An amoeba (; less commonly spelled ameba or amœba; : amoebas (less commonly, amebas) or amoebae ( amebae) ),[ "Amoeba" at Oxforddictionaries.com] often called an amoeboid, is a type of cell or unicellular organism with the ability to alter its shape, primarily by extending and retracting pseudopodia.
often use the terms "amoeboid" and "amoeba" interchangeably for any organism that exhibits amoeboid movement.
In older classification systems, most amoebae were placed in the class or subphylum Sarcodina, a grouping of single-celled organisms that possess pseudopods or move by 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 Chaos carolinense and Amoeba proteus, both of which have been widely cultivated and studied in classrooms and laboratories.
Biology
Pseudopods and movement
Amoebae do not have cell walls, which allows for free movement. Amoebae move and feed by using pseudopods, which are bulges of
cytoplasm formed by the coordinated action of
actin microfilaments pushing out the
plasma membrane that surrounds the cell.
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 amoebae" (enclosed within a hard shell), or "naked" (also known as , 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.[Kudo, Richard Roksabro. "Protozoology." Protozoology 4th Edit (1954). p. 83]
Nutrition
The food sources of amoebae vary. Some amoebae are predatory and live by consuming bacteria and other
. Some are
and eat dead organic material.
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.[Thorp, James H. (2001). Ecology and Classification of North American Freshwater Invertebrates. San Diego: Academic. p. 71. .]
Some amoebae also feed by pinocytosis, imbibing dissolved nutrients through vesicles formed within the cell membrane.
Size range
The size of amoeboid cells and species is extremely variable. The marine amoeboid
Massisteria is just 2.3 to 3
in diameter,
within the size range of many bacteria.
At the other extreme, the shells of deep-sea
can attain 20 cm in diameter. Most of the free-living freshwater amoebae commonly found in
pond life, ditches, and lakes are microscopic, but some species, such as the so-called "giant amoebae"
Pelomyxa and
Chaos carolinense, can be large enough to see with the naked eye.
Sexual reproduction
Recent evidence indicates that several Amoebozoa lineages undergo
meiosis.
Orthologs of genes employed in meiosis of sexual have recently been identified in the Acanthamoeba genome. These genes included Spo11, Mre11, Rad50, Rad51, Rad52, Mnd1, Dmc1, Msh and Mlh.
The meiosis-specific recombinase, Dmc1, is required for efficient meiotic homologous recombination, and Dmc1 is expressed in Entamoeba histolytica.
Studies of Entamoeba invadens found that, during the conversion from the Polyploid uninucleate
trophozoite to the tetranucleate cyst, homologous recombination is enhanced.
Dictyostelium discoideum in the supergroup Amoebozoa can undergo mating and sexual reproduction including meiosis when food is scarce.
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.
Ecology
Pathogenic amoebae
Some amoebae can infect other organisms
, causing disease:
[Casadevall A (2008) Evolution of intracellular pathogens. Annu Rev Microbiol 62: 19–33. 10.1146/annurev.micro.61.080706.093305 PubMed CrossRef Google][Guimaraes AJ, Gomes KX, Cortines JR, Peralta JM, Peralta RHS (2016) Acanthamoeba spp. as a universal host for pathogenic microorganisms: One bridge from environment to host virulence. Microbiological Research 193: 30–38. 10.1016/j.micres.2016.08.001 PubMed CrossRef Google][Hilbi H, Weber SS, Ragaz C, Nyfeler Y, Urwyler S (2007) Environmental predators as models for bacterial pathogenesis. Environmental microbiology 9: 563–575. 10.1111/j.1462-2920.2007.01238.x PubMed CrossRef Google]
-
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.
Amoebae have been found to harvest and grow the bacteria implicated in plague.
History of knowledge and classification
Conceptual origins
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 Ancient Greek ἀμοιβή amoibe, meaning "change") was erected by the French naturalist Bory de Saint-Vincent.[Bory de Saint-Vincent, J. B. G. M. "Essai d'une classification des animaux microscopiques." Agasse, Paris (1826).p. 28][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]
In 1841, Félix Dujardin coined the term " sarcode" (from Greek σάρξ sarx, "flesh", and εἶδος eidos, "form") for the "thick, glutinous, homogeneous substance" which fills protozoan cell bodies.
Traditional classification
For convenience, all amoebae were grouped as Sarcodina and generally divided into morphological categories, on the basis of the form and structure of their
. Amoebae with pseudopods supported by regular arrays of
(such as the freshwater
Heliozoa and marine
Radiolaria) were classified as
Actinopoda, whereas those with unsupported pseudopods were classified as
Rhizopoda.
The Rhizopods were further subdivided into lobose, filose, plasmodial and reticulose, according to the morphology of their pseudopods. During the 1980s, taxonomists reached the following classification, based exclusively on morphological comparisons:
- * Rhizopoda : amorphous amoebae that lack axopodia and move through pseudopodia.
- :*Heterolobosea : amoebae with eruptive pseudopodia, similar to the lobose ones but with a distinct movement, and usually with flagellate life stages. It was traditionally divided into those which aggregate to form fruiting bodies (Acrasida) and those that do not (Schizopyrenida).
- :*Lobosea : amoebae with lobose pseudopodia. This paraphyletic group is now represented by a big portion of the current phylum Amoebozoa, in particular the classes Tubulinea, Discosea and Cutosea.
