View source for Paramecium

{{Short description|Genus of unicellular ciliates, commonly studied as a representative of the ciliate group}}
{{Redirect|Paramecia|the prehistoric protist|Paramecia (alga)}}
{{Automatic taxobox
| fossil_range = 
| image = Paramecium.jpg
| image_alt = "Paramecium aurelia"
| image_caption = ''[[Paramecium aurelia]]''
| taxon = Paramecium
| authority = [[Otto Friedrich Müller|Müller]], 1773
| synonyms = {{ubl|''Paramoecium'' <small>[[Johann Hermann|Hermann]], 1783</small>|''Paramœcium'' <small>[[Johann Hermann|Hermann]], 1783</small>|''Chloroparamecium'' <small>Fokin, Przybos, Chivilev, Beier, Horn, Skotarczak, Wodecka & Fujishima, 2004</small>|''Viridoparamecium'' <small>Kreutz, Stoeck & Foissner, 2012</small>}}
| synonyms_ref = <ref>[https://www.gbif.org/species/3237260 GBIF]</ref>
| subdivision_ranks = Species
| subdivision = See text
}}

'''''Paramecium''''' ({{IPAc-en|ˌ|p|ær|ə|ˈ|m|iː|s|(|i|)|ə|m}} {{respell|PARR|ə|MEE|s(ee-)əm}}, {{IPAc-en|-|s|i|ə|m}} {{respell|-|see|əm}}, plural "paramecia" only when used as a [[Common name|vernacular name]])<ref>{{cite Merriam-Webster|paramecium}}</ref> is a genus of [[eukaryotic]], [[unicellular]] [[ciliate]]s, widespread in [[freshwater]], [[brackish]], and [[Ocean|marine]] environments. Paramecia are often abundant in stagnant basins and ponds. Because some species are readily cultivated and easily induced to [[sexual conjugation|conjugate]] and divide, they have been widely used in classrooms and laboratories to study [[biological process]]es. ''Paramecium'' species are commonly studied as [[model organism]]s of the ciliate group and have been characterized as the "[[laboratory rat|white rat]]s" of the phylum [[Ciliophora]].<ref name="CCG">{{cite book|last=Lynn|first=Denis|url=https://books.google.com/books?id=Hd3jKGDBR48C&q=editions:JeZk3WJRIIUC|title=The Ciliated Protozoa: Characterization, Classification, and Guide to the Literature|publisher=Springer Science & Business Media|year=2008|isbn=9781402082399|page=30}}</ref>

==Historical background==
[[File:Muller paramecium aurelia.jpg|left|300px|thumb|Paramecia, illustrated by [[Otto Friedrich Müller|Otto Müller]], 1773]]
[[File:Paramecium from Philosophical Transactions 1703.jpg|right|280px|thumb|Earliest known illustration of ''Paramecium'']]
[[File:Joblot chausson.jpg|280px|right|thumb|"Slipper animalcule," illustrated by [[Louis Joblot]], 1718]]
''Paramecium'' were among the first ciliates to be observed by [[microscopy|microscopists]], in the late 17th century. They were most likely known to the Dutch pioneer of [[protozoology]], [[Antonie van Leeuwenhoek]], and were clearly described by his contemporary [[Christiaan Huygens]] in a letter from 1678.<ref>{{cite book| last= Dobell| first= Clifford| title= Antony van Leeuwenhoek and his "Little Animals"| place= New York| publisher= Dover| year= 1932| edition= 1960| pages= 164–165 | isbn= 978-0-486-60594-4}}</ref> The earliest known illustration of a ''Paramecium'' species was published anonymously in [[Philosophical Transactions of the Royal Society]] in 1703.<ref>{{Cite journal|last=Dolan|first=John R.|date=2019-08-01|title=Unmasking "The Eldest Son of The Father of Protozoology": Charles King|journal=Protist|volume=170|issue=4|pages=374–384|doi=10.1016/j.protis.2019.07.002|pmid=31479910|issn=1434-4610|url=https://hal.sorbonne-universite.fr/hal-02283230|doi-access=free}}</ref>

