Paramecium tetraurelia
Taxonomic lineage
Paramecium tetraurelia belongs to Ciliophora, a group related to the unicellular parasites named apicomplexans, which include Plasmodium falciparum, the main causative agent of malaria.
cellular organisms - Eukaryota - Alveolata - Ciliophora - Intramacronucleata - Oligohymenophorea - Peniculida - Parameciidae - Paramecium - Paramecium tetraurelia
Brief facts
- Paramecia are a large unicellular protozooans that are found in stagnant warm freshwater.
- The surface of a Paramecium exhibits a highly regular structure with alternating cilia and trichocysts. One unit, a kinetid, is ~1-2 μm in size and harbors a cilium or two in its central depression. Trichocysts are installed in the middle of perpendicular ridges, and, thus, form a line with intermittent cilia. The number of kinetids in P. tetraurelia has been estimated as ~3,000. The regular spacing of cilia causes their hydrodynamic coupling and allows a quasi-synchronous beat which is called metachronic because it moves like waves over a grain field.
- The tricocysts are secretory organelles in which proteins are packaged with crystalline organization. Unlike any known metazoan secretory granule, the trichocyst contents not only remains insoluble upon secretion, but undergoes a rapid change of state (mediated by Ca++ and involving uptake of H2O by the structure) to give an extended extracellular form which is also highly structured. The crystalline contents of the secretory vesicle have a precise shape. On average ~3 μm long and 1 μm at the widest point, the body of intracellular trichocyst is carrot-shaped with an elaborate tip at the wide end, by which it is attached to preformed cortical sites ready for exocytosis. Quantitative freeze-fracture analysis showed that exocytosis occurs within 80 milliseconds, followed by slightly slower membrane re-sealing, all within ~350 ms. This is the fastest dense-core vesicle secretion known. The extracellular, or extended trichocyst looks like needle, on average 25 μm long and 0.7 μm wide.
- Paramecium feeds on bacteria. They track bacteria by dissipated metabolites, like folic acid, using anteriorly localized receptors, and engulf them with a cytostome (cell mouth). From there, membrane-bounded phagosomes are pinched off and quickly acidified with help from acidocomes to kill the bacteria and to make them amenable to digestion by lysosomal enzymes. This well-regulated intracellular cycle ends at the cytoproct (cell anus).
- In their natural habitat these organisms are heavily preyed upon by another ciliates, Didinium and Dileptus. Paramecia developed effective mechanism against predation. Upon an encounter with its natural enemy, Paramecium cell initiates explosive local release of trichocysts, which forms a barrier while pushing the cell away from the predator. After this, the cell can activate its "back-gears" by performing ciliary reversal. In the experiments, the escape rate was close to 100%, while none of the paramecia lacking this ability survived.
- As all Ciliates, Paramecium 's cells possess two nuclei. A germinal nucleus (the micronucleus) is responsible for the transmission of genetic information via sexual processes, whereas a somatic nucleus (the macronucleus) ensures expression of this information. At each sexual generation, a new somatic nucleus is produced by programmed rearrangements of the whole genome contained in the germinal nucleus. Both the macronucleus and the micronucleus are derived from copies of the zygotic nucleus.
- As other ciliates P. tetraurelia may serve as a host for numerous endosymbiotic bacteria. Being relatively large, the ciliate cell represents a well-structured econiche for the bacteria. By infecting macronucleus or other cell compartments they avoid danger of lytic enzymes contained in lysosomes and get access to substances and energy required for their life activities. One of most intriguing and interesting aspects of the bacteria-ciliate symbiosis is that a number of ciliate endosymbionts, mostly cytoplasmic ones, confer killer traits on the host - the ability to kill the cells of clones lacking these bacteria either at a distance or upon a contact (for example, during conjugation). The genus Caedibacter includes four species of such cytoplasmic bacteria wide-spread in the populations of P. tetraurelia. These bacteria form so-called R-bodies inside their host. The R-bodies look like proteinaceous ribbons coiled inside the ciliate cell that form a hollow cylindrical structure. Namely, the R-bodies are believed to play a critical role in killing mediation.
Life cycle
- Vegetative Or asexual reproduction by fission: splitting in two by pinching of in the middle of the long axis of the cell; the macronucleus does not divide by mitosis: it splts in two, roughly equal parts which are going into each of the daughter cells.
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Sexual
Sexual pathway in paramecia can be entered in two ways: via cell interaction leading to
mating (conjugation) or via an endogenous processing leading to autogamy
(self-fertilization).
- Autogamy Autogamy is a process of meiosis and fertilization which takes place in unpaired Paramecium cells, and which is triggered by starvation.
- Conjugation Mating between two individuals of different mating strains; two paramecia line up side by side and then fuse together; each of the micronuclei of the fused cells divide meiotically into four; all but one disintegrate in the both groups; each remaining haploid micronucleus divides mitotically and one of the resultant pair goes into partner paramecium; cells separate, their macronuclei disintegrate and their haploid micronuclei fuse; new macronuclei are formed from diploid micronuclei.
References
- Berger JD.Autogamy in Paramecium. Cell cycle stage-specific commitment to meiosis. Exp Cell Res. 1986 Oct;166(2):475-85.
- Fokin SI. Bacterial endocytobionts of ciliophora and their interactions with the host cell. Int Rev Cytol. 2004;236:181-249.
- Plattner H. My favorite cell--Paramecium. Bioessays. 2002 Jul;24(7):649-58.
- Sperling L, Tardieu A, Gulik-Krzywicki T. The crystal lattice of Paramecium trichocysts before and after exocytosis by X-ray diffraction and freeze-fracture electron microscopy. J Cell Biol. 1987 Oct;105(4):1649-62.
- PubMed free full-text articles: major topic "Paramecium"
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