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An amoeba (; less commonly spelt ameba or amœba; plural ''am(o)ebas'' or ''am(o)ebae'' ), often called an amoeboid, is a type of cell or
unicellular organism A unicellular organism, also known as a single-celled organism, is an organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemica ...
which has the ability to alter its shape, primarily by extending and retracting pseudopods. Amoebae do not form a single taxonomic group; instead, they are found in every major lineage of
eukaryotic Eukaryotes () are organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interact ...

eukaryotic
organisms. Amoeboid cells occur not only among the
protozoa Protozoa (singular protozoon or protozoan, plural protozoa or protozoans) is an informal term for a group of single-celled eukaryote Eukaryotes () are organism In biology, an organism () is any organic, life, living system that ...

protozoa
, but also in
fungi A fungus (plural The plural (sometimes abbreviated An abbreviation (from Latin ''brevis'', meaning ''short'') is a shortened form of a word or phrase, by any method. It may consist of a group of letters, or words taken from the full ...

fungi
,
algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic Photosynthesis is a process used by plants and other organisms to convert Conversion or convert may refer to: Arts, entertainment, and media * Co ...

algae
, and
animals Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells ...

animals
.
Microbiologist A microbiologist (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is ...

Microbiologist
s often use the terms "amoeboid" and "amoeba" interchangeably for any organism that exhibits
amoeboid movementAmoeboid movement is the most common mode of locomotion in eukaryotic cells. It is a crawling-like type of movement accomplished by protrusion of cytoplasm In cell biology, the cytoplasm is all of the material within a Cell (biology), cell, enclos ...
. In older classification systems, most amoebae were placed in the
class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used differently f ...
or
subphylum In zoological nomenclature The International Code of Zoological Nomenclature (ICZN) is a widely accepted Convention (norm), convention in zoology that rules the formal scientific name, scientific naming of organisms treated as animals. It is als ...
Sarcodina, a grouping of single-celled organisms that possess pseudopods or move by
protoplasmProtoplasm (/prəʊtə(ʊ)ˌplaz(ə)m/, plural protoplasms) is the living part of a cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or ...

protoplasm
ic flow. However,
molecular phylogenetic Molecular phylogenetics () is the branch of phylogeny that analyzes genetic, hereditary molecular differences, predominately in DNA sequences, to gain information on an organism's evolutionary relationships. From these analyses, it is possible to ...
studies have shown that Sarcodina is not a
monophyletic In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic grou ...

monophyletic
group whose members share
common descent Common descent is a concept in evolutionary biology Evolutionary biology is the subfield of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chem ...
. Consequently, amoeboid organisms are no longer classified together in one group.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)
The best known amoeboid
protists A protist () is any eukaryotic organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties of life. It is a synonym f ...
are ''
Chaos carolinense ''Chaos'' is a genus of Unicellular organism, single-celled Amoeba, amoeboid Organism, organisms in the family Amoebidae. The largest and best-known species, the so-called "giant amoeba" ''Chaos carolinensis'', can reach lengths of 5 mm, alth ...

Chaos carolinense
'' and ''
Amoeba proteus ''Amoeba proteus (type species of Amoeba)'', of which ''Chaos diffluens'' is one of many synonyms, is a large amoeba related to another genus of giant amoebae, '' Chaos''. It can be bought at science supply stores. This protozoan uses extensions ...

Amoeba proteus
'', both of which have been widely cultivated and studied in classrooms and laboratories. Other well known species include the so-called "brain-eating amoeba" ''
Naegleria fowleri ''Naegleria fowleri'', colloquially known as a "brain-eating amoeba", is a species of the genus ''Naegleria'', belonging to the phylum Percolozoa, which is technically not classified as true amoeba, but a shapeshifting amoeboflagellate Excavata ...
'', the intestinal parasite ''
Entamoeba histolytica ''Entamoeba histolytica'' is an anaerobic organism, anaerobic parasitic amoebozoan, part of the genus ''Entamoeba''. Predominantly infecting humans and other primates causing amoebiasis, ''E. histolytica'' is estimated to infect about 35-50 mill ...

