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Amoeba

From Wikipedia, the free encyclopedia
This article is about the cellular body type. For the genus, seeAmoeba (genus).For other uses, seeAmoeba (disambiguation).

Clockwise from top right:Amoeba proteus,Actinophrys sol,Acanthamoebasp.,Nuclearia thermophila.,Euglypha acanthophora,neutrophilingesting bacteria.

Anamoeba(/əˈmiːbə/;less commonly spelledamebaoramœba;pl.:amoebas(less commonly,amebas) oramoebae(amebae)/əˈmiːbi/),[1]often called anamoeboid,is a type ofcellorunicellular organismwith the ability to alter its shape, primarily by extending and retractingpseudopods.[2]Amoebae do not form a singletaxonomic group;instead, they are found in every majorlineageofeukaryoticorganisms. Amoeboid cells occur not only among theprotozoa,but also infungi,algae,andanimals.[3][4][5][6][7]

Microbiologistsoften use the terms "amoeboid" and "amoeba" interchangeably for any organism that exhibitsamoeboid movement.[8][9]

In older classification systems, most amoebae were placed in theclassorsubphylumSarcodina, a grouping ofsingle-celled organismsthat possess pseudopods or move byprotoplasmicflow. However,molecular phylogeneticstudies have shown that Sarcodina is not amonophyleticgroup whose members sharecommon descent.Consequently, amoeboid organisms are no longer classified together in one group.[10]

The best known amoeboidprotistsareChaos carolinenseandAmoeba proteus,both of which have been widely cultivated and studied in classrooms and laboratories.[11][12]Other well known species include the so-called "brain-eating amoeba"Naegleria fowleri,the intestinal parasiteEntamoeba histolytica,which causesamoebic dysentery,and the multicellular "social amoeba" orslime mouldDictyostelium discoideum.

Shape, movement and nutrition

[edit]
The forms ofpseudopodia,from left: polypodial and lobose; monopodial and lobose; filose; conical; reticulose; tapering actinopods; non-tapering actinopods

Amoeba do not have cell walls, which allows for free movement. Amoeba move and feed by using pseudopods, which are bulges ofcytoplasmformed by the coordinated action ofactinmicrofilamentspushing out theplasma membranethat surrounds the cell.[13]The appearance and internal structure of pseudopods are used to distinguish groups of amoebae from one another.Amoebozoanspecies, such as those in the genusAmoeba,typically have bulbous (lobose) pseudopods, rounded at the ends and roughly tubular in cross-section.Cercozoanamoeboids, such asEuglyphaandGromia,have slender, thread-like (filose) pseudopods.Foraminiferaemit fine, branching pseudopods that merge with one another to form net-like (reticulose) structures. Some groups, such as theRadiolariaandHeliozoa,have stiff, needle-like, radiatingaxopodia(actinopoda) supported from within by bundles ofmicrotubules.[3][14]

Naked amoeba in the genus Mayorella
Shell of the testate amoeba Cylindrifflugia acuminata
"Naked" amoeba of the genusMayorella(left) and shell of thetestate amoebaCylindrifflugia acuminata(right)

Free-living amoebae may be "testate"(enclosed within a hard shell), or" naked "(also known asgymnamoebae,lacking any hard covering). The shells of testate amoebae may be composed of various substances, includingcalcium,silica,chitin,or agglutinations of found materials like small grains of sand and thefrustulesofdiatoms.[15]

To regulateosmotic pressure,most freshwater amoebae have a contractilevacuolewhich expels excess water from the cell.[16]Thisorganelleis necessary because freshwater has a lower concentration ofsolutes(such as salt) than the amoeba's own internal fluids (cytosol). Because the surrounding water ishypotonicwith respect to the contents of the cell, water is transferred across the amoeba's cell membrane byosmosis.Without a contractile vacuole, the cell would fill with excess water and, eventually, burst. Marine amoebae do not usually possess a contractile vacuole because the concentration of solutes within the cell are in balance with thetonicityof the surrounding water.[17]

Diet

[edit]
Amoebaphagocytosisof abacterium

The food sources of amoebae vary. Some amoebae are predatory and live by consuming bacteria and otherprotists.Some aredetritivoresand eat dead organic material.

Amoebae typically ingest their food byphagocytosis,extending pseudopods to encircle and engulf live prey or particles of scavenged material. Amoeboid cells do not have a mouth orcytostome,and there is no fixed place on the cell at which phagocytosis normally occurs.[18]

Some amoebae also feed bypinocytosis,imbibing dissolved nutrients throughvesiclesformed within the cell membrane.[19]

Size range

[edit]
Foraminiferahave reticulose (net-like) pseudopods, and many species are visible with the naked eye

The size of amoeboid cells and species is extremely variable. The marine amoeboidMassisteria voersiis just 2.3 to 3micrometresin diameter,[20]within the size range of many bacteria.[21]At the other extreme, the shells of deep-seaxenophyophorescan attain 20 cm in diameter.[22]Most of the free-living freshwater amoebae commonly found inpond water,ditches, and lakes aremicroscopic,but some species, such as the so-called "giant amoebae"Pelomyxa palustrisandChaos carolinense,can be large enough to see with the naked eye.

