An amoeba (/əˈmiːbə/; rarely spelled amœba, US English commonly ameba; plural am(o)ebas or am(o)ebae /əˈmiːbiː/), often called amoeboid, is a type of cell or organism which has the ability to alter its shape, primarily by extending and retracting pseudopods. Amoebas do not form a single taxonomic group; instead, they are found in every major lineage of eukaryotic organisms. Amoeboid cells occur not only among the protozoa, but also fungi, algae, and animals.
In older classification systems, most amoebas were placed in the class or subphylum Sarcodina, a grouping of single-celled organisms that possess pseudopods or move by protoplasmic flow. However, molecular phylogenetic studies have shown that Sarcodina is not a monophyletic group whose members share common descent. Consequently, amoeboid organisms are no longer classified together in one group.
The best known amoeboid protists are the "Giant Amoebae" Chaos carolinense and Amoeba proteus, both of which are widely cultivated and studied in classrooms and laboratories. Other well known species include the so-called "brain-eating amoeba" Naegleria fowleri, the intestinal parasite Entamoeba histolytica, which causes amoebic dysentery, and the multicellular "social amoeba" Dictyostelium discoideum.
Shape, movement and nutrition
The appearance and internal structure of pseudopods are used to distinguish groups of amoebae from one another. Amoebozoan species, such as those in the genus Amoeba, typically have bulbous (lobose) pseudopods, rounded at the ends and roughly tubular in cross-section. Cercozoan amoeboids, such as Euglypha and Gromia, have slender, thread-like (filose) pseudopods. Foraminifera emit fine, branching pseudopods that merge with one another to form net-like (reticulose) structures. Some groups, such as the Radiolaria and Heliozoa, have stiff, needle-like, radiating axopodia (actinopoda) supported from within by bundles of microtubules.
Free-living amoebae may be "testate" (enclosed within a hard shell), or "naked" (lacking any hard covering). The shells of testate amoebae may be composed of various substances, including calcium, silica, chitin, or agglutinations of found materials like small grains of sand and the frustules of diatoms.
To regulate osmotic pressure, most freshwater amoebae have a contractile vacuole which expels excess water from the cell. This organelle is necessary because freshwater has a lower concentration of solutes (such as salt) than the amoeba's own internal fluids (cytosol). Because the surrounding water is hypotonic with respect to the contents of the cell, water is transferred across the amoeba's cell membrane by osmosis. Without a contractile vacuole, the cell would fill with excess water and, eventually, burst.
Amoebae typically ingest their food by phagocytosis, extending pseudopods to encircle and engulf live prey or particles of scavenged material. Amoeboid cells do not have a mouth or cytostome, and there is no fixed place on the cell at which phagocytosis normally occurs.
Amoebae as specialized cells and life cycle stages
Some multicellular organisms have amoeboid cells only in certain phases of life, or use amoeboid movements for specialized functions. In the immune system of humans and other animals, amoeboid white blood cells pursue invading organisms, such as bacteria and pathogenic protists, and engulf them by phagocytosis.
Amoeboid stages also occur in the multicellular fungus-like protists, the so-called slime molds. Both the plasmodial slime molds, currently classified in the class Myxogastria, and the cellular slime molds of the groups Acrasida and Dictyosteliida, live as amoebae during their feeding stage. The amoeboid cells of the former combine to form a giant multinucleate organism, 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.
Other organisms may also present amoeboid cells during certain life-cycle stages, e.g., the gametes of some green algae (Zygnematophyceae), of pennate diatoms, of some foraminiferans, or the spores (or dispersal phases) of some Mesomycetozoea.
Amoebae as organisms
Early history and origins of Sarcodina
The earliest record of an amoeboid organism was produced in 1755 by August Johann Rösel von Rosenhof, who named his discovery "Der Kleine Proteus" ("the Little Proteus"). Rösel's illustrations show an unidentifiable freshwater amoeba, similar in appearance to the common species now known as Amoeba proteus. The term "Proteus animalcule" remained in use throughout the 18th and 19th centuries, as an informal name for any large, free-living amoeboid.
In 1822, the genus Amiba (from the Greek amoibè, meaning "change") was erected by the French naturalist Bory de Saint-Vincent. Bory's contemporary, C. G. Ehrenberg, adopted the genus in his own classification of microscopic creatures, but changed the spelling to Amoeba.