Entamoeba histolytica
'', which causes
amoebic dysentery Amoebiasis or amoebic dysentery Dysentery () is a type of gastroenteritis that results in bloody diarrhea. Other symptoms may include fever, abdominal pain, and a feeling of incomplete defecation. Complications may include dehydration In ...
, and the multicellular "social amoeba" or
slime mould Slime mold or slime mould is an informal name given to several kinds of unrelated eukaryotic Eukaryotes () are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual ...
''
Dictyostelium discoideum ''Dictyostelium discoideum'' is a species of soil-dwelling amoeba An amoeba (; less commonly spelt ameba or amœba; plural ''am(o)ebas'' or ''am(o)ebae'' ), often called an amoeboid, is a type of cell Cell most often refers to: * Cell (bio ...
''.


Shape, movement and nutrition

Amoebae do not have cell walls, which allows for free movement. Amoebae move and feed by using pseudopods, which are bulges of
cytoplasm In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes ...
formed by the coordinated action of
actin Actin is a protein family, family of Globular protein, globular multi-functional proteins that form microfilaments. It is found in essentially all Eukaryote, eukaryotic cells, where it may be present at a concentration of over 100 Micromolar, μ ...
microfilaments Actin cytoskeleton of mouse embryo fibroblasts, stained with Fluorescein isothiocyanate-phalloidin, 250px Microfilaments, also called actin filaments, are protein filaments in the cytoplasm of eukaryotic cell (biology), cells that form part of the ...

microfilaments
pushing out the
plasma membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane A biological membrane, biomembrane or cell membrane is a selectively permeable membra ...
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 An amoeba (; less commonly spelt ameba or amœba; plural ''am(o)ebas'' or ''am(o)ebae'' ), often called an amoeboid, is a type of cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Cl ...
'', typically have bulbous (lobose) pseudopods, rounded at the ends and roughly tubular in cross-section. Cercozoan amoeboids, such as ''
Euglypha
Euglypha
'' and ''
Gromia ''Gromia'' is a genus of protists, closely related to foraminifera Foraminifera (; Latin for "hole bearers"; informally called "forams") are single-celled organisms, members of a phylum or class (biology), class of Amoeba, amoeboid protists cha ...
'', have slender, thread-like (filose) pseudopods.
Foraminifera Foraminifera (; Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the ...
emit fine, branching pseudopods that merge with one another to form net-like (reticulose) structures. Some groups, such as the
Radiolaria The Radiolaria, also called Radiozoa, are protozoa Protozoa (also protozoan, plural protozoans) is an informal term for a group of single-celled eukaryotes, either free-living or parasitic, which feed on organic matter such as other microor ...

Radiolaria
and
Heliozoa Heliozoa, commonly known as sun-animalcules, are microbial eukaryotes Eukaryotes () are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embod ...
, have stiff, needle-like, radiating
axopodia
axopodia
(actinopoda) supported from within by bundles of
microtubules Microtubules are polymer A polymer (; Greek ''poly- Poly, from the Greek :wikt:πολύς, πολύς meaning "many" or "much", may refer to: Businesses * China Poly Group Corporation, a Chinese business group, and its subsidiaries: ** Po ...

microtubules
. Free-living amoebae may be "
testate Intestacy is the condition of the estate (law), estate of a person who dies without having in force a valid Will and testament, will or other binding declaration. Alternatively this may also apply where a will or declaration has been made, but o ...
" (enclosed within a hard shell), or "naked" (also known as gymnamoebae, lacking any hard covering). The shells of testate amoebae may be composed of various substances, including
calcium Calcium is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elem ...

calcium
,
silica Silicon dioxide, also known as silica, is an oxide An oxide () is a chemical compound A chemical compound is a chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any su ...