In 1718, the French mathematics teacher and microscopist [[Louis Joblot]] published a description and illustration of a microscopic {{lang|fr|poisson}} (fish), which he discovered in an [[infusion]] of oak bark in water. Joblot gave this creature the name {{lang|fr|"Chausson"}}, or "slipper", and the phrase "slipper animalcule" remained in use as a colloquial epithet for ''Paramecium'', throughout the 18th and 19th centuries.<ref>{{cite book|title=Description et usages de Plusieurs Nouveaux Microscopes, tant simple que composez|last1=Joblot|first1=Louis|publisher=Jacques Collombat|year=1718|volume=2|location=Paris|page=79|language=fr}}</ref>

The name "''Paramecium''" – constructed from the [[Greek language|Greek]] {{Lang|grc|παραμήκης}} (''paramēkēs'', "oblong") – was coined in 1752 by the English microscopist [[John Hill (botanist)|John Hill]], who applied the name generally to "[[Animalcules]] which have no visible limbs or tails, and are of an irregularly oblong figure."<ref>{{cite book | last1 = Hill | first1 = John | title = An History of Animals | url = https://archive.org/details/historyanimals00hill | publisher = Thomas Osborne | year = 1752 | location = Paris | page = [https://archive.org/details/historyanimals00hill/page/5 5]}}</ref> In 1773, [[Otto Friedrich Müller|O. F. Müller]], the first researcher to place the genus within the [[Linnaean taxonomy|Linnaean]] system of [[Taxonomy (biology)|taxonomy]], adopted the name ''Paramecium'' but changed the spelling to ''Paramæcium.''<ref>{{Cite book |last=Müller |first=Otto Frederik |url=https://www.biodiversitylibrary.org/item/100435 |title=Vermivm terrestrium et fluviatilium, seu, Animalium infusoriorum, helminthicorum et testaceorum, non marinorum, succincta historia |last2=Müller |first2=Otto Frederik |date=1773 |publisher=apud Heineck et Faber |volume=v.1 (1773-1774) |location=et Lipsiae}}</ref> In 1783, [[Johann Hermann]] changed the spelling once more, to ''Paramœcium''.<ref>[https://www.gbif.org/species/8336773 ''Paramoecium'' Hermann, 1783]</ref> [[Christian Gottfried Ehrenberg|C. G. Ehrenberg]], in a major study of the [[infusoria]] published in 1838, restored Hill's original spelling for the name, and most researchers have followed his lead.<ref>{{cite journal|last=Woodruff|first=Lorande Loss|title=The structure, life history, and intrageneric relationships of ''Paramecium calkinsi'', sp. nov.|journal=The Biological Bulletin|date=September 1921|volume=41|issue=3|pages=171–180|doi=10.2307/1536748|jstor=1536748|doi-access=free}}</ref>

==Description==

[[File:Paramecium diagram.png|left|370px|A diagram of ''Paramecium caudatum'']]
Species of ''Paramecium'' range 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 the [[pellicle (biology)|pellicle]]. The pellicle consists of an outer [[cell membrane]] (plasma membrane), a layer of flattened membrane-bound sacs called ''alveoli'', and an inner membrane called the ''epiplasm''. 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 of ''Paramecium'' have closely spaced spindle-shaped [[trichocyst]]s, explosive organelles that discharge thin, non-toxic filaments, often used for defensive purposes.<ref>{{Cite book|url=https://www.springer.com/gp/book/9781402082382|title=The Ciliated Protozoa: Characterization, Classification, and Guide to the Literature|last=Lynn|first=Denis|date=2008|publisher=Springer Netherlands|isbn=9781402082382|edition=3|language=en}}</ref><ref>{{Cite book|url=https://books.google.com/books?id=JB_pBwAAQBAJ&q=biology+of+paramecium&pg=PA1|title=The Biology of ''Paramecium''|last=Wichterman|first=R.|date=2012-12-06|publisher=Springer Science & Business Media|isbn=9781475703726|language=en}}</ref>