In 1841, Félix Dujardin coined the term "sarcode" (from Greek sarx, flesh, and eidos, form) for the "thick, glutinous, homogenous substance" which fills protozoan cell bodies. 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. Thirty years later, the Austrian zoologist Ludwig Karl Schmarda used "sarcode" as the conceptual basis for his Division Sarcodea, a phylum-level group made up of "unstable, changeable" organisms with bodies largely composed of 'sarcode.' Later workers, including the influential taxonomist Otto Bütschli, emended this group to create the class or subphylum Sarcodina, a taxon that remained in wide use throughout most of the 20th century.
Within the traditional Sarcodina, amoebae were generally divided into morphological categories, on the basis of the form and structure of their pseudopods. Amoebae with pseudopods supported by regular arrays of microtubules (such as the freshwater Heliozoa and marine Radiolaria) were classified as Actinopods; whereas those with unsupported pseudopods were classified as Rhizopods. The Rhizopods were further subdivided into lobose, filose, and reticulose amoebae, according to the morphology of their pseudopods.
Dismantling of Sarcodina
In the final decade of the 20th century, a series of molecular phylogenetic analyses confirmed that Sarcodina was not a monophyletic group. 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 eukaryote supergroups: Amoebozoa and Rhizaria. The rest have been distributed among the excavates, opisthokonts, and stramenopiles. Some, like the Centrohelida, have yet to be placed in any supergroup.
Recent classification places the various amoeboid genera in the following groups:
|Supergroups||Major Groups and Genera||Morphology|
|Chromalveolate||Heterokont: Hyalodiscus, Labyrinthula
|Adelphamoeba, Astramoeba, Cashia, Dinamoeba, Flagellipodium, Flamella, Gibbodiscus, Gocevia, Hollandella, Iodamoeba, Malamoeba, Nollandia, Oscillosignum, Paragocevia, Parvamoeba, Pernina, Pontifex, Protonaegleria, Pseudomastigamoeba, Rugipes, Striamoeba, Striolatus, Subulamoeba, Theratromyxa, Trienamoeba, Trimastigamoeba, Vampyrellium|
Pathogenic interactions with other organisms
Some amoebae can infect other organisms pathogenically, causing disease:
- Entamoeba histolytica is the cause of amoebiasis, or amoebic dysentery.
- Naegleria fowleri (the "brain-eating amoeba") is a fresh-water-native species that can be fatal to humans if introduced through the nose.
- Acanthamoeba can cause amoebic keratitis and encephalitis in humans.
- Balamuthia mandrillaris is the cause of (often fatal) granulomatous amoebic meningoencephalitis
Recent evidence indicates that several Amoebozoa lineages undergo meiosis.
Orthologs of genes employed in meiosis of sexual eukaryotes have recently been identified in the Acanthamoeba genome. These genes included Spo11, Mre11, Rad50, Rad51, Rad52, Mnd1, Dmc1, Msh and Mlh. This finding suggests that the ‘’Acanthamoeba‘’ are capable of some form of meiosis and may be able to undergo sexual reproduction.
The meiosis-specific recombinase, Dmc1, is required for efficient meiotic homologous recombination, and Dmc1 is expressed in Entamoeba histolytica. The purified Dmc1 from E. histolytica forms presynaptic filaments and catalyzes ATP-dependent homologous DNA pairing and DNA strand exchange over at least several thousand base pairs. The DNA pairing and strand exchange reactions are enhanced by the eukaryotic meiosis-specific recombination accessory factor (heterodimer) Hop2-Mnd1. These processes are central to meiotic recombination, suggesting that E. histolytica undergoes meiosis.
Studies of Entamoeba invadens found that, during the conversion from the tetraploid uninucleate trophozoite to the tetranucleate cyst, homologous recombination is enhanced. Expression of genes with functions related to the major steps of meiotic recombination also increase during encystations. These findings in E. invadens, combined with evidence from studies of E. histolytica indicate the presence of meiosis in the Entamoeba.
Since the Amoebozoa diverged early from the eukaryotic family tree, these results suggest that meiosis was present early in eukaryotic evolution. Furthermore, these findings are consistent with the proposal of Lahr et al. that the majority of amoeboid lineages are anciently sexual.