silica
,
chitin Chitin (carbon, C8hydrogen, H13oxygen, O5nitrogen, N)n ( ) is a long-chain polymer of N-Acetylglucosamine, ''N''-acetylglucosamine, an amide derivative of glucose. The second most abundant polysaccharide in nature (behind only cellulose), it ...

chitin
, or agglutinations of found materials like small grains of sand and the frustules of
diatoms Diatoms (''diá-tom-os'' 'cut in half', from ''diá'', 'through' or 'apart', and the root of ''tém-n-ō'', 'I cut') are a major group of algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic ...

diatoms
. To regulate
osmotic pressure Osmotic pressure is the minimum pressure Pressure (symbol: ''p'' or ''P'') is the force In physics, a force is an influence that can change the motion (physics), motion of an Physical object, object. A force can cause an object with ...

osmotic pressure
, most freshwater amoebae have a contractile
vacuole A vacuole () is a membrane-bound organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts ...
which expels excess water from the cell. This
organelle In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, ...
is necessary because freshwater has a lower concentration of
solutes In chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence i ...
(such as salt) than the amoeba's own internal fluids (
cytosol The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a s ...
). Because the surrounding water is
hypotonic Tonicity is a measure of the effective osmotic pressure gradient; the water potential of two solution Image:SaltInWaterSolutionLiquid.jpg, Making a saline water solution by dissolving Salt, table salt (sodium chloride, NaCl) in water. The ...

hypotonic
with respect to the contents of the cell, water is transferred across the amoeba's cell membrane by
osmosis Osmosis (, ) is the spontaneous net movement or diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration In chemistry Chemistry is the ...

osmosis
. 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 the
tonicity Tonicity is a measure of the effective osmotic pressure gradient; the water potential Water potential is the potential energy In physics, potential energy is the energy held by an object because of its position relative to other objects, ...

tonicity
of the surrounding water.


Diet

The food sources of amoebae vary. Some amoebae are predatory and live by consuming bacteria and other
protist A protist () is any eukaryotic Eukaryotes () are organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are c ...
s. Some are
detritivore Detritivores (also known as detrivores, detritophages, detritus feeders, or detritus eaters) are heterotrophs that obtain nutrients by consuming detritus (decomposing plant and animal parts as well as feces). There are many kinds of invertebrates, ...
s and eat dead organic material. Amoebae typically ingest their food by
phagocytosis Phagocytosis () is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis Endocytosis is a cellular process i ...

phagocytosis
, extending pseudopods to encircle and engulf live prey or particles of scavenged material. Amoeboid cells do not have a mouth or
cytostome A cytostome (from ''cyto-'', cell and ''stome-'', mouth) or cell mouth is a part of a cell specialized for phagocytosis Phagocytosis () is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving ri ...
, and there is no fixed place on the cell at which phagocytosis normally occurs. Some amoebae also feed by
pinocytosisImage:Pinocytosis.svg, 250px, Pinocytosis In cellular biology, pinocytosis, otherwise known as fluid endocytosis and bulk-phase pinocytosis, is a mode of endocytosis in which small particles suspended in extracellular fluid are brought into the cell ...

pinocytosis
, imbibing dissolved nutrients through
vesicles Vesicle may refer to: ; In cellular biology or chemistry * Vesicle (biology and chemistry), a supramolecular assembly of lipid molecules, like a cell membrane * Synaptic vesicle ; In human embryology * Vesicle (embryology), bulge-like features of ...
formed within the cell membrane.


