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What Is a Paramecium?
Aparna Vidyasagar, Live Science Contributor
Paramecia are single-celled protists that are naturally found in aquatic habitats. They are typically oblong or slipper-shaped and are covered with short hairy structures called cilia. Certain paramecia are also easily cultured in labs and serve as useful model organisms.
Paramecia cells are characteristically elongated. Historically, based on cell shape, these organisms were divided into two groups: aurelia and bursaria, according to the " The Biology of Paramecium, 2nd Ed. " (Springer, 1986). The aurelia morphological type is oblong, or "cigar" shaped, with a somewhat tapered posterior end. Bursaria, on the other hand, represents cells that are "slipper" shaped. They tend to be shorter, and their posterior end is rounded.
Paramecia are a part of a group of organisms known as ciliates . As the name suggests, their bodies are covered in cilia, or short hairy protrusions. Cilia are essential for movement of paramecia. As these structures whip back and forth in an aquatic environment, they propel the organism through its surroundings. Paramecia can move forward at rates up to 2 millimeters per second, as José de Ondarza, an associate professor in the Department of Biological Sciences at SUNY Plattsburgh notes on his research website . Sometimes the organism will perform "avoidance reactions" by reversing the direction in which the cilia beat. This results in stopping, spinning or turning, after which point the paramecium resumes swimming forward. If multiple avoidance reactions follow one another, it is possible for a paramecium to swim backward, though not as smoothly as swimming forward.
Cilia also aid in feeding by pushing food into a rudimentary mouth opening known as the oral groove. Paramecia feed primarily on bacteria, but are known to eat yeast, unicellular algae and even some non-living substances such as milk powder, starch and powdered charcoal, according to "Biology of Paramecium."
Paramecia are eukaryotes. In contrast to prokaryotic organisms, such as bacteria and archaea, eukaryotes have well-organized cells. The defining features of eukaryotic cells are the presence of specialized membrane-bound cellular machinery called organelles and the nucleus, which is a compartment that holds DNA. Paramecia have many organelles characteristic of all eukaryotes, such as the energy-generating mitochondria . However, the organism also contains some unique organelles.
Under an external covering called the pellicle is a layer of somewhat firm cytoplasm called the ectoplasm. This region consists of spindle-shaped organelles known as trichocysts . When they discharge their contents, they become long, thin and spiky, according to "Biology of Paramecium." The exact function of trichocysts is not quite clear, though a popular theory is that they are important for defense against predators. This has been tested over the years and has held true for certain Paramecium species against particular predators. For example, a 2013 article published in the journal Zoological Science found that trichocysts of Paramecium tetraurelia were effective against two of the three predators that were tested: the Cephalodella species of rotifers and the Eucypris species of arthropods .
Below the ectoplasm lies a more fluid type of cytoplasm: the endoplasm. This region contains the majority of cell components and organelles, including vacuoles. These are membrane-enclosed pockets within a cell. According to a 2013 paper published in the journal Bioarchitecture, the name "vacuole" describes the fact that they appear transparent, and empty. In actuality, these organelles tend to be filled with fluid and other materials. Vacuoles take on specific functions with a paramecium cell. Food vacuoles encapsulate food consumed by the paramecium. They then fuse with organelles called lysosomes , whose enzymes break apart food molecules and conduct a form of digestion. Contractile vacuoles are responsible for osmoregulation, or the discharge of excess water from the cell, according to the authors of " Advanced Biology, 1st Ed. " (Nelson, 2000). Depending on the species, water is fed into the contractile vacuoles via canals, or by smaller water-carrying vacuoles. When the contractile vacuole collapses, this excess water leaves the paramecium body through a pore in the pellicle ("Biology of Paramecium").
Perhaps the most unusual characteristic of paramecia is their nuclei. "Paramecium along with the other ciliates have this rather unique feature," said James Forney , a professor of biochemistry at Purdue University. "They have two types of nuclei, which differ in their shape, their content and function."
The two types of nuclei are the micronucleus and macronucleus. The micronucleus is diploid ; that is, it contains two copies of each paramecium chromosome. Forney notes that the micronucleus contains all of the DNA that is present in the organism. "It’s the DNA that is passed from one generation to the another during sexual reproduction," he said. On the other hand, the macronucleus contains a subset of DNA from the micronucleus, according to Forney. "It is the transcriptionally active nucleus," he added. "So it’s the nucleus that is transcribed to make mRNAs and proteins from those mRNAs." The macronucleus is polyploid , or contains multiple copies of each chromosome, sometimes up to 800 copies.