- "amoeba" at Oxforddictionaries.com
- Singleton, Paul (2006). Dictionary of Microbiology and Molecular Biology, 3rd Edition, revised. Chichester, UK: John Wiley & Sons. p. 32. ISBN 978-0-470-03545-0.
- Marée, Athanasius FM, and Paulien Hogeweg. "How amoeboids self-organize into a fruiting body: multicellular coordination in Dictyostelium discoideum." Proceedings of the National Academy of Sciences 98.7 (2001): 3879–3883.
- Mackerras, M. J., and Q. N. Ercole. "Observations on the action of paludrine on malarial parasites." Transactions of the Royal Society of Tropical Medicine and Hygiene 41.3 (1947): 365–376.
- Jan Pawlowski: The twilight of Sarcodina: a molecular perspective on the polyphyletic origin of amoeboid protists. Protistology, Band 5, 2008, S. 281–302. (pdf, 570 kB)
- Alberts Eds.; et al. (2007). Molecular Biology of the Cell 5th Edition. New York: Garland Science. p. 1037. ISBN 9780815341055.
- Margulis, Lynn (2009). Kingdoms and Domains. Academic Press. pp. 206–7. ISBN 978-0-12-373621-5.
- Ogden, C. G. (1980). An Atlas of Freshwater Testate Amoeba. Oxford, London, Glasgow: Oxford University Press, for British Museum (Natural History). pp. 1–5. ISBN 0198585020.
- Alberts Eds.; et al. (2007). Molecular Biology of the Cell 5th Edition. New York: Garland Science. p. 663. ISBN 9780815341055.
- Kudo, Richard Roksabro. "Protozoology." Protozoology 4th Edit (1954). p. 83
- Thorp, James H. (2001). Ecology and Classification of North American Freshwater invertebrates. San Diego: Academic. p. 71. ISBN 0-12-690647-5.
- Jeon, Kwang W. (1973). Biology of Amoeba. New York: Academic Press. p. 100.
- Friedl, Peter, Stefan Borgmann, and Eva-B. Bröcker. "Amoeboid leukocyte crawling through extracellular matrix: lessons from the Dictyostelium paradigm of cell movement." Journal of leukocyte biology 70.4 (2001): 491–509.
- Nakagaki; et al. (2000). "Intelligence: Maze-solving by an amoeboid organism". Nature 407 (6803): 470. doi:10.1038/35035159. PMID 11028990. Retrieved Sep 14, 2014.
- Wehr, John D. (2003). Freshwater Algae of North America. San Diego and London: Academic Press. p. 353. ISBN 978-0-12-741550-5.
- "Algae World: diatom sex and life cycles". Algae World. Royal Botanic Garden Edinburgh. Retrieved March 1, 2015.
- Sen Gupta, Barun K. (2002). Modern Foraminifera. Springer. p. 16. ISBN 978-1-4020-0598-5.
- Valle, L.G. (2014). "New species of Paramoebidium (trichomycetes, Mesomycetozoea) from the Mediterranean, with comments about the amoeboid cells in Amoebidiales". Mycologia. doi:10.3852/13-153. Retrieved March 1, 2015.
- Taylor, J. W. & Berbee, M. L. (2014). Fungi from PCR to Genomics: The Spreading Revolution in Evolutionary Biology. In: Systematics and Evolution. Springer Berlin Heidelberg. p. 52, 
- Rosenhof, R. (1755). Monatlich herausgegebene Insektenbelustigungen, vol. 3, p. 621, .
- Jeon, Kwang W. (1973). Biology of Amoeba. New York: Academic Press. pp. 2–3, .
- McAlpine, Daniel (1881). Biological atlas: a guide to the practical study of plants and animals. Edinburgh and London: W. & A. K. Johnston. p. 17.
- Bory de Saint-Vincent, J. B. G. M. "Essai d'une classification des animaux microscopiques." Agasse, Paris (1826).p. 28
- McGrath, Kimberley; Blachford, Stacey, eds. (2001). Gale Encyclopedia of Science Vol. 1: Aardvark-Catalyst (2nd ed.). Gale Group. ISBN 0-7876-4370-X. OCLC 46337140.