Size range

The size of amoeboid cells and species is extremely variable. The marine amoeboid ''
Massisteria voersi ''Massisteria'' is a genus of Cercozoa. The genus has two described species, ''Massisteria marina'' and ''Massisteria voersi''. References

Sarcomonadea Cercozoa genera {{Cercozoa-stub ...
'' is just 2.3 to 3
micrometre The micrometre ( international spelling as used by the International Bureau of Weights and Measures The International Bureau of Weights and Measures (french: Bureau international des poids et mesures, BIPM) is an intergovernmental orga ...
s in diameter, within the size range of many bacteria. At the other extreme, the shells of deep-sea
xenophyophore Xenophyophorea is a clade of foraminifera Foraminifera (; Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, kn ...

xenophyophore
s can attain 20 cm in diameter. Most of the free-living freshwater amoebae commonly found in pond water, ditches, and lakes are
microscopic The microscopic scale (from , ''mikrós'', "small" and σκοπέω, ''skopéō'' "look") is the scale of objects and events smaller than those that can easily be seen by the naked eye Naked eye, also called bare eye or unaided eye, is the pr ...
, but some species, such as the so-called "giant amoebae" ''Pelomyxa palustris'' and ''Chaos carolinense'', can be large enough to see with the naked eye.


Amoebae as specialized cells and life cycle stages

Some
multicellular organisms Multicellular organisms are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties of life. It is a synonym for "Out ...
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 White blood cells, also called leukocytes or leucocytes, are the cell (biology), cells of the immune system that are involved in protecting the body against both infectious disease and foreign invaders. All white blood cells are produced and de ...
pursue invading organisms, such as bacteria and pathogenic protists, and engulf them by
phagocytosis Phagocytosis () is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis Endocytosis is a cellular process i ...

phagocytosis
. Amoeboid stages also occur in the multicellular fungus-like protists, the so-called
slime mould Slime mold or slime mould is an informal name given to several kinds of unrelated eukaryotic Eukaryotes () are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual ...

slime mould
s. Both the plasmodial slime moulds, currently classified in the class
Myxogastria Myxogastria/Myxogastrea (myxogastrids, ICZN The International Code of Zoological Nomenclature (ICZN) is a widely accepted convention Convention may refer to: * Convention (norm), a custom or tradition, a standard of presentation or conduct ...
, 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 Multinucleate cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious recluse lives * Prison cell, a roo ...
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 Zygnematophyceae (or Conjugatophyceae) is a class of green algae The green algae (singular: green alga) are a large, informal grouping of algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic ...
) and pennate
diatoms Diatoms (''diá-tom-os'' 'cut in half', from ''diá'', 'through' or 'apart', and the root of ''tém-n-ō'', 'I cut') are a major group of algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic ...

diatoms
, the spores (or dispersal phases) of some
Mesomycetozoea The Mesomycetozoea (or DRIP clade A clade (; from grc, , ''klados'', "branch"), also known as a monophyletic group or natural group, is a group of organisms that are monophyly, monophyletic—that is, composed of a common ancestor and all it ...
, and the
sporoplasm Sporoplasm is an infectious material present in the cytoplasm of various Fungus, fungi-like organisms, such as members of class ''Microsporidia''. Sporoplasm is defined as a mass of protoplasm that gives rise to or forms a spore. The protoplasmic ...
stage of
Myxozoa Myxozoa (etymology Etymology ()The New Oxford Dictionary of English ''The'' () is a grammatical article Article often refers to: * Article (grammar) An article is any member of a class of dedicated words that are used with noun phras ...
and of
Ascetosporea The Ascetosporea are a group of eukaryote Eukaryotes () are organisms whose Cell (biology), cells have a cell nucleus, nucleus enclosed within a nuclear envelope. Eukaryotes belong to the Domain (biology), domain Eukaryota or Eukarya; their ...
.


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 ''Amoeba proteus (type species of Amoeba)'', of which ''Chaos diffluens'' is one of many synonyms, is a large amoeba related to another genus of giant amoebae, '' Chaos''. It can be bought at science supply stores. This protozoan uses extensions ...