All Paramecium species have one macronucleus, according to Forney. However the number of micronuclei can vary by species. He gives the example of the Paramecium aurelia species complex, which have two micronuclei and Paramecium multimicronucleatum, which have several.
Why the presence of two distinct nuclei? One evolutionary reason is that it is a mechanism by which paramecia and other ciliates can stave off genetic intruders: pieces of DNA that embed themselves into the genome. "In the case of ciliates, there’s a mechanism in which, if a piece of DNA is in the micronucleus but it’s not in the macronucleus, it will be removed from the next macronucleus that is made," Forney explained. "In other words, if something foreign got into the micronuclear genome, then when the next macronucleus is made, it would removed and not included in the expressed version [transcribed] of the genome." Forney notes that this has been described by some as a primitive DNA immune system; that is, surveying the genome and trying to keep out invading elements.
Paramecia can reproduce either asexually or sexually, depending on their environmental conditions. Asexual reproduction takes place when ample nutrients are available, while sexual reproduction takes place under conditions of starvation. In addition, paramecia can also undergo "autogamy" or self-fertilization under conditions of prolonged starvation, according to de Ondarza’s research website .
Asexual reproduction (binary fission)
During binary fission, one paramecium cell divides into two genetically identical offspring, or daughter cells. According to Forney, the micronucleus undergoes mitosis , but the macronucleus divides another way, called an amitotic, or non-mitotic, mechanism. "It is not based on mitosis but it [macronucleus] divides between the two cells and somehow is able to keep approximately the same number of copies of each gene," he said.
Sexual reproduction (conjugation)
Conjugation among paramecia is akin to mating. Forney said that there are two mating types for paramecia, which are referred to as odd and even. This reflects the fact that the mating types for various Paramecium species are denoted by either an odd or even number. For example, according to Forney, Paramecium tetraurelia have mating types 7 and 8. "Odd will mate with the even mating type but you cannot mate if you are the same mating type," he said. Moreover, only cells within a single Paramecium species can mate with one another.
The process is easily distinguishable under laboratory conditions. "The cells stick together. They can actually form rather dramatic clumps of cells when they are initially mixed," Forney said. "Then those gradually pair off into individual pairs in culture."
During sexual reproduction, the micronuclei of each paramecium undergo meiosis , ultimately halving the genetic content to create a haploid nucleus. These are exchanged between the two connected mates. The haploid nuclei from each mate fuse to create a new, genetically varied, micronucleus. In turn, the new micronucleus replicates to give rise to a new macronucleus, according to de Ondarza’s research website .
"Autogamy is essentially the same thing as conjugation, but it is only happening with a single cell," Forney said. During this process, the micronucleus replicates multiple times. One of these new micronuclei undergo rearrangement of their genetic content. Some DNA is fragmented and some DNA sequences, known as "Internal Eliminated Sequences," are removed, according to de Ondarza’s research website .
The general term "paramecium" refers to a single organism within the genus Paramecium. A genus , according to Oregon State University, refers to a closely related group of organisms that share similar characteristics. The genus Paramecium is further divided in groups known as subgenera, which each contain one or more species.
The methods of classifying paramecia have changed over the years. The earliest methods were through visual observation and were based on morphology, ultimately describing all paramecia as either aurelia or bursaria. More recently, classification has combined morphological observation with molecular and genetic information. This has helped to develop a family tree, known as a phylogenetic tree , that represents evolutionary relationships. This shift from morphology to molecular phylogenetics has affected the understanding of relationships within theParamecium genus and species diversity, according to Michaela Strüder-Kypke , manager of advanced light microscopy at the Molecular and Cellular Imaging Facility at the University of Guelph in Ontario, Canada. She said that as of 2012, there are five subgenera generally supported by molecular phylogeny to varying degrees: Chloroparamecium, Helianter, Cypriostomum, Viridoparamecium and Paramecium.
Strüder-Kypke said that a method of identifying species known as "DNA barcoding" has been used for Paramecium. "Identification of species based on the sequence of a particular fragment of DNA has been referred to as DNA barcoding," she explained. "Just like a barcode in the stores identifies each product, a short DNA sequence that is sufficiently divergent, can identify each species." One such barcode, the cox1 gene, has been "extensively utilized for the genus Paramecium," Strüder-Kypke said.