- Ehrenberg, Christian Gottfried. Organisation, systematik und geographisches verhältniss der infusionsthierchen: Zwei vorträge, in der Akademie der wissenschaften zu Berlin gehalten in den jahren 1828 und 1830. Druckerei der Königlichen akademie der wissenschaften, 1832. p. 59
- Dujardin, Felix (1841). Histoire Naturelle des Zoophytes Infusoires. Paris: Librarie Encyclopedique de Roret. p. 26.
- Schmarda, Ludwig Karl (1871). Zoologie. W. Braumüller. p. 156.
- Bütschli, Otto (1882). Klassen und Ordnungen des Thier-Reichs I. Abteilung: Sarkodina und Sporozoa. Paleontologische Entwicklung der Rhisopoda von C. Scwager. p. 1.
- Calkins, Gary N. (1909). Protozoölogy. New York: Lea & Febiger. pp. 38–40.
- Adl, Sina M.; et al. (2012). "The Revised Classification of Eukaryotes". Journal of Eukaryotic Microbiology. doi:10.1111/j.1550-7408.2012.00644.x.
- Park, J. S.; Simpson, A. G. B.; Brown, S.; Cho, B. C. (2009). "Ultrastructure and Molecular Phylogeny of two Heterolobosean Amoebae, Euplaesiobystra hypersalinica gen. Et sp. Nov. And Tulamoeba peronaphora gen. Et sp. Nov., Isolated from an Extremely Hypersaline Habitat". Protist 160 (2): 265–283. doi:10.1016/j.protis.2008.10.002. PMID 19121603.
- Khan NA, Siddiqui R (2015). "Is there evidence of sexual reproduction (meiosis) in Acanthamoeba?". Pathog Glob Health 109 (4): 193–5. doi:10.1179/2047773215Y.0000000009. PMID 25800982.
- Kelso AA, Say AF, Sharma D, Ledford LL, Turchick A, Saski CA, King AV, Attaway CC, Temesvari LA, Sehorn MG (2015). "Entamoeba histolytica Dmc1 Catalyzes Homologous DNA Pairing and Strand Exchange That Is Stimulated by Calcium and Hop2-Mnd1". PLoS ONE 10 (9): e0139399. doi:10.1371/journal.pone.0139399. PMC 4589404. PMID 26422142.
- Singh N, Bhattacharya A, Bhattacharya S (2013). "Homologous recombination occurs in Entamoeba and is enhanced during growth stress and stage conversion". PLoS ONE 8 (9): e74465. doi:10.1371/journal.pone.0074465. PMC 3787063. PMID 24098652.
- Flowers JM, Li SI, Stathos A, Saxer G, Ostrowski EA, Queller DC, Strassmann JE, Purugganan MD (2010). "Variation, sex, and social cooperation: molecular population genetics of the social amoeba Dictyostelium discoideum". PLoS Genet. 6 (7): e1001013. doi:10.1371/journal.pgen.1001013. PMC 2895654. PMID 20617172.
- O'Day DH, Keszei A (2012). "Signalling and sex in the social amoebozoans". Biol Rev Camb Philos Soc 87 (2): 313–29. doi:10.1111/j.1469-185X.2011.00200.x. PMID 21929567.
- Lahr DJ, Parfrey LW, Mitchell EA, Katz LA, Lara E (2011). "The chastity of amoebae: re-evaluating evidence for sex in amoeboid organisms". Proc. Biol. Sci. 278 (1715): 2081–90. doi:10.1098/rspb.2011.0289. PMC 3107637. PMID 21429931.
- The Amoebae website brings together information from published sources.
- Amoebas are more than just blobs
- Sun Animacules and Amoebas
- Molecular Expressions Digital Video Gallery: Pond Life – Amoeba (Protozoa) Some good, informative Amoeba videos.
- Amoebae: Protists Which Move and Feed Using Pseudopodia at the Tree of Life web project
- Siemensma, F. Microworld: world of amoeboid organisms. Arcella.nl.
- Pawlowski, J. & Burki, F. (2009). Untangling the Phylogeny of Amoeboid Protists. Journal of Eukaryotic Microbiology 56.1: 16–25, .
- Walochnik, J. & Aspöck, H. (2007). Amöben: Paradebeispiele für Probleme der Phylogenetik, Klassifikation und Nomenklatur. Denisia 20: 323–350, .