Amoebozoa
Encyclopedia
The Amoebozoa are a major group of amoeboid
protozoa, including the majority that move by means of
internal cytoplasm
ic flow. Their pseudopod
ia are characteristically blunt and finger-like,
called lobopodia. Most are unicellular, and are common in soils and aquatic habitats, with some found as symbiotes
of other organisms, including several pathogen
s. The Amoebozoa also include the slime moulds, multinucleate
or
multicellular forms that produce spore
s and are usually visible to the unaided eye.
Amoebozoa vary greatly in size. Many are only 10-20 μm in size, but they also include many of the larger
protozoa. The famous species Amoeba proteus
may reach 800 μm in length, and partly on account of
its size is often studied as a representative cell
. Multinucleate amoebae like Chaos
and
Pelomyxa
may be several millimetres in length, and some slime moulds cover several square feet.
, and a clear outer layer, called ectoplasm. During locomotion the endoplasm flows forwards and the ectoplasm runs backwards along the outside of the cell. Many amoeba move with a definite anterior and posterior; in essence the cell functions as a single pseudopod. They usually produce numerous clear projections called subpseudopodia (or determinate pseudopodia), which have a defined length and are not directly involved in locomotion.
Other amoebozoans may form multiple indeterminate pseudopodia, which are more or less tubular and are mostly filled with granular endoplasm. The cell mass flows into a leading pseudopod, and the others ultimately retract unless it changes direction. Subpseudopodia are usually absent. In addition to a few naked forms like Amoeba and Chaos, this includes most amoebae that produce shells. These may be composed of organic materials, as in Arcella
, or of collected particles cemented together, as in Difflugia
, with a single opening through which the pseudopodia emerge.
The primary mode of nutrition is by phagocytosis: the cell surrounds potential food particles, sealing them into vacuole
s where the may be digested and absorbed. Some amoebae have a posterior bulb called a uroid, which may serve to accumulate waste, periodically detaching from the rest of the cell. When food is scarce, most species can form cyst
s, which may be carried aerially and introduce them to new environments. In slime moulds, these structures are called spores, and form on stalked structures called fruiting bodies or sporangia
.
Most Amoebozoa lack flagella
and more generally do not form microtubule
-supported structures except during mitosis
. However, flagella occur among some archamoebae
, and many slime moulds produce biflagellate gamete
s . The flagellum is generally anchored by a cone of microtubules, suggesting a close relationship to the opisthokont
s. The mitochondria
characteristically have branching tubular crista
e, but have been lost among archamoebae.
It appears (based in proteome
s) that the amoebozoa form a sister group to animals and fungi, diverging from this lineage after it had split from the other groups,
as illustrated below:
Strong similarities between Amoebozoa and Opisthokont
s (including animals and fungi) lead to the proposal that they form a clade called Unikont
s.
, placed with other amoeboids in the phylum Sarcodina or Rhizopoda, but these were considered to be unnatural groups. Structural and genetic studies identified the percolozoa
ns and several archamoebae as independent groups. In phylogenies based on rRNA their representatives were separate from other amoebae, and appeared to diverge near the base of eukaryotic
evolution, as did most slime molds.
However, revised trees by Cavalier-Smith
and Chao in 1996 suggested that the remaining lobosans do form a monophyletic group, and that the archamoebae and Mycetozoa are closely related to it, although the percolozoans are not. Subsequently they emended the older phylum Amoebozoa to refer to this supergroup. Studies based on other genes have provided strong support for the unity of this group. Patterson
treated most with the testate filose amoebae as the ramicristates, based on mitochondrial similarities, but the latter are now removed to the Cercozoa
.
The subphylum Lobosa is paraphyletic
. Two major classes of Lobosa have been identified, the Tubulinea
and Flabellinea
, but various others remain of uncertain placement.
. This classification receives some support from molecular phylogenies.
Era. The fossil species Melanocyrillium hexodiadema, Palaeoarcella athanata, and Hemisphaeriella ornata come from rocks 750 million years old. All three VSMs share a hemispherical shape, invaginated aperture, and regular indentations, that strongly resemble modern arcellinids, which are shell-bearing amoeboids
. P. athanata in particular looks the same as the extant genus Arcella.