Typically, an [[anal pore]] (cytoproct) is located on the [[ventral]] surface, in the [[Posterior (anatomy)|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 [[Cilium|cilia]] which beat continuously, drawing food into the cell.<ref name="Curds">{{cite book|title=British and other freshwater ciliated protozoa|last1=Curds|first1=Colin R.|last2=Gates|first2=Michael|last3=Roberts|first3=David McL.|publisher=Cambridge University Press|year=1983|volume=2|page=126}}</ref> ''Paramecium'' are primarily [[heterotroph]]ic, feeding on [[bacteria]] and other small organisms. A few species are [[mixotroph]]s, deriving some nutrients from [[endosymbiont|endosymbiot]]ic [[algae]] ([[chlorella]]) carried in the [[cytoplasm]] of the cell.<ref>{{cite journal|last1=Esteban|first1=Genoveva F.|last2=Fenchel|first2=Tom|last3=Finlay|first3=Bland J.|year=2010|title=Mixotrophy in Ciliates|journal=Protist|volume=161|issue=5|pages=621–641|doi=10.1016/j.protis.2010.08.002|pmid=20970377}}</ref>

[[Osmoregulation]] is carried out by [[contractile vacuole]]s, which actively expel water from the cell to compensate for fluid absorbed by [[osmosis]] from its surroundings.<ref name= Reece134>{{cite book | last1 = Reece | first1 = Jane B. | title = Campbell Biology | publisher = Pearson Education | year = 2011 | location = San Francisco | page = [https://archive.org/details/campbellbiologyj00reec/page/134 134] | isbn = 9780321558237 | url-access = registration | url = https://archive.org/details/campbellbiologyj00reec/page/134 }}</ref> The number of contractile vacuoles varies depending on the species.<ref name= Curds />

== Movement ==

A ''Paramecium'' propels 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.<ref>{{cite journal|last1=Blake|first1=John R.|last2=Sleigh|first2=Michael A.|title=Mechanics of ciliary locomotion|journal=Biological Reviews|date=February 1974|volume=49|issue=1|pages=85–125|doi=10.1111/j.1469-185x.1974.tb01299.x|pmid=4206625|s2cid=41907168}}</ref>

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.<ref>Ogura, A., and K. Takahashi. "Artificial deciliation causes loss of calcium-dependent responses in ''Paramecium''" (1976): 170–172.</ref>

It has been calculated that a ''Paramecium'' expends more than half of its energy in propelling itself through the water.<ref>{{cite journal|last1=Katsu-Kimura|first1=Yumiko|display-authors=et al.|year=2009|title=Substantial energy expenditure for locomotion in ciliates verified by means of simultaneous measurement of oxygen consumption rate and swimming speed|journal=Journal of Experimental Biology|volume=212|issue=12|pages=1819–1824|doi=10.1242/jeb.028894|pmid=19482999|doi-access=free}}</ref> 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.''<ref>{{cite journal|last1=Osterman|first1=Natan|last2=Vilfan|first2=Andrej|title=Finding the ciliary beating pattern with optimal efficiency|url=http://www.pnas.org/content/pnas/108/38/15727.full.pdf|journal=[[Proceedings of the National Academy of Sciences]]|date=September 20, 2011|volume=108|issue=38|pages=15727–15732|doi=10.1073/pnas.1107889108|pmid=21896741|pmc=3179098|bibcode=2011PNAS..10815727O|arxiv=1107.4273|doi-access=free}}</ref>

==Gathering food==
[[File:Инфузория туфелька поедает бактерии!.gif|thumb|Paramecium feeding on Bacteria]]
''Paramecium'' feed on microorganisms such as bacteria, algae, and [[yeast]]s. To gather food, the ''Paramecium'' makes 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 the [[cytostome]], or cell mouth, and move into the interior of the cell. As food enters the cell, it is gathered into food [[vacuole]]s, which are periodically closed off and released into the [[cytoplasm]], where they begin circulating through the cell body by the streaming movement of the cell contents, a process called cyclosis or [[cytoplasmic streaming]]. As a food vacuole moves along, [[enzyme]]s from 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, the [[pH]] of its contents drops from 7 to 3.<ref>{{Cite book|url=https://www.springer.com/gp/book/9781475703740|title=The Biology of ''Paramecium''|last=Wichterman|first=R.|date=1986|publisher=Springer US|isbn=9781475703740|pages=200–1|language=en}}</ref> 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.<ref>{{Cite book|title=The Biology of ''Paramecium''|last=Wichterman|first=Ralph|publisher=Plenum Press|year=1985|isbn=978-1-4757-0374-0|location=New York|pages=88–90}}</ref><ref name= Reece584>{{cite book | last1 = Reece | first1 = Jane B. | title = Campbell Biology | publisher = Pearson Education | year = 2011 | location = San Francisco | page = [https://archive.org/details/campbellbiologyj00reec/page/584 584] | isbn = 9780321558237 | display-authors = etal | url-access = registration | url = https://archive.org/details/campbellbiologyj00reec/page/584 }}</ref><ref>{{cite journal|last=Mast|first=S. O.|title=The food-vacuole in ''Paramecium''|journal=[[The Biological Bulletin]]|date=February 1947|volume=92|issue=1|pages=31–72|doi=10.2307/1537967|jstor=1537967|pmid=20284992|url=https://www.biodiversitylibrary.org/part/31627}}</ref>

