Paramecium

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Paramecium
Paramecium aurelia

Paramecium aurelia

Systematics
without rank: Ciliates (ciliophora)
without rank: Intramacronucleata
without rank: Conthreep
without rank: Oligohymenophorea
without rank: Peniculia
Genre : Paramecium
Scientific name
Paramecium
OV Müller , 1773
Paramecium
Paramecium, food vacuoles colored blue

The paramecium ( Paramecium ) are a kind of all-round ciliated protozoa . The characteristic "cilia" ("eyelashes") have the paramecia in common with some other unicellular organisms that are traditionally classified as eyelashes ( ciliata , ciliophora ). As the first protists, the German Society for Protozoology named the representatives of the genus Paramecium as unicellular of the year in 2007 .

Occurrence

Paramecium live mainly in fresh water, but some species such as Paramecium woodruffi are euryhaline and also occur in the brackish water of estuaries, rarely even in the sea. Most representatives can be found in waters such as B. ponds , ponds , lakes , rivers , but also in puddles of water . Thus they are an important part of the freshwater ecosystem. Paramecium are common ciliates . In contrast to many other unicellular organisms, the genus Paramecium does not develop cysts as persistence stages.

Appearance

Paramecia are free-living, actively moving protozoa with a length between 50 and 300 micrometers . When viewed from above, their body shape is elongated oval ("cigar-shaped") to rounded, with the front end being more or less asymmetrically cut off obliquely (that is, sole or "slipper-shaped"). In the water they are able to move quickly while swimming freely, almost always turning to the left in a spiral around their axis. The cell body is flattened from top to bottom (dorsoventral). On the underside (ventral side) there is a pit-shaped mouth area. This pit-shaped depression extends - approximately half the length of the body - asymmetrically from the left front end to the middle of the body. At the end, the depression merges into the mouth funnel (vestibule), at the pointed end of which there is a mouth opening (the cell mouth ). From the cell mouth, a tubular buccal cavity , which when viewed from above appears mostly S-shaped and ends blind with a pointed end, extends inward. The arrangement and morphology of the cilia in the buccal cavity is important for the precise identification of the species. The cell surface is reinforced by a stiff but elastic shell called a pellicle . This is divided into elongated hexagonal (hexagonal) fields on the top. One, rarely two cilia ("eyelashes") arise from each of the fields and sit in rows on the entire surface. In some species they are elongated like tufts at the rear end. Under the pellicle is a thin area of ​​the cell plasma called the cortex, in which the trichocysts are located (see below in the Defense section). The underlying endoplasm appears thinner and contains numerous organelles , often also crystals, which move with the plasma flow in the cell body. There are usually two contractile vacuoles . These pulsate alternately (alternately) and release their interior to the outside through a pore. Typically, there are two types of cell nuclei: a single, large macronucleus, elliptical to kidney-shaped, located a little laterally in the middle of the body, and a number of much smaller micronuclei, which vary depending on the type, and are usually in the immediate vicinity.

The largest species are counted among the “giants” among the unicellular organisms, as they can be recognized with the naked eye as small, white dots in a drop of water.

Movement with the help of eyelashes

Paramecia are noticeable under the microscope by swiftly swimming around. They are surrounded on the outside by many (around 10,000) eyelashes ( cilia ), which are used for locomotion . Through the rhythmic curving and straightening of the eyelashes, shock waves run over the body of the paramecium. Due to the spiral arrangement of these eyelashes, the paramecium is rotated about its longitudinal axis, so that the lateral deviation of the path caused by the body does not turn into a circular movement but into a helical path. For a single cell it has a very high speed of 1 to 1.4 mm / s. The paramecium passes obstacles or bottlenecks with ease, as it can effortlessly move past or through them due to an elastic cell membrane (more precisely: pellicula ). Rotations in any direction are possible, and in the case of startle reactions, a sudden backward swim even by reversing the beat of the cilia.

Food intake, digestion and elimination

The paramecium perceives its prey and food through its “chemical” sense ( chemoreceptors ) and through tactile stimuli (other molecular receptors). It feeds primarily on bacteria that are carried to the mouth area by the blink of an eye . So the eyelashes also act in the food intake with by her close strudeln food particles. With the help of the mouth field, the bacteria reach the cell mouth via the mouth field , where they are then imported into a food vacuole in the cell throat . This process is called endocytosis .

