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Paramecium

"Paramecia" redirects here. For the prehistoric protist, seeParamecia (alga).

Paramecium(/ˌpærəˈmiːs(i)əm/PARR-ə-MEE-s(ee-)əm,/-siəm/-⁠see-əm,plural "paramecia" only when used as avernacular name)[2]is a genus ofeukaryotic,unicellularciliates,widespread infreshwater,brackish,andmarineenvironments. Paramecia are often abundant in stagnant basins and ponds. Because some species are readily cultivated and easily induced toconjugateand divide, they have been widely used in classrooms and laboratories to studybiological processes.Parameciumspecies are commonly studied asmodel organismsof the ciliate group and have been characterized as the "white rats"of the phylumCiliophora.[3]

Paramecium
"Paramecium aurelia"
Paramecium aurelia
Scientific classificationEdit this classification
Domain: Eukaryota
Clade: Diaphoretickes
Clade: SAR
Clade: Alveolata
Phylum: Ciliophora
Class: Oligohymenophorea
Order: Peniculida
Family: Parameciidae
Genus: Paramecium
Müller,1773
Species

See text

Synonyms[1]
  • ParamoeciumHermann,1783
  • ParamœciumHermann,1783
  • ChloroparameciumFokin, Przybos, Chivilev, Beier, Horn, Skotarczak, Wodecka & Fujishima, 2004
  • ViridoparameciumKreutz, Stoeck & Foissner, 2012

Historical background

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Paramecia, illustrated byOtto Müller,1773
Earliest known illustration ofParamecium
"Slipper animalcule," illustrated byLouis Joblot,1718

Parameciumwere among the first ciliates to be observed bymicroscopists,in the late 17th century. They were most likely known to the Dutch pioneer ofprotozoology,Antonie van Leeuwenhoek,and were clearly described by his contemporaryChristiaan Huygensin a letter from 1678.[4]The earliest known illustration of aParameciumspecies was published anonymously inPhilosophical Transactions of the Royal Societyin 1703.[5]

In 1718, the French mathematics teacher and microscopistLouis Joblotpublished a description and illustration of a microscopicpoisson(fish), which he discovered in aninfusionof 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 forParamecium,throughout the 18th and 19th centuries.[6]

The name "Paramecium"– constructed from theGreekπαραμήκης(paramēkēs,"oblong" ) – was coined in 1752 by the English microscopistJohn Hill,who applied the name generally to "Animalculeswhich have no visible limbs or tails, and are of an irregularly oblong figure. "[7]In 1773,O. F. Müller,the first researcher to place the genus within theLinnaeansystem oftaxonomy,adopted the nameParameciumbut changed the spelling toParamæcium.[8]In 1783,Johann Hermannchanged the spelling once more, toParamœcium.[9]C. G. Ehrenberg,in a major study of theinfusoriapublished in 1838, restored Hill's original spelling for the name, and most researchers have followed his lead.[10]

Description

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A diagram ofParamecium caudatum

Species ofParameciumrange in size from 0.06 mm to 0.3 mm in length. Cells are typically ovoid, elongate, or foot- or cigar-shaped.

The body of the cell is enclosed by a stiff but elastic structure called thepellicle.The pellicle consists of an outercell membrane(plasma membrane), a layer of flattened membrane-bound sacs calledalveoli,and an inner membrane called theepiplasm.The pellicle is not smooth, but textured with hexagonal or rectangular depressions. Each of these polygons is perforated by a central aperture through which a single cilium projects. Between the alveolar sacs of the pellicle, most species ofParameciumhave closely spaced spindle-shapedtrichocysts,explosive organelles that discharge thin, non-toxic filaments, often used for defensive purposes.[11][12]

Typically, ananal pore(cytoproct) is located on theventralsurface, in theposteriorhalf of the cell. In all species, there is a deep oral groove running from theanteriorof the cell to its midpoint. This is lined with inconspicuousciliawhich beat continuously, drawing food into the cell.[13]Parameciumare primarilyheterotrophic,feeding onbacteriaand other small organisms. A few species aremixotrophs,deriving some nutrients fromendosymbioticalgae(chlorella) carried in thecytoplasmof the cell.[14]

Osmoregulationis carried out bycontractile vacuoles,which actively expel water from the cell to compensate for fluid absorbed byosmosisfrom its surroundings.[15]The number of contractile vacuoles varies depending on the species.[13]

Movement

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AParameciumpropels itself by whip-like 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.[16]

TheParameciumspirals 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 theavoidance reaction.If it runs into the solid object again, it repeats this process, until it can get past the object.[17]

It has been calculated that aParameciumexpends more than half of its energy in propelling itself through the water.[18]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 ofParamecium.[19]

Gathering food

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Paramecium feeding on Bacteria

Parameciumfeed on microorganisms such as bacteria, algae, andyeasts.To gather food, theParameciummakes movements with cilia to sweep prey organisms, along with some water, through the oral groove (vestibulum, or vestibule), and into the cell. The food passes from the cilia-lined oral groove into a narrower structure known as the buccal cavity (gullet). From there, food particles pass through a small opening called thecytostome,or cell mouth, and move into the interior of the cell. As food enters the cell, it is gathered into foodvacuoles,which are periodically closed off and released into thecytoplasm,where they begin circulating through the cell body by the streaming movement of the cell contents, a process called cyclosis orcytoplasmic streaming.As a food vacuole moves along,enzymesfrom the cytoplasm enter it, to digest the contents. As enzymatic digestion proceeds, the vacuole contents become more acidic. Within five minutes of a vacuole's formation, thepHof its contents drops from 7 to 3.[20]As digested nutrients pass into the cytoplasm, the vacuole shrinks. When the fully digested vacuole reaches the anal pore, it ruptures, expelling its waste contents outside the cell.[21][22][23]