- ::*Gymnamoebia : lobose naked amoebae. This polyphyletic group included the classic amorphous amoebae with big, blunt pseudopodia, such as Euamoebida, Leptomyxida, Centramoebida, Echinamoebida, Entamoebida, etc.
- ::*Testacealobosia : lobose testate amoebae. This polyphyletic group included three unrelated lineages of amoebozoans enclosed by tests or other complex coverings: Arcellinida, Himatismenida and Trichosida.
- :*Caryoblastea : amoebae with sparse, non-motile flagella on the surface. This group only includes the order Pelobiontida,
- :*Eumycetozoa : plasmodial amoebae with filiform subpseudopodia that produce fruiting bodies.
- :*Plasmodiophorid : endoparasitic plasmodial amoebae with minute pseudopodia. This group is now an order within Rhizaria, closely related to the endoparasites Phagomyxida.
- :*Filosa : amoebae with filose pseudopodia.
- ::*Aconchulinia : filose naked amoebae, sometimes covered in scales. This group included two unrelated taxa: the nucleariid amoebae, closely related to fungi; and most of the Vampyrellida, found in Rhizaria.
- ::*Testaceafilosia : filose testate amoebae. This group included taxa now found throughout Rhizaria, such as Gromiida and Euglyphida.
- :*Granuloreticulosea : amoebae with delicate granular pseudopodia. This group included both the Foraminifera (now in Rhizaria) and some members of Vampyrellida.
- :*Xenophyophorea : plasmodial amoebae enclosed in a branched-tube system composed of a transparent organic substance. This group is now fully integrated into the Foraminifera.
- * Actinopoda : spherical amoebae that float in the water column. This group included those organisms that have a -type appearance, with radially positioned filopodia, reticulopodia or axopodia surrounding the cell body. These were the Radiolaria, Phaeodaria, Proteomyxidea (all three now in Rhizaria), Centroplasthelida (now in Haptista), and Actinophryida (now in Stramenopiles).
Transitional period
In the final decades of the 20th century, a series of molecular phylogenetic analyses confirmed that Sarcodina was not a
monophyletic group, and that amoebae evolved from flagellate ancestors.
The protozoologist Thomas Cavalier-Smith proposed that the ancestor of most eukaryotes was an
amoeboflagellate much like modern
, which in turn gave rise to a paraphyletic Sarcodina from which other groups (e.g., alveolates, animals, plants) evolved by a secondary loss of the amoeboid phase. In his scheme, the Sarcodina were divided into the more primitive
Eosarcodina (with the phyla Reticulosa and Mycetozoa) and the more derived
Neosarcodina (with the phyla
Amoebozoa for lobose amoebae and Rhizopoda for filose amoebae).
Shortly after, phylogenetic analyses disproved this hypothesis, as non-amoeboid and amoeboflagellates were found to be completely intermingled with amoebae. With the addition of many flagellates to Rhizopoda and the removal of some amoebae, the name was rejected in favour of a new name Cercozoa. As such, both names Rhizopoda and Sarcodina were finally abandoned as formal taxa, but they remained useful as descriptive terms for amoebae.
Current classification
Today, amoebae are dispersed among many high-level taxonomic groups. The majority of traditional sarcodines are placed in two eukaryote supergroups:
Amoebozoa and
Rhizaria. The rest have been distributed among the
Excavata,
opisthokonts,
stramenopiles and minor clades.
-
Amoebozoa : includes all naked and testate lobose amoebae (traditional Lobosea) as well as the and , and a few flagellates.
-
Rhizaria : includes amoebae bearing reticulose or filose pseudopodia, the majority of which were traditionally classified as Filosea, Granuloreticulosea and Actinopoda, such as Euglyphida, Gromiida, Radiolaria, Proteomyxidea, Phaeodarea and Foraminifera (including Xenophyophorea). It also houses a large diversity of free-living flagellates, amoeboflagellates and parasites like the Plasmodiophorida.
-
Heterolobosea : amoebae with lobose pseudopodia but eruptive flow of cytoplasm. Currently it includes the aggregative Acrasida, as well as several other amoeboflagellates. They are a class of excavata closely related to Euglenozoa, with whom they share their characteristic discoidal crista.
-
Stramenopiles : although primarily composed by flagellates, it contains a few groups of amoebae. For example: the Actinophryida, an order with typical heliozoan morphology;
-
Nucleariid : also known as , includes a few filose amoebae traditionally classified within the Filosea, positioned as the sister group of Fungi.
-
Centroplasthelida : heliozoans with a centroplast from which axopodia arise.
-
Rigifilida : a small order of filose amoebae previously interpreted as nucleariids.
-
Breviatea : includes enigmatic free-living related to .
The following cladogram shows the sparse positions of amoeboid groups (in bold), based on molecular phylogenetic analyses:
Amoeboid cells in other organisms
Amoeboid cell types in multicellular organisms
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.
exhibit a
totipotent cell type known as
, capable of transforming into the feeding cells or
.
Amoeboid dispersal stages
Amoeboid stages also occur in the multicellular fungus-like protists, the so-called
slime mold. Both the plasmodial slime moulds, currently classified in the class
Myxogastria, and the cellular slime moulds 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)[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, [2] ] and the sporoplasm stage of Myxozoa and of Ascetosporea.
Further reading
External links