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#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 million as of ...
ἀμοιβή ''amoibe'', meaning "change") was erected by the French naturalist
Bory de Saint-Vincent Jean-Baptiste Geneviève Marcellin Bory de Saint-Vincent was a France, French natural history, naturalist, Officer (armed forces), officer and politician. He was born on July 6, 1778 in Agen (Lot-et-Garonne) and died on December 22, 1846 in Paris. B ...
. 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 σάρξ ''sarx'', "flesh," and εἶδος ''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 Ludwig Karl Schmarda (23 August 1819 – 7 April 1908) was an Austrians, Austrian natural science, naturalist and traveler, born at Olomouc, Olmütz, Moravia. Early life and education Schmarda was born at Olomouc, Olmütz where he attended the Gr ...
used "sarcode" as the conceptual basis for his division Sarcodea, a
phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of number. The plural of a noun typically denotes a q ...
-level group made up of "unstable, changeable" organisms with bodies largely composed of "sarcode". Later workers, including the influential taxonomist , emended this group to create the class Sarcodina, a
taxon In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechani ...
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
pseudopod A pseudopod or pseudopodium (plural: pseudopods or pseudopodia) is a temporary arm-like projection of a eukaryotic Eukaryotes () are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') ...
s. Amoebae with pseudopods supported by regular arrays of
microtubule Microtubules are polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules, or macromolecules, composed of many Repeat unit, rep ...

microtubule
s (such as the freshwater
Heliozoa Heliozoa, commonly known as sun-animalcules, are microbial eukaryotes Eukaryotes () are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embod ...
and marine
Radiolaria The Radiolaria, also called Radiozoa, are protozoa Protozoa (also protozoan, plural protozoans) is an informal term for a group of single-celled eukaryotes, either free-living or parasitic, which feed on organic matter such as other microor ...

Radiolaria
) were classified as
Actinopoda An amoeba (; less commonly spelt ameba or amœba; plural ''am(o)ebas'' or ''am(o)ebae'' ), often called an amoeboid, is a type of Cell (biology), cell or unicellular organism which has the ability to alter its shape, primarily by extending and r ...
; whereas those with unsupported pseudopods were classified as Rhizopoda. 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 In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic grou ...

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 Amoebozoa is a major Taxonomy (biology), taxonomic group containing about 2,400 described species of Amoeba, amoeboid protists, often possessing blunt, fingerlike, Pseudopod#Morphology, lobose pseudopods and tubular mitochondrial cristae. In most ...
and
Rhizaria The Rhizaria are an ill-defined but species-rich supergroup of mostly unicellular A unicellular organism, also known as a single-celled organism, is an organism that consists of a single cell (biology), cell, unlike a multicellular organism that ...
. The rest have been distributed among the excavates,
opisthokonts The opisthokonts (Greek language , Greek: (''opísthios'')="rear, posterior" + (''kontós'')="pole" i.e. "flagellum") are a broad group of eukaryotes, including both the animal and fungus kingdom (biology) , kingdoms. The opisthokonts, previousl ...
, and
stramenopiles Stramenopile is a clade A clade (; from grc, , ''klados'', "branch"), also known as a monophyletic group or natural group, is a group of organisms that are monophyly, monophyletic—that is, composed of a common ancestor and all its lineage ( ...
. Some, like the
Centrohelida The centrohelids or centroheliozoa are a large group of heliozoan protist A protist () is any eukaryotic organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous ...
, have yet to be placed in any supergroup.


Classification

Recent classification places the various amoeboid genera in the following groups: Some of the amoeboid groups cited (e.g., part of chrysophytes, part of xanthophytes,
chlorarachniophyte The chlorarachniophytes are a small group of exclusively marine alga Algae (; singular alga ) is an informal term for a large and diverse group of photosynthesis, photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes ...
s) were not traditionally included in Sarcodina, being classified as
algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic Photosynthesis is a process used by plants and other organisms to convert Conversion or convert may refer to: Arts, entertainment, and media * Co ...

algae
or
flagellate 's '' Artforms of Nature'', 1904 (''Giardia lamblia'') ('' Chlamydomonas'') A flagellate is a cell or organism with one or more whip-like Appendage, appendages called flagellum, flagella. The word ''flagellate'' also describes a particular const ...
d protozoa.