== Symbiosis ==
Some species of ''Paramecium'' form [[Mutualism (biology)|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#Didinium nasutum|Didinium nasutum]]''.<ref>{{cite journal|last=Berger|first=Jacques|year=1980|title=Feeding Behaviour of ''Didinium nasutum'' on ''Paramecium bursaria'' with Normal or Apochlorotic Zoochlorellae|journal=[[Journal of General Microbiology]]|volume=118|issue=2|pages=397–404|doi=10.1099/00221287-118-2-397|doi-access=free}}</ref><ref>{{cite journal|last1=Kreutz|first1=Martin|last2=Stoeck|first2=Thorsten|last3=Foissner|first3=Wilhelm|year=2012|title=Morphological and Molecular Characterization of ''Paramecium'' (''Viridoparamecium'' nov. subgen.) ''chlorelligerum'' Kahl (Ciliophora)|journal=Journal of Eukaryotic Microbiology|volume=59|issue=6|pages=548–563|doi=10.1111/j.1550-7408.2012.00638.x|pmc=3866650|pmid=22827482}}</ref> Numerous bacterial endosymbionts have been identified in species of ''Paramecium''.<ref name="Preer-1974">{{cite journal|last1=Preer|first1=John R. Jr.|last2=Preer|first2=Louise B.|last3=Jurand|first3=Artur|date=June 1974|title=Kappa and Other Endosymbionts in ''Paramecium aurelia''|journal=Bacteriological Reviews|volume=38|issue=2|pages=113–163|doi=10.1128/MMBR.38.2.113-163.1974|pmid=4599970|pmc=413848}}</ref> Some intracellular bacteria, known as [[kappa particle]]s, give ''Paramecium'' the ability to kill other strains of ''Paramecium'' that lack kappa particles.<ref name="Preer-1974" />

== Genome ==
The [[genome]] of the species ''[[Paramecium tetraurelia]]'' has been sequenced, providing evidence for three whole-[[genome duplications]].<ref>{{cite journal|last1=Aury|first1=Jean-Marc|last2=Jaillon|first2=Oliver|last3=Wincker|first3=Patrick|display-authors=et al.|date=November 2006|title=Global trends of whole-genome duplications revealed by the ciliate ''Paramecium tetraurelia''|journal=[[Nature (journal)|Nature]]|volume=444|issue=7116|pages=171–8|doi=10.1038/nature05230|pmid=17086204|bibcode=2006Natur.444..171A|doi-access=free}}</ref>

In some ciliates, like ''[[Stylonychia]]'' and ''Paramecium'', only UGA is decoded as a [[Genetic code#Start and stop codons|stop codon]], while UAG and UAA are reassigned as sense codons (that is, codons that code for standard amino acids), coding for the [[amino acid]] [[glutamic acid]].<ref>{{cite journal|last1=Lekomtsev|first1=Sergey|last2=Kolosov|first2=Petr|last3=Bidou|first3=Laure|last4=Frolova|first4=Ludmila|last5=Rousset|first5=Jean-Pierre|last6=Kisselev|first6=Lev|date=June 26, 2007|title=Different modes of stop codon restriction by the ''Stylonychia'' and ''Paramecium'' eRF1 translation termination factors|journal=[[Proceedings of the National Academy of Sciences of the United States of America]]|volume=104|issue=26|pages=10824–9|doi=10.1073/pnas.0703887104|pmc=1904165|pmid=17573528|bibcode=2007PNAS..10410824L|doi-access=free}}</ref>