Numerous ribbon-like structures lead to this food vacuole, with the help of which many membrane vesicles are transported that enlarge the food vacuole. Once it has reached a certain size, it constricts inside the cell. Excess water is withdrawn from the food vacuole.

First, acidosomes get into the food vacuole and lower the pH value to 1.2. About lysosomes get digestive enzymes into the bubble that is now called digestive vacuole. While digestion takes place and the bacteria are broken down, the digestive vacuole is transported through the cell on an oval-shaped path. This is called cyclose .

The usable nutrients are absorbed and the indigestible substances are excreted via the cytopyge (the so-called cell extractor ). In order for this to happen, the digestive vesicle has to make contact with the cell membrane as a contact vacuole on the cell after. This process is called exocytosis . Since it almost constantly gushes in food, a paramecium can double its body mass within a few hours.

Excess water that has penetrated the cell is fed into the collecting vesicles of two pulsating vesicles ( contractile vacuoles ) with the help of supply channels arranged in a star shape and actively excreted from them via an excretion pore.

Multiplication

Video microscopy: conjugation of paramecia
Exchange of the small cores

Paramecium normally reproduce asexually by dividing vertically into two daughter cells. The paramecium elongates and the mouth is divided. Another pulsating vacuole is formed in each case . The micronucleus and the macronucleus double. The paramecium constricts the cell body so that each of the two newly created individuals contains a small nucleus, a large nucleus, two pulsating vacuoles and a mouth field. The paramecium divides up to seven times a day under favorable conditions.

Occasionally there are also sexual processes called conjugation in which the paramecia exchange genetic information with other individuals of the same species. To do this, two paramecia lie next to each other on the mouth panels. The cell membranes fuse in this area, the eyelashes disappear. The large nuclei gradually dissolve. The small nuclei are divided into four haploid daughter nuclei through reduction division . Of these four, three die and one divides into two haploid nuclei. One daughter nucleus in each case migrates into the other paramecium in order to fuse with the daughter nucleus that has remained there to form a new diploid nucleus. In each individual, the new nucleus created by fusion divides into two daughter nuclei. One is the new small nucleus, the other develops (with multiple duplication and complete reorganization of the chromosomes ) into a large nucleus. Now the conjugation partners separate again, eyelashes and mouth fields are added in the missing section.

The conjugation is stimulated, among other things, by seasonal changes or deteriorating environmental conditions.

behavior

The paramecium reacts very strongly to stimuli from the environment (touch, temperature, chemical stimuli, exposure). In closed glass tubes, they always swim towards the surface of the water, although they are heavier than water. They react to gravity ( gravitaxis ). Paramecium perceive chemical and thermal stimuli only with the front part of the body. If a paramecium encounters an obstacle, it swims back a little by reversing the blink of an eye and performs a slight turn. Then it swims forward again. If it hits the obstacle again, the paramecium tries this method until it gets past the obstacle. The paramecium works in a coordinated manner, since not only the affected area but the entire living being carries out the reaction.

Enemies

The enemies of the paramecia include amoebas and protozoa of the genus Didinium . The latter also belong to the ciliates. Didinium seizes the paramecium with the help of a nose-like protuberance and then incorporates it. Amoebas and sun animals cover the paramecium with pseudopods and then digest it in a food vacuole .

The paramecium tries to defend itself against attackers with the help of trichocysts . These are hair vesicles containing rod-shaped structures, the tips of which are calcified, which lie directly below the cell membrane and, if there is danger, throw out long, sticky protein threads. As soon as an attacker touches a paramecium, these structures expand explosively and shoot the protein threads into the water. A whole tuft of the protein threads can help the paramecium keep enemies at bay. Some predators can get caught in the protein threads that are shot off and ultimately die. Ejected trichocysts are replaced by new ones that arise in vesicles in the cytoplasm.

Use by humans

Paramecia are used as an indicator of water pollution. They are also used as fish feed.