Pathogenic interactions with other organisms

Some amoebae can infect other organisms
pathogen In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a Germ theory ...
ically, causing disease: *''
Entamoeba histolytica ''Entamoeba histolytica'' is an anaerobic organism, anaerobic parasitic amoebozoan, part of the genus ''Entamoeba''. Predominantly infecting humans and other primates causing amoebiasis, ''E. histolytica'' is estimated to infect about 35-50 mill ...

Entamoeba histolytica
'' is the cause of
amoebiasis Amoebiasis or amoebic dysentery Dysentery () is a type of gastroenteritis Gastroenteritis, also known as infectious diarrhea and gastro, is inflammation of the gastrointestinal tract The gastrointestinal tract, (GI tract, GIT, digest ...
, or amoebic dysentery. *''
Naegleria fowleri ''Naegleria fowleri'', colloquially known as a "brain-eating amoeba", is a species of the genus ''Naegleria'', belonging to the phylum Percolozoa, which is technically not classified as true amoeba, but a shapeshifting amoeboflagellate Excavata ...
'' (the "brain-eating amoeba") is a fresh-water-native species that can be fatal to humans if introduced through the nose. *''
Acanthamoeba ''Acanthamoeba'' is a genus of amoebae that are commonly recovered from soil, fresh water, and other habitats. ''Acanthamoeba'' has two evolutive forms, the metabolically active trophozoite and a dormant, stress-resistant cyst. Trophozoites ar ...
'' can cause amoebic
keratitis Keratitis is a condition in which the eye Eyes are organs of the visual system The visual system comprises the sensory organ (the eye) and parts of the central nervous system (the retina containing photoreceptor cells, the optic nerve, ...
and
encephalitis Encephalitis is inflammation Inflammation (from la, inflammatio) is part of the complex biological response of body tissues to harmful stimuli, such as pathogen In biology Biology is the natural science that studies life and living ...
in humans. *''
Balamuthia mandrillaris ''Balamuthia mandrillaris'' is a free-living amoeba An amoeba (; less commonly spelt ameba or amœba; plural ''am(o)ebas'' or ''am(o)ebae'' ), often called an amoeboid, is a type of cell or unicellular organism which has the ability to alte ...
'' is the cause of (often fatal) granulomatous amoebic meningoencephalitis. * Amoeba have been found t
harvest and grow
the bacteria implicated in plague. *Amoebae can likewise play host to microscopic organisms that are pathogenic to people and help in spreading such microbes. Bacterial pathogens (for example, ''
Legionella ''Legionella'' is a genus Genus (plural genera) is a taxonomic rank Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classification. The term may also refer to ...
'') can oppose absorption of food when devoured by amoebae. *The presently generally utilized and best-explored amoebae that host other organisms are Acanthamoeba castellanii and Dictyostelium discoideum. *Microorganisms that can overcome one-celled critters' guards increase a shelter wherein to multiply, where they are shielded from unfriendly outside conditions by their accidental hosts.


Meiosis

Recent evidence indicates that several Amoebozoa lineages undergo
meiosis Meiosis (; , because it is a reductional division) is a special type of of in organisms used to produce the , such as or . It involves two rounds of division that ultimately result in four cells with only one copy of each (). Additionall ...

meiosis
.
Orthologs Sequence homology is the biological homology between DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of f ...
of genes employed in
meiosis Meiosis (; , because it is a reductional division) is a special type of of in organisms used to produce the , such as or . It involves two rounds of division that ultimately result in four cells with only one copy of each (). Additionall ...

meiosis
of sexual
eukaryote Eukaryotes () are organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interact ...