Symbiosis

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Some species ofParameciumformmutualisticrelationships with other organisms.Paramecium bursariaandParamecium chlorelligerumharbour endosymbiotic green algae, from which they derive nutrients and a degree of protection from predators such asDidinium nasutum.[24][25]Numerous bacterial endosymbionts have been identified in species ofParamecium.[26]Some intracellular bacteria, known askappa particles,giveParameciumthe ability to kill other strains ofParameciumthat lack kappa particles.[26]

Genome

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Thegenomeof the speciesParamecium tetraureliahas been sequenced, providing evidence for three whole-genome duplications.[27]

In some ciliates, likeStylonychiaandParamecium,only UGA is decoded as astop codon,while UAG and UAA are reassigned as sense codons (that is, codons that code for standard amino acids), coding for theamino acidglutamic acid.[28]

Learning

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The question of whetherParameciumexhibit learning has been the object of a great deal of experimentation, yielding equivocal results. However, a study published in 2006 seems to show thatParamecium caudatummay be trained, through the application of a 6.5voltelectric current,to discriminate between brightness levels.[29]This experiment has been cited as a possible instance of cell memory, orepigenetic learningin organisms with nonervous system.[30]

Reproduction and sexual phenomena

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Reproduction

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Like all ciliates,Parameciumhave a dualnuclear apparatus,consisting of apolyploidmacronucleus,and one or morediploidmicronuclei.The macronucleus controls non-reproductive cell functions, expressing thegenesneeded for daily functioning. The micronucleus is the generative, orgermlinenucleus, containing the genetic material that is passed along from one generation to the next.[31]

Parameciumreproduction isasexual,bybinary fission,which has been characterized as "the sole mode of reproduction in ciliates" (conjugationbeing a sexual phenomenon, not directly resulting in increase of numbers).[3][32]During fission, the macronucleus splits by a type ofamitosis,and the micronuclei undergomitosis.The cell then divides transversally, and each new cell obtains a copy of the micronucleus and the macronucleus.[3]

Fission may occur spontaneously, in the course of the vegetativecell cycle.Under certain conditions, it may be preceded by self-fertilization (autogamy),[33]or it may immediately followconjugation,in whichParameciumof compatible mating types fuse temporarily and exchange genetic material.

Conjugation

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In ciliates such asParamecium,conjugation is a sexual phenomenon that results ingenetic recombinationand nuclear reorganization within the cell.[31][26]During conjugation, twoParameciumof a compatible mating type come together and form a bridge between theircytoplasms.Their respective micronuclei undergomeiosis,andhaploidmicronuclei are exchanged over the bridge. Following conjugation, the cells separate. The old macronuclei are destroyed, and both post-conjugants form new macronuclei, by amplification of DNA in their micronuclei.[31]Conjugation is followed by one or more "exconjugant divisions."[34]

Stages of conjugation
Stages of conjugation inParamecium caudatum

InParamecium caudatum,the stages of conjugation are as follows (see diagram at right):

  1. Compatible mating strains meet and partly fuse
  2. The micronuclei undergo meiosis, producing four haploid micronuclei per cell.
  3. Three of these micronuclei disintegrate. The fourth undergoes mitosis.
  4. The two cells exchange a micronucleus.
  5. The cells then separate.
  6. The micronuclei in each cell fuse, forming a diploid micronucleus.
  7. Mitosis occurs three times, giving rise to eight micronuclei.
  8. Four of the new micronuclei transform into macronuclei, and the old macronucleus disintegrates.
  9. Binary fission occurs twice, yielding four identical daughter cells.

Aging

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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 studiedParamecium tetraurelia,the asexual line of clonally agingParameciumloses vitality and expires after about 200 fissions if the cells fail to undergo autogamy or conjugation. The basis for clonal aging was clarified bytransplantationexperiments of Aufderheide in 1986.[35]When macronuclei of clonally youngParameciumwere injected intoParameciumof standard clonal age, the lifespan (clonal fissions) of the recipient was prolonged. In contrast, transfer ofcytoplasmfrom clonally youngParameciumdid 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,[36]Holmes and Holmes,[37]and Gilley and Blackburn[38]demonstrated that, during clonal aging,DNA damageincreases dramatically.[39]Thus, DNA damage in the macronucleus appears to be the cause of aging inP. tetraurelia.In this single-celledprotist,aging appears to proceed as it does in multicellulareukaryotes,as described inDNA damage theory of aging.

Meiosis and rejuvenation

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When clonally agedP. tetraureliaare stimulated to undergo meiosis in association with either conjugation orautomixis,the genetic descendants are rejuvenated, and are able to have many more mitotic binary fission divisions. During conjugation orautomixis,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 further support the idea that clonal aging is due, in large part, to a progressive accumulation of DNA damage; 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 inP. tetraurelia.[40]InP. tetraurelia,CtlP protein is a key factor needed for the completion ofmeiosisduringsexual reproductionand recovery of viable sexual progeny.[40]The CtlP and Mre11 nuclease complex are essential for accurate processing and repair of double-strand breaks during homologous recombination.[40]

The adaptive benefit ofmeiosisandself-fertilizationin response to starvation appears to be independent of the generation of any new genetic variation inP. tetraurelia.[41]This observation suggests that the underlying molecular mechanism of meiosis provides a fitness advantage regardless of any concomitant effect of sex ongenetic diversity.[41][42]

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  • Paramecium bursaria,a species with symbiotic algae
  • Paramecium putrinum
  • Parameciumbinary fission
  • Paramecium in conjugation
  • Paramecium caudatum

List of species

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Paramecium aureliaspecies complex:

Other species:

References

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  1. ^GBIF
  2. ^"paramecium".Merriam-Webster.com Dictionary.Merriam-Webster.
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