There are currently 19 recognized morphospecies of Paramecium, according to Strüder-Kypke. She explained that a morphospecies is a species defined only by distinct morphological characteristics, not by genetics or the ability to produce fertile offspring. Of this, 15 sibling species form what is known as the Paramecium aurelia species complex. Sibling species, according to Strüder-Kypke, look alike with no morphologically distinguishing characteristics, but they differ in biochemical and genetic aspects and cannot conjugate with one another. The Paramecium aurelia complex counts as a single morphospecies.
New insights into Paramecium taxonomy and the existence of new species continue to be described even today. The 19th morphospecies,Paramecium buetschlii, was discovered in a freshwater pool in Norway and described in a 2015 research paper , published in the journal Organisms Diversity & Evolution. The same paper also described three new "cryptic species" found in Germany, Hungary and Brazil. The authors explain that they were treated as cryptic species because they were difficult to distinguish morphologically from other members of theParamecium genus. However, taxonomic markers in their DNA [DNA barcodes] indicate that they are a separate species.
"The idea is that, if we look in unusual habitats or "under sampled" regions of this world, we may still find new species," Strüder-Kypke told LiveScience.
- Rutgers: Videos of paramecia moving
- Jose de Ondarza’s Paramecium website
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Müller , 1773
Paramecium (also Paramoecium /( ) / PARR-ə-MEE-sh(ee-)əm , /-/ , -see-əm )  is a genus of unicellular ciliates , commonly studied as a representative of the ciliate group. Paramecia are widespread in freshwater , brackish , and marine environments and are often very abundant in stagnant basins and ponds. Because some species are readily cultivated and easily induced to conjugate and divide, it has been widely used in classrooms and laboratories to study biological processes .  Its usefulness as a model organism has caused one ciliate researcher to characterize it as the ” white rat ” of the phylum Ciliophora . 
- 1 Historical background
- 2 Description
- 3 Movement
- 4 Gathering food
- 5 Symbiosis
- 6 Genome
- 7 Learning
- 8 Reproduction and sexual phenomena
- 9 Aging
- 10 Meiosis and rejuvenation
- 11 Video gallery
- 12 List of species
- 13 References
- 14 External links
Historical background[ edit ]
Paramecia, illustrated by Otto Müller , 1773
“Slipper animalcule”, illustrated by Louis Joblot , 1718
Paramecia were among the first ciliates to be seen by microscopists , in the late 17th century. They were probably known to the Dutch pioneer of protozoology , Antonie van Leeuwenhoek , and were clearly described by his contemporary Christiaan Huygens in a letter of 1678.  In 1718, the French mathematics teacher and microscopist Louis Joblot published a description and illustration of a microscopic poisson (fish), which he discovered in an infusion of oak bark in water. Joblot gave this creature the name “Chausson”, or “slipper”, and the phrase “slipper animalcule” remained in use as a colloquial epithet for Paramecium, throughout the 18th and 19th centuries. 
The name “Paramecium” – constructed from the Greek παραμήκης (paramēkēs, “oblong”) – was coined in 1752 by the English microscopist John Hill , who applied the name generally to “Animalcules which have no visible limbs or tails, and are of an irregularly oblong figure”.  In 1773, O. F. Müller , the first researcher to place the genus within the Linnaean system of taxonomy , adopted the name Paramecium, but changed the spelling to Paramœcium. C. G. Ehrenberg , in a major study of the infusoria published in 1838, restored Hill’s original spelling for the genus name, and most researchers have followed his lead. 
Description[ edit ]
Species of Paramecium range in size from 50 to 330 micrometres (0.0020 to 0.0130 in) in length. Cells are typically ovoid, elongate, foot- or cigar-shaped. The body of the cell is enclosed by a stiff but elastic membrane ( pellicle ), uniformly covered with simple cilia , hairlike organelles which act like tiny oars to move the organism in one direction. Nearly all species have closely spaced spindle-shaped trichocysts embedded deeply in the cellular envelope ( cortex ) that surrounds the organism. Typically, an anal pore (cytoproct) is located on the ventral surface, in the posterior half of the cell. In all species, there is a deep oral groove running from the anterior of the cell to its midpoint. This is lined with inconspicuous cilia which beat continuously, drawing food inside the cell.  Paramecia live mainly by heterotrophy , feeding on bacteria and other small organisms. A few species are mixotrophs , deriving some nutrients from endosymbiontic algae ( chlorella ) carried in the cytoplasm of the cell. 