== Learning ==
The question of whether ''Paramecium'' 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.<ref>{{cite journal|last1=Armus|first1=Harvard L.|last2=Montgomery|first2=Amber R.|last3=Jellison|first3=Jenny L.|date=Fall 2006|title=Discrimination Learning in Paramecia (''P. caudatum'')|url=http://opensiuc.lib.siu.edu/cgi/viewcontent.cgi?article=1156&context=tpr|journal=The Psychological Record|volume=56|issue=4|pages=489–498|doi=10.1007/BF03396029|s2cid=142785414}}</ref> This experiment has been cited as a possible instance of cell memory, or [[epigenetics|epigenetic learning]] in organisms with no [[nervous system]].<ref>{{cite journal|last1=Ginsburg|first1=Simona|last2=Jablonka|first2=Eva|year=2009|title=Epigenetic learning in non-neural organisms|journal=[[Journal of Biosciences]]|volume=34|issue=4|pages=633–646|doi=10.1007/s12038-009-0081-8|pmid=19920348|s2cid=15507569}}</ref>

==Reproduction and sexual phenomena==

=== Reproduction ===
Like all ciliates, ''Paramecium'' have a dual [[Cell nucleus|nuclear apparatus]], consisting of a [[polyploid]] [[macronucleus]], and one or more [[diploid]] [[micronuclei]]. The macronucleus controls non-reproductive cell functions, expressing the [[gene]]s 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.<ref name="Prescott-1971">{{cite journal|last1=Prescott|first1=D. M.|display-authors=et al.|year=1971|title=DNA of ciliated protozoa|journal=Chromosoma|volume=34|issue=4|pages=355–366|doi=10.1007/bf00326311|s2cid=5013543}}</ref>

''Paramecium'' reproduction is [[Asexual reproduction|asexual]], by [[fission (biology)|binary fission]], which has been characterized as "the sole mode of reproduction in ciliates" ([[Ciliate#Reproduction and sexual phenomena|conjugation]] being a sexual phenomenon, not directly resulting in increase of numbers).<ref name="CCG" /><ref name="Raikov-1972">{{Cite journal|last=Raikov|first=I.B|date=1972|title=Nuclear phenomena during conjugation and autogamy in ciliates|url=http://www.sciencedirect.com/science/book/9780080164373|journal=Research in Protozoology|volume=4|pages=149}}</ref> During fission, 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.<ref name=CCG />

Fission may occur spontaneously, in the course of the vegetative [[cell cycle]]. Under certain conditions, it may be preceded by self-fertilization ([[autogamy]]),<ref>{{cite journal|last=Berger|first=James D.|date=October 1986|title=Autogamy in'' Paramecium'' cell cycle stage-specific commitment to meiosis|journal=[[Experimental Cell Research]]|volume=166|issue=2|pages=475–485|doi=10.1016/0014-4827(86)90492-1|pmid=3743667}}</ref> or it may immediately follow [[isogamy|conjugation]], in which ''Paramecium'' of compatible mating types fuse temporarily and exchange genetic material.

==== Conjugation ====

In ciliates such as ''Paramecium'', conjugation is a sexual phenomenon that results in [[genetic recombination]] and nuclear reorganization within the cell.<ref name="Prescott-1971" /><ref name="Preer-1974" />  During conjugation, two ''Paramecium'' of a compatible mating type come together and form a bridge between their [[cytoplasm]]s. Their respective micronuclei undergo [[meiosis]], and [[haploid]] micronuclei 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.<ref name="Prescott-1971" />  Conjugation is followed by one or more "exconjugant divisions."<ref>{{Cite journal|last=Raikov|first=I.B|date=1972|title=Nuclear phenomena during conjugation and autogamy in ciliates|url=http://www.sciencedirect.com/science/book/9780080164373|journal=Research in Protozoology|volume=4|pages=151, 240}}</ref>

;Stages of conjugation[[File:Stages of ciliate conjugation.svg|thumb|562x562px|Stages of conjugation in ''Paramecium caudatum'']]

In ''[[Paramecium caudatum]]'', the stages of conjugation are as follows (see diagram at right):
# Compatible mating strains meet and partly fuse
# The micronuclei undergo meiosis, producing four haploid micronuclei per cell.
# Three of these micronuclei disintegrate. The fourth undergoes mitosis.
# The two cells exchange a micronucleus.
# The cells then separate.
# The micronuclei in each cell fuse, forming a diploid micronucleus. 
# Mitosis occurs three times, giving rise to eight micronuclei.
# Four of the new micronuclei transform into macronuclei, and the old macronucleus disintegrates.
# Binary fission occurs twice, yielding four identical daughter cells.