Nuclear Dimorphism and Genome

Like many ciliates, paramecia are kernimorph, i. H. the nucleus comes in two different forms. A large cell nucleus, called a macronucleus, is somatically active; genes are read and transcribed in it . One or more small cell nuclei, or micronuclei, are resting stages during this time. Your genes are not transcribed. In normal, asexual cell division , macro- and micronucleus are, as usual , doubled separately, via mitosis , and distributed to the daughter cells. During sexual reproduction (a meiosis ), either via conjugation between two cells or via autogamy , the macronucleus is not doubled but destroyed. A new macronucleus is then formed from a micronucleus.

The genetic content in the macro- and micronucleus is identical, i.e. H. in principle, the same genes are present in them . However, the genome is completely rebuilt when the macronucleus is formed. All chromosomes are duplicated more than eight hundred times. Functional genes are then assembled from numerous short pieces of sequence. Repetitive DNA and transposons are cut out. As a result of the complete remodeling, the macronucleus is then homozygous .

Since all copies of the macronucleus are lost during sexual reproduction, ultimately only mutations in the micronucleus are permanently inherited. As a result, paramecia have an extremely low mutation rate. Perhaps facilitated by the peculiar mode of reproduction, but an unusually high number of genes with almost 40,000 were found in the genome of Paramecium (the species Paramecium tetraurelia from the aurelia species complex was examined). This paramecium therefore has roughly twice the number of genes as humans, Homo sapiens . The unusually high number of genes is attributed to several rounds (at least three) of doubling the entire genome, in which numerous paralogous genes are created.

species

The genus Paramecium belongs to the peniculia of a group of ciliate animals, which, depending on the author, is classified as a suborder, an order or a subclass . Within the group, according to genetic data, it is relatively isolated and at a great distance from others, so it is formally the only genus in the monotypical family Parameciidae, but most of the processors completely dispense with specifying a family. Possible sister group could be the predominantly marine genus Frontonia .

According to morphological and genetic data, the genus is divided into four and, according to more recent results, even five sub-genera. It is difficult to state the delimitation and number of species. A little less than 20 morpho species are distinguished, which can be differentiated under the microscope according to their anatomical features. The species can be differentiated in groups on the basis of their general body shape, from elongated “cigar-shaped” to “slipper-shaped” with a rounded shape when viewed from above with the front end being cut off at an angle. Two species are colored green by endosymbiotic algae. An exact determination is based on the shape and arrangement of the cilia, the number of micronuclei and various vaculoa. Within the morphospecies, however, an additional distinction is made between cryptospecies , which are species that look morphologically the same (or almost the same), but can be differentiated on the basis of genetic, biological and ecological data. The “ Paramecium aurelia ” species complex is particularly well known. What morphologically represents a morpho species could be divided into fifteen “tribes” or “lines” according to the mating behavior, each of which has two mating types (o, according to English odd, unequal and e, according to English even, equal). During conjugation, each of these strains (essentially) only accepts individuals of the same strain, with only o and e pairing with each other - this makes it possible in the laboratory to suppress the conjugation entirely through appropriate selection in the culture medium. The strains were later described as species by their discoverer, whereby he simply numbered them consecutively (Latin) in their names: Paramecium primaurelia , Paramecium biaurelia , Paramecium triaurelia etc. Since these are isolated from each other in reproduction , an essential criterion for the delimitation of a Species, despite the lack of morphological features. Genetically, the independence of the cryptic species of the aurelia species complex could essentially be confirmed. Unexpectedly, however, some strains existed which, according to their genetic relationship, belonged to a certain line, but mated with members of a "wrong" strain; here a later change is removed by a subsequent mutation . Other morpho species such as Paramecium multimicronucleatum have also been shown to be composed of cryptic species, but have been poorly researched.

Only the morpho species are included in the following list.

Subgenus Paramecium .

Subgenus Chloroparamecium

Subgenus Helianter

Subgenus Cypriostomum

Subgenus Viridoparamecium

literature

  • Heinz Streble , Dieter Krauter : Life in a drop of water. Microflora and microfauna of freshwater. An identification book. 10th edition. Kosmos, Stuttgart 2006, ISBN 3-440-10807-4 .
  • H.-D. Görtz (Ed.): Paramecium. Springer Verlag, Berlin 1988.
  • R. Wichterman: The Biology of Paramecium. 2nd ed., Plenum Press, New York 1986.