Osmoregulation is carried out by contractile vacuoles , which actively expel water from the cell to compensate for fluid absorbed by osmosis from its surroundings.  The number of contractile vacuoles varies from one, to many, depending on species. 
Movement[ edit ]
A Paramecium propels itself by whiplash movements of the cilia, which are arranged in tightly spaced rows around the outside of the body. The beat of each cilium has two phases: a fast “effective stroke”, during which the cilium is relatively stiff, followed by a slow “recovery stroke”, during which the cilium curls loosely to one side and sweeps forward in a counter-clockwise fashion. The densely arrayed cilia move in a coordinated fashion, with waves of activity moving across the “ciliary carpet”, creating an effect sometimes likened to that of the wind blowing across a field of grain. 
The Paramecium spirals through the water as it progresses. When it happens to encounter an obstacle, the “effective stroke” of its cilia is reversed and the organism swims backward for a brief time, before resuming its forward progress. This is called the avoidance reaction . If it runs into the solid object again, it repeats this process, until it can get past the object. 
It has been calculated that a Paramecium expends more than half of its energy in propelling itself through the water.  This ciliary method of locomotion has been found to be less than 1% efficient. This low percentage is nevertheless close to the maximum theoretical efficiency that can be achieved by an organism equipped with cilia as short as those of the members of Paramecium. 
Gathering food[ edit ]
Paramecia feed on microorganisms like bacteria, algae, and yeasts. To gather food, the Paramecium makes movements with cilia to sweep prey organisms, along with some water, through the oral groove, and inside the mouth opening. The food passes through the cell mouth into the gullet. When enough food has accumulated at the gullet base, it forms a vacuole in the cytoplasm, which then begins circulating through the cell. As it moves along, enzymes from the cytoplasm enter the vacuole to digest the contents; digested nutrients then pass into the cytoplasm, and the vacuole shrinks. When the vacuole, with its fully digested contents, reaches the anal pore, it ruptures, expelling its waste contents to the environment.  
Symbiosis[ edit ]
Some species of Paramecium form mutualistic relationships with other organisms. Paramecium bursaria and Paramecium chlorelligerum harbour endosymbiotic green algae, from which they derive nutrients and a degree of protection from predators such as Didinium nasutum .   Numerous bacterial endosymbionts have been identified in species of Paramecium.   Some intracellular bacteria, known as Kappa particles , give Paramecia that have them the ability to kill other strains of Paramecium that lack Kappa. 
Genome[ edit ]
The genome of the species Paramecium tetraurelia has been sequenced, providing evidence for three whole-genome duplications. 
In some ciliates, like Stylonychia and Paramecium, only UGA is decoded as a stop codon , while UAG and UAA are reassigned as sense codons, coding for the amino acid, Glutamic acid . 
Learning[ edit ]
The question of whether paramecia exhibit learning has been the object of a great deal of experimentation, yielding equivocal results. However, a study published in 2006 seems to show that Paramecium caudatum may be trained, through the application of a 6.5 volt electric current, to discriminate between brightness levels.  This experiment has been cited as a possible instance of cell memory, or epigenetic learning in organisms with no nervous system . 
Reproduction and sexual phenomena[ edit ]
Like all ciliates, Paramecium has a dual nuclear apparatus, consisting of a polyploid macronucleus , and one or more diploid micronuclei . The macronucleus controls non-reproductive cell functions, expressing the genes needed for daily functioning. The micronucleus is the generative, or germline nucleus, containing the genetic material that is passed along from one generation to the next. 
Paramecium reproduces asexually, by binary fission . During reproduction, the macronucleus splits by a type of amitosis , and the micronuclei undergo mitosis . The cell then divides transversally, and each new cell obtains a copy of the micronucleus and the macronucleus. 
Fission may occur spontaneously, in the course of the vegetative cell cycle . Under certain conditions, it may be preceded by self-fertilization (autogamy),  or it may follow conjugation , a sexual phenomenon in which Paramecium of compatible mating types fuse temporarily and exchange genetic material. During conjugation, the micronuclei of each conjugant divide by meiosis and the haploid gametes pass from one cell to the other. The gametes of each organism then fuse to form diploid micronuclei. The old macronuclei are destroyed, and new ones are developed from the new micronuclei. 
