Amoeboid

Foraminiferan (Ammonia tepida)

Amoeboids are single-celled life-forms characterized by an irregular shape.^[1]

"Amoeboid" and "amœba" are often used interchangeably even by biologists,^[2] and especially refer to a creature moving by using pseudopodia. Most references to "amoebas" or "amoebae" are to amoeboids in general rather than to the specific genus Amoeba. The genus Amoeba and amoeboids in general both derive their names from the ancient Greek word for change.

Structure

Amoeboids move using pseudopodia, which are bulges of cytoplasm.

Amoebas breathe using their entire cell membrane that is constantly immersed in water. Excess water can cross into the cytosol. Amoebas have a contractile vacuole to expel excess water.

Food sources vary in rhizopoda. They may consume bacteria or other protists. Some are detritivores and eat dead organic material. They extend a pair of pseudopodia around food. They fuse to make a food vacuole which then fuses with a lysosome to add digestive chemicals. Undigested food is expelled at the cell membrane.

Amoebas use pseudopodia to move and feed. They are powered by flexible microfilaments near the membrane. Microfilaments are at least 50% of the cytoskeleton. The other parts are more stiff and are composed of intermediate filaments and macrotubules. These are not used in amoeboid movement, but are stiff skeletons on which organelles are supported or can move on.

The shells of amoebas are often composed of calcium. The proteins or materials are synthesised in the cell and exported just outside the cell membrane.

Amoebas seem to have connections with two phyla of the lineage fungus-like protists. The two phyla are myxomycota (plasmodial slime molds), and acrasiomycota (cellular slime molds). These two phyla use amoeboid movement in their feeding stage. One is basically a giant multinucleate amoeba, while the other lives solitary until food runs out; in which a colony of these functions as a unit. Myxomycotes use amoeboid gametes, as well.

Diversity

They have appeared in a number of different groups. Some cells in multicellular animals may be amoeboid, for instance human white blood cells, which consume pathogens. Many protists also exist as individual amoeboid cells, or take such a form at some point in their life-cycle. The most famous such organism is Amoeba proteus; the name amoeba is variously used to describe its close relatives, other organisms similar to it, or the amoeboids in general.

As amoebas themselves are polyphyletic and subject to some imprecision in definition, the term "amoeboid" does not provide identification of an organism, and is better understood as description of locomotion.

When used in the broader sense, the term can include many different groups. One source includes 97 different genera.^[3] Others include far fewer.

In older classification systems, amoeboids have been divided into several morphological categories based on the form and structure of the pseudopods. Those where the pseudopods are supported by regular arrays of microtubules are called actinopods, and forms where they are not are called rhizopods, further divided into lobose, filose, and reticulose amoebae. There is also a strange group of giant marine amoeboids, the xenophyophores, that do not fall into any of these categories.

More modern classifications are based upon cladistics. It has been stated that most amoeboid are now grouped in Amoebozoa or Rhizaria.^[4] However, in contexts where "amoeboid" is defined more loosely, there are many amoeboid species that are in the Excavata clade.

Phylogenetic analyses place these genera into the following groups (not all of these are considered amoeboid (or "amoebas") by all sources):

Grouping	Genus	Morphology
Chromalveolate	Heterokont: Hyalodiscus, Labyrinthula Alveolata: Pfiesteria	Hyalodiscus and Pfiesteria are sometimes considered to have amoeboid characteristics.
Rhizaria	Filosa: Gyromitus Endomyxa: Vampyrella	Filose pseudopods (Filosa): Filose pseudopods are narrow and tapering. The vast majority of filose amoebae, including all those that produce shells, are placed within the Cercozoa together with various flagellates that tend to have amoeboid forms. The naked filose amoebae also includes vampyrellids. Reticulose pseudopods (Endomyxa): Reticulose pseudopods are cytoplasmic strands that branch and merge to form a net. They are found most notably among the Foraminifera, a large group of marine protists that generally produce multi-chambered shells. There are only a few sorts of naked reticulose amoeboids, notably the gymnophryids, and their relationships are not certain. Radiolarians are a subgroup of actinopods that are now grouped with rhizarians.
Excavata	Vahlkampfiidae: Naegleria, Neovahlkampfia, Paratetramitus, Paravahlkampfia, Psalteriomonas, Sawyeria, Tetramitus, Vahlkampfia, Willaertia Parabasalidea: Dientamoeba, Histomonas Other: Stachyamoeba, Rosculus, Acrasis, Heteramoeba, Learamoeba, Monopylocystis, Stygamoeba	The Percolozoa, includes protists that can transform between amoeboid and flagellate forms.
Amoebozoa	Lobosea: Acanthamoeba, Amoeba, Balamuthia, Chaos, Clydonella, Discamoeba, Echinamoeba, Filamoeba, Flabellula, Gephyramoeba, Glaeseria, Hartmannella, Hydramoeba, Korotnevella, Leptomyxa, Lingulamoeba, Mastigina, Mayorella, Metachaos, Neoparamoeba, Paramoeba, Polychaos, Phreatamoeba, Platyamoeba, Protoacanthamoeba, Rhizamoeba, Saccamoeba, Sappinia, Stereomyxa, Thecamoeba, Trichamoeba, Trichosphaerium, Unda, Vannella, Vexillifera Conosa: Endamoeba, Entamoeba, Hyperamoeba, Mastigamoeba, Mastigella, Pelomyxa	Lobose pseudopods (Lobosea): Lobose pseudopods 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.
Nucleariid	Micronuclearia, Nuclearia	Nucleariids appear to be close relatives of animals and fungi.
Ungrouped/ unknown	Adelphamoeba, Astramoeba, Cashia, Dactylamoeba, Dinamoeba, Flagellipodium, Flamella, Gibbodiscus, Gocevia, Gruberella, Hollandella, Iodamoeba, Malamoeba, Nollandia, Oscillosignum, Paragocevia, Parvamoeba, Pernina, Pontifex, Protonaegleria, Pseudomastigamoeba, Plaesiobystra, Rugipes, Striamoeba, Striolatus, Subulamoeba, Theratromyxa, Trienamoeba, Trimastigamoeba, Vampyrellium

While most morphologies can be mapped to modern classification systems, the older grouping "actinopods" is polyphyletic. Actinopods are divided into the radiolaria and heliozoa (itself a polyphyletic grouping).

Pathogenic interactions with other organisms

Some amoeboids 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

References

1. ^ "amoeboid". Memidex (WordNet) Dictionary/Thesaurus. http://www.memidex.com/amoeboid. Retrieved 2010-12-02. 
2. ^ Eric Bapteste, Henner Brinkmann, Jennifer A. Lee, Dorothy V. Moore, Christoph W. Sensen, Paul Gordon, Laure Duruflé, Terry Gaasterland, Philippe Lopez, Miklós Müller & Hervé Philippe (2001). "The analysis of 100 genes supports the grouping of three highly divergent amoebae: Dictyostelium, Entamoeba, and Mastigamoeba". Proceedings of the National Academy of Sciences 99 (3): 1414–1419. doi:10.1073/pnas.032662799. PMC 122205. PMID 11830664. http://www.pnas.org/content/99/3/1414.full.pdf. 
3. ^ "The Amoebae". http://www.bms.ed.ac.uk/research/others/smaciver/amoebae.htm. Retrieved 2009-05-02. 
4. ^ Pawlowski J, Burki F (2009). "Untangling the phylogeny of amoeboid protists". J. Eukaryot. Microbiol. 56 (1): 16–25. doi:10.1111/j.1550-7408.2008.00379.x. PMID 19335771. 

External links

• 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