Individual evidence

  1. ↑ Alexei O. Smurov & Sergei I. Fokin (1999): Resistance of Paramecium Species (Ciliophora, Peniculia) to Salinity of Environment. Protistology 1: 43-53.
  2. ^ R. Wichterman: The Biology of Paramecium. Springer, 2012, ISBN 978-1-4757-0372-6 . Page 345–355.
  3. ^ R. Wichterman: The Biology of Paramecium. Springer, 2012, ISBN 978-1-4757-0372-6 . Pages 6 to 8.
  4. ↑ H. Linder: Herman Linder Biology.
  5. ↑ paramecium unicellular of the year - German Society for Protozoology honors paramecium on scinexx.de
  6. ↑ Food for aquarium fish - keeping and breeding of paramecia on aqua-spider.de
  7. ↑ Way Sung, Abraham E. Tucker, Thomas G. Doak, Eunjin Choi, W. Kelley Thomas, Michael Lynch (2012): Extraordinary genome stability in the ciliate Paramecium tetraurelia. PNAS Proceedings of the National Academy of Sciences USA 109 (47): 19339-19344, doi: 10.1073 / pnas.1210663109 .
  8. ↑ Jean-Marc Aury et al. (2006): Global trends of whole-genome duplications revealed by the ciliate Paramecium tetraurelia. Nature 444: 171-178, doi: 10.1038 / nature05230 .
  9. ↑ Sina M. Adl et al. (2012): The Revised Classification of Eukaryotes. Journal of Eukaryotic Microbiology 59 (5): 429-493. doi: 10.1111 / j.1550-7408.2012.00644.x .
  10. ↑ Michaela C. Strüder-Kypke, André-Denis G. Wright, Sergei I. Fokin, Denis H. Lynn (2000): Phylogenetic Relationships of the Subclass Peniculia (Oligohymenophorea, Ciliophora) Inferred from Small Subunit rRNA Gene Sequences. Journal of Eukaryotic Microbiology 47 (4): 419-429. doi: 10.1111 / j.1550-7408.2000.tb00069.x .
  11. ↑ Yan Zhao, Zhenzhen Yi, Eleni Gentekaki, Aibin Zhan, Saleh A. Al-Farraj, Weibo Song (2015): Utility of combining morphological characters, nuclear and 4 mitochondrial genes: An attempt to resolve the conflicts of species 5 identification for ciliated protists. Molecular Phylogenetics and Evolution 94: 718-729. doi: 10.1016 / j.ympev.2015.10.017 .
  12. ↑ a b Sergei I. Fokin (2011): Paramecium genus: biodiversity, some morphological features and the key to the main morphospecies discrimination. Protistology 6 (4): 227-235.
  13. ↑ Martin Kreutz, Thorsten Stoeck, Wilhelm Foissner (2012): Morphological and Molecular Characterization of Paramecium (Viridoparamecium nov. Subgen.) Chlorelligerum Kahl, 1935 (Ciliophora). Journal of Eukaryotic Microbiology 59 (6): 548-563 doi: 10.1111 / j.1550-7408.2012.00638.x .
  14. ^ A b R. Wichterman: The Biology of Paramecium. Springer, 2012, ISBN 978-1-4757-0372-6 .
  15. ↑ Francesco Catania, François Wurmser, Alexey A. Potekhin, Ewa Przyboś, Michael Lynch (2009): Genetic Diversity in the Paramecium aurelia Species Complex. Molecular Biology and Evolution 26 (2): 421-431, doi: 10.1093 / molbev / msn266 .
  16. ↑ Sebastian Tarcz, Alexey Potekhin, Maria Rautian, Ewa Przyboś (2012): Variation in ribosomal and mitochondrial DNA sequences demonstrates the existence of intraspecific groups in Paramecium multimicronucleatum (Ciliophora, Oligohymenophorea). Molecular Phylogenetics and Evolution 63 (2): 500-509, doi: 10.1016 / j.ympev.2012.01.024 .
  17. ↑ based on SIFokin: Paramecium - the genus. German Society for Protozoology: Protozoa of the year 2007 , with additions

See also

Web links

Commons : paramecium ( Paramecium )  - collection of pictures, videos and audio files
Wiktionary: Paramecium  - explanations of meanings, word origins, synonyms, translations