Autogamy or conjugation can be induced by shortage of food at certain points in the Paramecium life cycle . 
Aging[ edit ]
In the asexual fission phase of growth, during which cell divisions occur by mitosis rather than meiosis, clonal aging occurs leading to a gradual loss of vitality. In some species, such as the well studied Paramecium tetraurelia, the asexual line of clonally aging paramecia loses vitality and expires after about 200 fissions if the cells fail to undergo autogamy or conjugation. The basis for clonal aging was clarified by transplantation experiments of Aufderheide.  When macronuclei of clonally young paramecia were injected into paramecia of standard clonal age, the lifespan (clonal fissions) of the recipient was prolonged. In contrast, transfer of cytoplasm from clonally young paramecia did not prolong the lifespan of the recipient. These experiments indicated that the macronucleus, rather than the cytoplasm, is responsible for clonal aging. Other experiments by Smith-Sonneborn,  Holmes and Holmes,  and Gilley and Blackburn  demonstrated that, during clonal aging, DNA damage increases dramatically (also reviewed by Bernstein and Bernstein).  Thus, DNA damage in the macronucleus appears to be the cause of aging in P. tetraurelia. In this single-celled protist, aging appears to proceed as it does in multicellular eukaryotes, as described in DNA damage theory of aging .
Meiosis and rejuvenation[ edit ]
When clonally aged P. tetraurelia are stimulated to undergo meiosis in association with either conjugation or automixis, the progeny are rejuvenated, and are able to have many more mitotic binary fission divisions. During either of these processes the micronuclei of the cell(s) undergo meiosis, the old macronucleus disintegrates and a new macronucleus is formed by replication of the micronuclear DNA that had recently undergone meiosis. There is apparently little, if any, DNA damage in the new macronucleus. These findings suggest that clonal aging is due, in large part, to a progressive accumulation of DNA damage (see DNA damage theory of aging ); and that rejuvenation is due to the repair of this damage in the micronucleus during meiosis. Meiosis appears to be an adaptation for DNA repair and rejuvenation in these paramecia. 
Video gallery[ edit ]
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Paramecium bursaria, a species with symbiotic algae
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Paramecium binary fission
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Paramecium caudatum in conjugation
List of species[ edit ]
- Paramecium africanum Dragesco, 1970
- Paramecium aurelia complex (includes biological species, P. primaurelia, P. biaurellia, etc.)
- Paramecium bursaria (Ehrenberg, 1831) Focke, 1836
- Paramecium calkinsi Woodruff, 1921
- Paramecium caudatum Ehrenberg, 1834
- Paramecium chlorelligerum Kahl, 1935
- Paramecium duboscqui Chatton & Brachon, 1933
- Paramecium jankowskii Dragesco, 1972
- Paramecium jenningsi Diller & Earl, 1958
- Paramecium nephridiatum Gelei, 1925
- Paramecium polycaryum Woodruf, 1923
- Paramecium pseudotrichium Dragesco, 1970
- Paramecium putrinum Claparède & Lachmann, 1859
- Paramecium schewiakoffi Fokin, Przybos, Chivilevc, §, 2004
- Paramecium sonneborni Aufderheide, Daggett & Nerad, 1983
- Paramecium ugandae Dragesco, 1972
- Paramecium wichtermani , Mohammed and Nashed, 1968–1969,
- Paramecium woodruffi Wenrich, 1928
References[ edit ]
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- ^ Woodruff, Lorande Loss (September 1921). “The structure, life history, and intrageneric relationships of Paramecium calkinsi, sp. nov” . The Biological Bulletin. 41 (3): 171–180. doi : 10.2307/1536748 .
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- ^ Reece, Jane B.; et al. (2011). Campbell Biology. San Francisco: Pearson Education. p. 584. ISBN 9780321558237 .
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- ^ Kreutz, Martin; Stoeck, Thorsten; Foissner, Wilhelm (2012). “Morphological and Molecular Characterization of Paramecium (Viridoparamecium nov. subgen.) chlorelligerum Kahl (Ciliophora)” . Journal of Eukaryotic Microbiology. 59 (6): 548–563. doi : 10.1111/j.1550-7408.2012.00638.x . PMC 3866650 . PMID 22827482 .
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External links[ edit ]
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