Species or cell type Size in micrometers
Massisteria voersi[20] 2.3–3
Naegleria fowleri[23] 8–15
Neutrophil(white blood cell)[24] 12–15
Acanthamoeba[25] 12–40
Entamoeba histolytica[26] 15–60
Arcella vulgaris[27] 30–152
Amoeba proteus[28] 220–760
Chaos carolinense[29] 700–2000
Pelomyxa palustris[30] up to 5000
Syringammina fragilissima[22] up to200000

Amoebae as specialized cells and life cycle stages

[edit]
Neutrophil(white blood cell) engulfing anthrax bacteria

Somemulticellular organismshave amoeboid cells only in certain phases of life, or use amoeboid movements for specialized functions. In the immune system of humans and other animals, amoeboidwhite blood cellspursue invading organisms, such as bacteria and pathogenic protists, and engulf them byphagocytosis.[31]

Amoeboid stages also occur in the multicellular fungus-like protists, the so-calledslime moulds.Both the plasmodial slime moulds, currently classified in the classMyxogastria,and the cellular slime moulds of the groupsAcrasidaandDictyosteliida,live as amoebae during their feeding stage. The amoeboid cells of the former combine to form a giantmultinucleateorganism,[32]while the cells of the latter live separately until food runs out, at which time the amoebae aggregate to form a multicellular migrating "slug" which functions as a single organism.[8]

Other organisms may also present amoeboid cells during certain life-cycle stages, e.g., the gametes of some green algae (Zygnematophyceae)[33]and pennatediatoms,[34]the spores (or dispersal phases) of someMesomycetozoea,[35][36]and thesporoplasmstage ofMyxozoaand ofAscetosporea.[37]

Amoebae as taxa

[edit]

Early history and origins of Sarcodina

[edit]
The first illustration of an amoeboid, from Roesel von Rosenhof'sInsecten-Belustigung(1755)
Amoeba proteus

The earliest record of an amoeboid organism was produced in 1755 byAugust Johann Rösel von Rosenhof,who named his discovery "Der Kleine Proteus" ( "the Little Proteus" ).[38]Rösel's illustrations show an unidentifiable freshwater amoeba, similar in appearance to the common species now known asAmoeba proteus.[39]The term "Proteus animalcule" remained in use throughout the 18th and 19th centuries, as an informal name for any large, free-living amoeboid.[40]

In 1822, the genusAmiba(from theGreekἀμοιβήamoibe,meaning "change" ) was erected by the French naturalistBory de Saint-Vincent.[41][42]Bory's contemporary,C. G. Ehrenberg,adopted the genus in his own classification of microscopic creatures, but changed the spelling toAmoeba.[43]

In 1841,Félix Dujardincoined the term "sarcode"(from Greek σάρξsarx,"flesh," and εἶδοςeidos,"form" ) for the "thick, glutinous, homogeneous substance" which fills protozoan cell bodies.[44]Although the term originally referred to the protoplasm of any protozoan, it soon came to be used in a restricted sense to designate the gelatinous contents of amoeboid cells.[10]Thirty years later, the Austrian zoologistLudwig Karl Schmardaused "sarcode" as the conceptual basis for his division Sarcodea, aphylum-level group made up of "unstable, changeable" organisms with bodies largely composed of "sarcode".[45]Later workers, including the influential taxonomistOtto Bütschli,amended this group to create the class Sarcodina,[46]ataxonthat remained in wide use throughout most of the 20th century.

Within the traditional Sarcodina, amoebae were generally divided intomorphological categories,on the basis of the form and structure of theirpseudopods.Amoebae with pseudopods supported by regular arrays ofmicrotubules(such as the freshwaterHeliozoaand marineRadiolaria) were classified asActinopoda;whereas those with unsupported pseudopods were classified asRhizopoda.[47]The Rhizopods were further subdivided into lobose, filose, and reticulose amoebae, according to the morphology of their pseudopods.