==Aging==
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 ''Paramecium'' 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 chimera|transplantation]] experiments of Aufderheide in 1986.<ref>{{cite journal|last=Aufderheide|first=Karl J.|year=1986|title=Clonal aging in ''Paramecium tetraurelia''. II. Evidence of functional changes in the macronucleus with age|journal=Mechanisms of Ageing and Development|volume=37|issue=3|pages=265–279|doi=10.1016/0047-6374(86)90044-8|pmid=3553762|s2cid=28320562}}</ref> When macronuclei of clonally young ''Paramecium'' were injected into ''Paramecium'' of standard clonal age, the lifespan (clonal fissions) of the recipient was prolonged. In contrast, transfer of [[cytoplasm]] from clonally young ''Paramecium'' 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,<ref>{{cite journal|last=Smith-Sonneborn|first=J.|year=1979|title=DNA repair and longevity assurance in ''Paramecium tetraurelia''|journal=[[Science (journal)|Science]]|volume=203|issue=4385|pages=1115–1117|doi=10.1126/science.424739|pmid=424739|bibcode=1979Sci...203.1115S}}</ref> Holmes and Holmes,<ref>{{cite journal|last1=Holmes|first1=George E.|last2=Holmes|first2=Norreen R.|title=Accumulation of DNA damages in aging ''Paramecium tetraurelia''|journal=Molecular and General Genetics|date=July 1986|volume=204|issue=1|pages=108–114|doi=10.1007/bf00330196|pmid=3091993|s2cid=11992591}}</ref> and Gilley and Blackburn<ref>{{cite journal|last1=Gilley|first1=David|last2=Blackburn|first2=Elizabeth H.|year=1994|title=Lack of telomere shortening during senescence in ''Paramecium''|url=http://www.pnas.org/content/pnas/91/5/1955.full.pdf|journal=[[Proceedings of the National Academy of Sciences of the United States of America]]|volume=91|issue=5|pages=1955–1958|doi=10.1073/pnas.91.5.1955|pmc=43283|pmid=8127914|bibcode=1994PNAS...91.1955G|doi-access=free}}</ref> demonstrated that, during clonal aging, [[DNA damage]] increases dramatically.<ref>{{cite book| last1=Bernstein |first1= H |last2= Bernstein |first2= C |year= 1991| title= Aging, Sex, and DNA Repair| publisher= Academic Press| place= San Diego| isbn=  978-0120928606| pages= 153–156}}</ref> 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 [[eukaryote]]s, as described in [[DNA damage theory of aging]].

==Meiosis and rejuvenation==

When clonally aged ''P. tetraurelia'' are stimulated to undergo meiosis in association with either conjugation or [[automixis]], the genetic descendants are rejuvenated, and are able to have many more mitotic binary fission divisions. During conjugation or [[automixis]], 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 in ''P. tetraurelia''.<ref name = Godau2019>{{Cite journal|doi=10.1016/j.dnarep.2019.03.011|title=Identification of a miniature Sae2/Ctp1/CtIP ortholog from ''Paramecium tetraurelia'' required for sexual reproduction and DNA double-strand break repair|year=2019|last1=Godau|first1=Julia|last2=Ferretti|first2=Lorenza P.|last3=Trenner|first3=Anika|last4=Dubois|first4=Emeline|last5=von Aesch|first5=Christine|last6=Marmignon|first6=Antoine|last7=Simon|first7=Lauriane|last8=Kapusta|first8=Aurélie|last9=Guérois|first9=Raphaël|last10=Bétermier|first10=Mireille|last11=Sartori|first11=Alessandro A.|journal=DNA Repair|volume=77|pages=96–108|pmid=30928893|s2cid=89619084 |url=https://hal.archives-ouvertes.fr/hal-02171008/file/20190322_Godau_DNAREP_accepted.pdf}}</ref> In ''P. tetraurelia'', CtlP protein is a key factor needed for the completion of [[origin and function of meiosis|meiosis]] during [[sexual reproduction]] and recovery of viable sexual progeny.<ref name = Godau2019/> The CtlP and Mre11 nuclease complex are essential for accurate processing and repair of double-strand breaks during homologous recombination.<ref name = Godau2019/>

