with the ciliate Paramecium has quite a long tradition in any areas of biology as it was found to be a perfect organism to study population dynamics as well as epigenetic effects. One of the most important subjects were the “surface antigens” of Paramecium, proteins located on the outer membrane with unidentified function in free living protists – in contrast to their obvious usage in parasitic protozoa, being able to evade the hosts immune system by changing the antigen-coat on the surface.

Paramecium is a free-living ciliate, occurring ubiquitously in freshwater habitats. The unicellular organism has a complex organisation, including a digestive membrane flow system founded on phagocytosis, contractile vacuoles for osmoregulation and an excretory unit. It reaches a remarkable length of about 150-200 µm, depending on the stages of cell cycle and feeding. Thus, Paramecium belongs to the most highly organised protozoa.

Several species of the genus Paramecium are known, its taxonomy having passed several revisions in the last hundred years, especially as a consequence of the great morphological similarity of individuals belonging to different species. Sonneborn (1938) characterised the species P. aurelia including several subspecies which he called “varieties” (or “syngens”), since no crossing events were observed between them, merely between individuals of the same variety but different mating type. Later on, after advanced rework of the species definition, varieties were accepted as true species according to the biological species concept. The aurelia-species were summarised to the species complex P. aurelia, containing the species P. primaurelia (previously variety 1), P. biaurelia, P. triaurelia, P. tetraurelia etc. Existing laboratory strains (or „stocks“) are called according to their original habitat from which they were isolated (e.g. strain 156, New Haven, Connecticut).

Using the serotype-systems of Paramecium, with all the sequences now identified, the worldwide collection of strains, and the antisera produced against different serotypes, Paramecium was characterized to be a perfect organism to study the phenomenon of serotype-expression.