Dismantling of Sarcodina

[edit]

In the final decade of the 20th century, a series of molecular phylogenetic analyses confirmed that Sarcodina was not amonophyleticgroup. In view of these findings, the old scheme was abandoned and the amoebae of Sarcodina were dispersed among many other high-level taxonomic groups. Today, the majority of traditional sarcodines are placed in two eukaryotesupergroups:AmoebozoaandRhizaria.The rest have been distributed among theexcavates,opisthokonts,andstramenopiles.Some, like theCentrohelida,have yet to be placed in any supergroup.[10][48]

Classification

[edit]

Recent classification places the various amoeboid genera in the following groups:

Supergroups Major groups and genera Morphology
Amoebozoa
  • Lobose pseudopods (Lobosa) are blunt, and there may be one or several on a cell, which is usually divided into a layer of clear ectoplasm surrounding more granular endoplasm.
Rhizaria
  • Filose pseudopods (Filosa) are narrow and tapering. The vast majority of filose amoebae, including all those that produce shells, are placed within theCercozoatogether with various flagellates that tend to have amoeboid forms. The naked filose amoebae also includesvampyrellids.
  • Reticulose pseudopods (Endomyxa) are cytoplasmic strands that branch and merge to form a net. They are found most notably among theForaminifera,a large group of marine protists that generally produce multi-chambered shells. There are only a few sorts of naked reticulose amoebae, notably thegymnophryids,and their relationships are not certain.
  • Radiolariansare a subgroup of actinopods that are now grouped with rhizarians.
Excavata
Heterokonta
  • The heterokont chrysophyte and xanthophyte algae include some amoeboid members, the latter being poorly studied.[50]
Alveolata
  • Parasite with amoeboid life cycle stages.
Opisthokonta
Ungrouped/
unknown
  • Adelphamoeba,Astramoeba,Dinamoeba,Flagellipodium,Flamella,Gibbodiscus,Gocevia,Malamoeba,Nollandia,Oscillosignum,Paragocevia,Parvamoeba,Pernina,Pontifex,Pseudomastigamoeba,Rugipes,Striamoeba,Striolatus,Subulamoeba,Theratromyxa,Trienamoeba,Trimastigamoeba,and over 40 other genera[51]

Some of the amoeboid groups cited (e.g., part ofchrysophytes,part ofxanthophytes,chlorarachniophytes) were not traditionally included in Sarcodina, being classified asalgaeorflagellatedprotozoa.

Pathogenic interactions with other organisms

[edit]
Trophozoitesof the pathogenicEntamoeba histolyticawith ingestedred blood cells

Some amoebae can infect other organismspathogenically,causing disease:[52][53][54][55]

Amoeba have been found to harvest and grow the bacteria implicated inplague.[56]Amoebae can likewise play host to microscopic organisms that are pathogenic to people and help in spreading such microbes. Bacterial pathogens (for example,Legionella) can oppose absorption of food when devoured by amoebae.[57] The currently generally utilized and best-explored amoebae that host other organisms are Acanthamoeba castellanii and Dictyostelium discoideum.[58] Microorganisms that can overcome the defenses of one-celled organisms can shelter and multiply inside them, where they are shielded from unfriendly outside conditions by their hosts.

Meiosis

[edit]

Recent evidence indicates that several Amoebozoa lineages undergomeiosis.

Orthologsof genes employed inmeiosisof sexualeukaryoteshave recently been identified in theAcanthamoebagenome.These genes includedSpo11,Mre11,Rad50,Rad51,Rad52,Mnd1,Dmc1,MshandMlh.[59]This finding suggests that the ‘'Acanthamoeba'’ are capable of some form of meiosis and may be able to undergo sexual reproduction.

The meiosis-specificrecombinase,Dmc1,is required for efficient meiotichomologous recombination,andDmc1is expressed inEntamoeba histolytica.[60]The purified Dmc1 fromE. histolyticaformspresynapticfilaments and catalysesATP-dependenthomologous DNA pairingand DNA strand exchange over at least several thousandbase pairs.[60]The DNA pairing and strand exchange reactions are enhanced by the eukaryotic meiosis-specific recombination accessory factor (heterodimer) Hop2-Mnd1.[60]These processes are central to meiotic recombination, suggesting thatE. histolyticaundergoes meiosis.[60]

Studies ofEntamoeba invadensfound that, during the conversion from thetetraploiduninucleate trophozoiteto the tetranucleate cyst,homologous recombinationis enhanced.[61]Expression of genes with functions related to the major steps of meiotic recombination also increase during encystations.[61]These findings inE. invadens,combined with evidence from studies ofE. histolyticaindicate the presence of meiosis in theEntamoeba.

Dictyostelium discoideumin the supergroupAmoebozoacan undergo mating andsexual reproductionincluding meiosis when food is scarce.[62][63]

Since the Amoebozoa diverged early from theeukaryoticfamily tree, these results suggest that meiosis was present early in eukaryotic evolution. Furthermore, these findings are consistent with the proposal of Lahr et al.[64]that the majority of amoeboid lineages are anciently sexual.

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Further reading

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Wikisourcehas the text of the 1905New International Encyclopediaarticle "Rhizopoda".
Wikimedia Commons has media related toAmoeba.
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