The adaptive benefit of [[meiosis]] and [[autogamy|self-fertilization]] in response to starvation appears to be independent of the generation of any new genetic variation in  ''P. tetraurelia''.<ref name = Thind2020>{{cite journal |vauthors=Thind AS, Vitali V, Guarracino MR, Catania F |title=What's Genetic Variation Got to Do with It? Starvation-Induced Self-Fertilization Enhances Survival in Paramecium |journal=Genome Biol Evol |volume=12 |issue=5 |pages=626–638 |date=May 2020 |pmid=32163147 |pmc=7239694 |doi=10.1093/gbe/evaa052 |url=}}</ref>  This observation suggests that the underlying molecular mechanism of meiosis provides a fitness advantage regardless of any concomitant effect of sex on [[genetic diversity]].<ref name = Thind2020/><ref>{{cite journal |vauthors=Bernstein H, Byerly HC, Hopf FA, Michod RE |title=Genetic damage, mutation, and the evolution of sex |journal=Science |volume=229 |issue=4719 |pages=1277–81 |date=September 1985 |pmid=3898363 |doi=10.1126/science.3898363 |url=}}</ref>

==Video gallery==
<gallery>
File:Paramecium bursaria.ogv|''Paramecium bursaria'', a species with symbiotic algae
File:Paramecium putrinum.ogv|''Paramecium putrinum''
File:Paramecium Dividing.ogv|''Paramecium'' binary fission
File:Conjugation in Paramecium.webm|''Paramecium in conjugation''
File:Инфузория туфелька 7.webm|''Paramecium caudatum''
</gallery>

==List of species==

''[[Paramecium aurelia]]'' species complex:<small></small>
*''[[Paramecium primaurelia]]'' <small></small>
*''[[Paramecium biaurelia]]'' <small></small>
*''[[Paramecium triaurelia]]'' <small></small>
*''[[Paramecium tetraurelia]]'' <small></small>
*''[[Paramecium pentaurelia]]'' <small></small>
*''[[Paramecium sexaurelia]]'' <small></small>
*''[[Paramecium septaurelia]]'' <small></small>
*''[[Paramecium octaurelia]]'' <small></small>
*''[[Paramecium novaurelia]]'' <small></small>
*''[[Paramecium decaurelia]]'' <small></small>
*''[[Paramecium undecaurelia]]'' <small></small>
*''[[Paramecium dodecaurelia]]'' <small></small>
*''[[Paramecium tredecaurelia]]'' <small></small>
*''[[Paramecium quadecaurelia]]'' <small></small>
*''[[Paramecium sonneborni]]'' <small></small>
Other species:
*''[[Paramecium buetschlii]]'' <small></small>
*''[[Paramecium bursaria]]'' <small></small>
*''[[Paramecium calkinsi]]'' <small></small>
*''[[Paramecium caudatum]]'' <small></small>
*''[[Paramecium chlorelligerum]]'' <small></small>
*''[[Paramecium duboscqui]]'' <small></small>
*''[[Paramecium grohmannae]]'' <small></small>
*''[[Paramecium jenningsi]]'' <small></small>
*''[[Paramecium multimicronucleatum]]'' <small></small>
*''[[Paramecium nephridiatum]]'' <small></small>
*''[[Paramecium polycaryum]]'' <small></small>
*''[[Paramecium putrinum]]'' <small></small>
*''[[Paramecium schewiakoffi]]'' <small></small>
*''[[Paramecium woodruffi]]'' <small></small>

== References ==
{{Reflist|2}}

== External links ==
* {{Commons category-inline|Paramecium|''Paramecium''}}
* {{Wikispecies-inline|Paramecium|''Paramecium''}}

{{Alveolata}}
{{Taxonbar|from=Q199456}}
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[[Category:Oligohymenophorea]]
[[Category:Ciliate species]]
[[Category:Ciliate genera]]
[[Category:Articles containing video clips]]
[[Category:Taxa named by Otto Friedrich Müller]]