Amoeboid
Amoeboids are single-celled life-forms characterized by an irregular shape."Amoeboid" and "amœba" are often used interchangeably even by biologists, and especially refer to a creature moving by using pseudopodia. Most references to "amoebas" or "amoebae" are to amoeboids in general rather than to...
protozoa, including the majority that move by means of
internal cytoplasm
Cytoplasm
The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...
ic flow. Their pseudopod
Pseudopod
Pseudopods or pseudopodia are temporary projections of eukaryotic cells. Cells that possess this faculty are generally referred to as amoeboids. Pseudopodia extend and contract by the reversible assembly of actin subunits into microfilaments...
ia are characteristically blunt and finger-like,
called lobopodia. Most are unicellular, and are common in soils and aquatic habitats, with some found as symbiotes
of other organisms, including several pathogen
Pathogen
A pathogen gignomai "I give birth to") or infectious agent — colloquially, a germ — is a microbe or microorganism such as a virus, bacterium, prion, or fungus that causes disease in its animal or plant host...
s. The Amoebozoa also include the slime moulds, multinucleate
Multinucleate
Multinucleate cells have more than one nucleus per cell, which is the result of nuclear division not being followed by cytokinesis. As a consequence, multiple nuclei share one common cytoplasm. This can be the consequence of a disturbed cell cycle control Multinucleate (also multinucleated,...
or
multicellular forms that produce spore
Spore
In biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions. Spores form part of the life cycles of many bacteria, plants, algae, fungi and some protozoa. According to scientist Dr...
s and are usually visible to the unaided eye.
Amoebozoa vary greatly in size. Many are only 10-20 μm in size, but they also include many of the larger
protozoa. The famous species Amoeba proteus
Amoeba proteus
Amoeba proteus, previously Chaos diffluens, is an amoeba closely related to the Giant Amoebae.This small protozoan uses tentacular protuberances called pseudopodia to move and phagocytose smaller unicellular organisms, which are enveloped inside the cell's cytoplasm in a food vacuole, where they...
may reach 800 μm in length, and partly on account of
its size is often studied as a representative cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
. Multinucleate amoebae like Chaos
Chaos (amoeba)
Chaos is a genus of giant amoebae. The largest and best known species, Chaos carolinensis, can reach lengths of 5mm, although most specimens fall between 1 and 3 mm....
and
Pelomyxa
Pelomyxa
Pelomyxa are giant amoebae, usually 500-800 μm but occasionally up to 5 mm in length. One notable species is P. palustris; other described species may be synonyms, or have been moved to the unrelated genus Chaos...
may be several millimetres in length, and some slime moulds cover several square feet.
Morphology
The cell is typically divided into a granular central mass, called endoplasmEndoplasm
Endoplasm generally refers to the inner part of a cell's cytoplasm. This is opposed to the ectoplasm which is the outer layer of the cytoplasm....
, and a clear outer layer, called ectoplasm. During locomotion the endoplasm flows forwards and the ectoplasm runs backwards along the outside of the cell. Many amoeba move with a definite anterior and posterior; in essence the cell functions as a single pseudopod. They usually produce numerous clear projections called subpseudopodia (or determinate pseudopodia), which have a defined length and are not directly involved in locomotion.
Other amoebozoans may form multiple indeterminate pseudopodia, which are more or less tubular and are mostly filled with granular endoplasm. The cell mass flows into a leading pseudopod, and the others ultimately retract unless it changes direction. Subpseudopodia are usually absent. In addition to a few naked forms like Amoeba and Chaos, this includes most amoebae that produce shells. These may be composed of organic materials, as in Arcella
Arcella
Arcella is a genus of testate amoebae or Arcellinida, usually found in freshwaters and mosses, and rarely in soils. A key characteristic of Arcella is the circular test with a hole on its center from where finger-like pseudopods emerge...
, or of collected particles cemented together, as in Difflugia
Difflugia
Difflugia is one of several genera of amoebozoa that produce shells or tests from granules of sand. These are swallowed by the cell and during the process of budding or fission they pass into the daughter, where they are joined by organic cement. The test has a single terminal opening. Difflugia...
, with a single opening through which the pseudopodia emerge.