eukaryote
s have recently been identified in the ''
Acanthamoeba ''Acanthamoeba'' is a genus of amoebae that are commonly recovered from soil, fresh water, and other habitats. ''Acanthamoeba'' has two evolutive forms, the metabolically active trophozoite and a dormant, stress-resistant cyst. Trophozoites ar ...
''
genome In the fields of molecular biology Molecular biology is the branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, M ...

genome
. These genes included ''
Spo11 Spo11 is a protein that in humans is encoded by the ''SPO11'' gene. Spo11, in a complex with mTopVIB, creates double strand breaks to initiate meiotic genetic recombination, recombination. Its active site contains a tyrosine which ligates and diss ...
,
Mre11 Double-strand break repair protein MRE11 is an enzyme Enzymes () are s that act as s (biocatalysts). Catalysts accelerate . The molecules upon which enzymes may act are called , and the enzyme converts the substrates into different molecule ...
,
Rad50 DNA repair protein RAD50, also known as RAD50, is a protein that in humans is encoded by the ''RAD50'' gene. Function The protein encoded by this gene is highly similar to ''Saccharomyces cerevisiae'' Rad50, a protein involved in DNA repair#Do ...
,
Rad51 DNA repair protein RAD51 homolog 1 is a protein encoded by the gene In biology, a gene (from ''genos'' "...Wilhelm Johannsen coined the word gene to describe the Mendelian_inheritance#History, Mendelian units of heredity..." (Greek language ...
,
Rad52 RAD52 homolog (S. cerevisiae), also known as RAD52, is a protein Proteins are large s and s that comprise one or more long chains of . Proteins perform a vast array of functions within organisms, including , , , providing and , and from on ...
, Mnd1,
Dmc1 Meiotic recombination protein DMC1/LIM15 homolog is a protein Proteins are large s and s that comprise one or more long chains of . Proteins perform a vast array of functions within organisms, including , , , providing and , and from one ...
, Msh'' and ''''. 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 Recombinases are genetic recombination Genetic recombination (also known as genetic reshuffling) is the exchange of DNA, genetic material between different organisms which leads to production of offspring with combinations of traits that differ fro ...
,
Dmc1 Meiotic recombination protein DMC1/LIM15 homolog is a protein Proteins are large s and s that comprise one or more long chains of . Proteins perform a vast array of functions within organisms, including , , , providing and , and from one ...
, is required for efficient meiotic
homologous recombination Homologous recombination is a type of genetic recombination in which genetic information is exchanged between two similar or identical molecules of double-stranded or single-stranded nucleic acids (usually DNA as in Cell (biology), cellular organi ...

homologous recombination
, and ''Dmc1'' is expressed in ''
Entamoeba histolytica ''Entamoeba histolytica'' is an anaerobic organism, anaerobic parasitic amoebozoan, part of the genus ''Entamoeba''. Predominantly infecting humans and other primates causing amoebiasis, ''E. histolytica'' is estimated to infect about 35-50 mill ...

Entamoeba histolytica
''. The purified Dmc1 from ''E. histolytica'' forms
presynaptic SyNAPSE is a DARPA program that aims to develop electronic neuromorphic machine technology, an attempt to build a new kind of cognitive computer with form, function, and architecture similar to the mammalian brain. Such artificial brains would be u ...
filaments and catalyses
ATP ATP may refer to: Companies and organizations * Association of Tennis Professionals * American Technical Publishers * ', a Danish pension * Armenia Tree Project * Association for Transpersonal Psychology * ATP architects engineers office * ATP ...

ATP
-dependent and DNA strand exchange over at least several thousand
base pairs A base pair (bp) is a fundamental unit of double-stranded nucleic acids consisting of two nucleobases bound to each other by hydrogen bonds. They form the building blocks of the DNA double helix and contribute to the folded structure of both DNA ...
. 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 '''' found that, during the conversion from the
tetraploid Polyploidy is a condition in which the cells of an organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecul ...
uninucleate {{Short pages monitor