The primary mode of nutrition is by phagocytosis: the cell surrounds potential food particles, sealing them into vacuole
Vacuole
A vacuole is a membrane-bound organelle which is present in all plant and fungal cells and some protist, animal and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic molecules including enzymes in solution, though in certain...
s where the may be digested and absorbed. Some amoebae have a posterior bulb called a uroid, which may serve to accumulate waste, periodically detaching from the rest of the cell. When food is scarce, most species can form cyst
Cyst
A cyst is a closed sac, having a distinct membrane and division on the nearby tissue. It may contain air, fluids, or semi-solid material. A collection of pus is called an abscess, not a cyst. Once formed, a cyst could go away on its own or may have to be removed through surgery.- Locations :* Acne...
s, which may be carried aerially and introduce them to new environments. In slime moulds, these structures are called spores, and form on stalked structures called fruiting bodies or sporangia
Sporangium
A sporangium is an enclosure in which spores are formed. It can be composed of a single cell or can be multicellular. All plants, fungi, and many other lineages form sporangia at some point in their life cycle...
.
Most Amoebozoa lack flagella
Flagellum
A flagellum is a tail-like projection that protrudes from the cell body of certain prokaryotic and eukaryotic cells, and plays the dual role of locomotion and sense organ, being sensitive to chemicals and temperatures outside the cell. There are some notable differences between prokaryotic and...
and more generally do not form microtubule
Microtubule
Microtubules are a component of the cytoskeleton. These rope-like polymers of tubulin can grow as long as 25 micrometers and are highly dynamic. The outer diameter of microtubule is about 25 nm. Microtubules are important for maintaining cell structure, providing platforms for intracellular...
-supported structures except during mitosis
Mitosis
Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...
. However, flagella occur among some archamoebae
Archamoebae
The Archamoebae are a group of Amoebozoa distinguished by the absence of mitochondria. They include genera that are internal parasites or commensals of animals . A few species are human pathogens, causing diseases such as amoebic dysentery...
, and many slime moulds produce biflagellate gamete
Gamete
A gamete is a cell that fuses with another cell during fertilization in organisms that reproduce sexually...
s . The flagellum is generally anchored by a cone of microtubules, suggesting a close relationship to the opisthokont
Opisthokont
The opisthokonts or "Fungi/Metazoa group" are a broad group of eukaryotes, including both the animal and fungus kingdoms, together with the eukaryotic microorganisms that are sometimes grouped in the paraphyletic phylum Choanozoa...
s. The mitochondria
Mitochondrion
In cell biology, a mitochondrion is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5 to 1.0 micrometers in diameter...
characteristically have branching tubular crista
Crista
Cristae are the internal compartments formed by the inner membrane of a mitochondrion. They are studded with proteins, including ATP synthase and a variety of cytochromes. The maximum surface for chemical reactions to occur is within the mitochondria...
e, but have been lost among archamoebae.
Classification
Amoebae are difficult to classify, and relationships within the phylum remain confused.It appears (based in proteome
Proteome
The proteome is the entire set of proteins expressed by a genome, cell, tissue or organism. More specifically, it is the set of expressed proteins in a given type of cells or an organism at a given time under defined conditions. The term is a portmanteau of proteins and genome.The term has been...
s) that the amoebozoa form a sister group to animals and fungi, diverging from this lineage after it had split from the other groups,
as illustrated below:
Strong similarities between Amoebozoa and Opisthokont
Opisthokont
The opisthokonts or "Fungi/Metazoa group" are a broad group of eukaryotes, including both the animal and fungus kingdoms, together with the eukaryotic microorganisms that are sometimes grouped in the paraphyletic phylum Choanozoa...
s (including animals and fungi) lead to the proposal that they form a clade called Unikont
Unikont
Unikonts are members of the Unikonta, a taxonomic group proposed by Thomas Cavalier-Smith.It includes amoebozoa, opisthokonts, and Apusozoa.-Clade:...
s.
Lobose amoebozoa
Traditionally all amoebae with lobose pseudopods were treated together as the LoboseaLobosea
Lobosea is a group of Amoebozoa. An example is Arcellinida.Sensu strico, it can refer to Tubulinea. However, sensu lato, it can also refer more generally to a paraphyletic group of lobose Amoebozoa. Formal classification of lobose amoebae remains speculative, and the older, less specific terms...
, placed with other amoeboids in the phylum Sarcodina or Rhizopoda, but these were considered to be unnatural groups. Structural and genetic studies identified the percolozoa
Percolozoa
The Percolozoa are a group of colourless protozoa, including many that can transform between amoeboid, flagellate, and encysted stages.-Terminology and classification:...
ns and several archamoebae as independent groups. In phylogenies based on rRNA their representatives were separate from other amoebae, and appeared to diverge near the base of eukaryotic
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
evolution, as did most slime molds.
However, revised trees by Cavalier-Smith
Thomas Cavalier-Smith
Professor Thomas Cavalier-Smith , FRS, FRSC, NERC Professorial Fellow, is a Professor of Evolutionary Biology in the Department of Zoology, at the University of Oxford...
and Chao in 1996 suggested that the remaining lobosans do form a monophyletic group, and that the archamoebae and Mycetozoa are closely related to it, although the percolozoans are not. Subsequently they emended the older phylum Amoebozoa to refer to this supergroup. Studies based on other genes have provided strong support for the unity of this group. Patterson
David J. Patterson
David J. Patterson is a taxonomist specializing in the protozoa and the use of taxonomy in biodiversity informatics. He was born in Belfast, Northern Ireland on the 19th April 1950 to Doris Mary and Samuel Patterson, with one elder brother and a sister . He was educated at Belmont Primary,...
treated most with the testate filose amoebae as the ramicristates, based on mitochondrial similarities, but the latter are now removed to the Cercozoa
Cercozoa
The Cercozoa are a group of protists. They are sometimes described as a kingdom.-Characteristics:The group includes most amoeboids and flagellates that feed by means of filose pseudopods. These may be restricted to part of the cell surface, but there is never a true cytostome or mouth as found in...
.
The subphylum Lobosa is paraphyletic
Paraphyly
A group of taxa is said to be paraphyletic if the group consists of all the descendants of a hypothetical closest common ancestor minus one or more monophyletic groups of descendants...
. Two major classes of Lobosa have been identified, the Tubulinea
Tubulinea
The Tubulinea are a major grouping of Amoebozoa, including most of the larger and more familiar amoebae likeAmoeba, Arcella, and Difflugia.-Characteristics:During locomotion most Tubulinea have a roughly cylindrical...
and Flabellinea
Flabellinea
The Flabellinea are a phylum of Amoebozoa. During locomotion the cells are flattened and have a clear layer called hyaloplasm along the front margin. Some form slender subpseudopodia projecting outward from the hyaloplasm, but the cell mass does not flow into these as in true pseudopodia, and...
, but various others remain of uncertain placement.
Other amoebozoa
Archamoebae and Mycetozoa have been placed in a subphylum ConosaConosa
Conosa is a grouping of Amoebozoa. It is subdivided into two infraphyla – Mycetozoa and Archamoebae.Conosa includes the species Dictyostelium discoideum and Entamoeba histolytica, among others....
. This classification receives some support from molecular phylogenies.
Fossil record
Vase-shaped microfossils (VSMs) discovered around the world show that amoebozoans have existed since the NeoproterozoicNeoproterozoic
The Neoproterozoic Era is the unit of geologic time from 1,000 to 542.0 ± 1.0 million years ago. The terminal Era of the formal Proterozoic Eon , it is further subdivided into the Tonian, Cryogenian, and Ediacaran Periods...
Era. The fossil species Melanocyrillium hexodiadema, Palaeoarcella athanata, and Hemisphaeriella ornata come from rocks 750 million years old. All three VSMs share a hemispherical shape, invaginated aperture, and regular indentations, that strongly resemble modern arcellinids, which are shell-bearing amoeboids
Testate amoebae
Testate amoebae are single-celled protists partially enclosed in a simple test .They are commonly found in soils, leaf litter, peat bogs and near/in fresh water....
. P. athanata in particular looks the same as the extant genus Arcella.
List of amoeboid protozoa pathogenic to humans
- Entamoeba histolyticaEntamoeba histolyticaEntamoeba histolytica is an anaerobic parasitic protozoan, part of the genus Entamoeba. Predominantly infecting humans and other primates, E. histolytica is estimated to infect about 50 million people worldwide...
- AcanthamoebaAcanthamoebaAcanthamoeba is a genus of amoebae, one of the most common protozoa in soil, and also frequently found in fresh water and other habitats. The cells are small, usually 15 to 35 μm in length and oval to triangular in shape when moving. The pseudopods form a clear hemispherical lobe at the anterior,...
- Balamuthia mandrillarisBalamuthia mandrillarisBalamuthia mandrillaris is a free-living leptomyxid amoeba which is known to cause amoebiasis in humans, especially the deadly neurological condition known as granulomatous amoebic encephalitis . Balamuthia has not been definitively isolated in nature, but it is believed to be distributed...