| Amoebozoa Temporal range:800–0 Ma[a] | |
|---|---|
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| Leocarpus fragilis | |
Scientific classification | |
| Domain: | Eukaryota |
| Clade: | Podiata |
| Clade: | Amorphea |
| Phylum: | Amoebozoa Lühe, 1913[3] emend.Cavalier-Smith, 1998[4] |
| Classes and subclades[5][6] | |
| Synonyms | |
EumycetozoaZopf 1884, emend Olive 1975[b] | |
Amoebozoa is a majortaxonomic group containing about 2,400 described species ofamoeboidprotists,[8] often possessing blunt, fingerlike,lobosepseudopods and tubularmitochondrialcristae.[7][9] In traditional classification schemes, Amoebozoa is usually ranked as aphylum within either thekingdomProtista[10] or the kingdomProtozoa.[11][12] In the classification favored by the International Society of Protistologists, it is retained as an unranked "supergroup" within Eukaryota.[7]Molecular genetic analysis supports Amoebozoa as amonophyleticclade. Modern studies of eukaryoticphylogenetic trees identify it as the sister group toOpisthokonta, another major clade which contains bothfungi andanimals as well as several other clades comprising some 300 species of unicellular eukaryotes.[8][9] Amoebozoa and Opisthokonta are sometimes grouped together in a high-leveltaxon, namedAmorphea.[7]Amoebozoa includes many of the best-known amoeboid organisms, such asChaos,Entamoeba,Pelomyxa and the genusAmoeba itself. Species of Amoebozoa may be either shelled (testate) or naked, and cells may possessflagella. Free-living species are common in both salt and freshwater as well as soil, moss and leaf litter. Some live asparasites orsymbionts of other organisms, and some are known to cause disease in humans and other organisms.
While the majority of amoebozoan species are unicellular, the group also includes several clades ofslime molds, which have a macroscopic, multicellular stage of life during which individual amoeboid cells remain together after multiple cell division to form a macroscopic plasmodium or, in cellular slime molds, aggregate to form one.
Amoebozoa vary greatly in size. Some are only 10–20 μm in diameter, while others are among the largest protozoa. The well-known speciesAmoeba proteus, which may reach 800 μm in length, is often studied in schools and laboratories as a representative cell ormodel organism, partly because of its convenient size.Multinucleate amoebae likeChaos andPelomyxa may be several millimetres in length, and some multicellular amoebozoa, such as the "dog vomit" slime moldFuligo septica, can cover an area of several square meters.[13]
Amoebozoa is a large and diverse group, but certain features are common to many of its members. The amoebozoan cell is typically divided into a granular central mass, calledendoplasm, and a clear outer layer, called ectoplasm. During locomotion, the endoplasm flows forwards and the ectoplasm runs backwards along the outside of the cell. In motion, many amoebozoans have a clearly defined anterior and posterior and may assume a "monopodial" form, with the entire cell functioning as a single pseudopod. Large pseudopods may produce numerous clear projections called subpseudopodia (or determinatepseudopodia), which are extended to a certain length and then retracted, either for the purpose of locomotion or food intake. A cell may also form multiple indeterminate pseudopodia, through which the entire contents of the cell flow in the direction of locomotion. These 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 the organism changes direction.[14]
While most amoebozoans are "naked," like the familiarAmoeba andChaos, or covered with a loose coat of minute scales, likeCochliopodium andKorotnevella, members of the orderArcellinida form rigid shells, ortests, equipped with a single aperture through which the pseudopods emerge. Arcellinid tests may be secreted from organic materials, as inArcella, or built up from collected particles cemented together, as inDifflugia.
In all amoebozoa, the primary mode of nutrition isphagocytosis, in which the cell surrounds potential food particles with its pseudopods, sealing them intovacuoles within which they may be digested and absorbed. Some amoebozoans have a posterior bulb called a uroid, which may serve to accumulate waste, periodically detaching from the rest of the cell.[citation needed] When food is scarce, most species can formcysts, which may be carried aerially and introduce them to new environments.[citation needed] In slime moulds, these structures are called spores, and form on stalked structures called fruiting bodies orsporangia.Mixotrophic species living in a symbiotic relationship with microalgae of the genusChlorella, which lives inside the cytoplasm of their host, have been found inArcellinida andMayorella.[15][16]
The majority of Amoebozoa lackflagella and more generally do not formmicrotubule-supported structures except duringmitosis. However, flagella do occur among theArchamoebae, and many slime moulds produce biflagellategametes[citation needed]. The flagellum is generally anchored by a cone of microtubules, suggesting a close relationship to theopisthokonts.[citation needed] Themitochondria in amoebozoan cells characteristically have branching tubularcristae. However, among theArchamoebae, which are adapted to anoxic or microaerophilic habitats, mitochondria have been lost.
It appears (based on molecular genetics) that the members of Amoebozoa form asister group to animals and fungi, diverging from this lineage after it had split from the other groups,[17]as illustrated below in a simplified diagram:
Strong similarities between Amoebozoa andOpisthokonts lead to the hypothesis that they form a distinct clade.[18] Thomas Cavalier-Smith proposed the name "unikonts" (formally, Unikonta) for this branch, whose members were believed to have been descended from a common ancestor possessing a single emergent flagellum rooted in onebasal body.[1][2] However, while the close relationship between Amoebozoa and Opisthokonta is robustly supported, recent work has shown that the hypothesis of a uniciliate ancestor is probably false. In their Revised Classification of Eukaryotes (2012), Adl et al. proposed Amorphea as a more suitable name for a clade of approximately the same composition, a sister group to theDiaphoretickes.[7] More recent work places the members of Amorphea together with themalawimonids andcollodictyonids in a proposed clade called Opimoda, which comprises one of two major lineages diverging at the root of the eukaryote tree of life, the other beingDiphoda.[19]
Traditionally all amoebozoa with lobose pseudopods were grouped together in the classLobosea, placed with other amoeboids in the phylum Sarcodina orRhizopoda, but these were considered to be unnatural groups. Structural and genetic studies identified thepercolozoans and several archamoebae as independent groups. In phylogenies based onrRNA their representatives were separate from other amoebae, and appeared to diverge near the base ofeukaryotic evolution, as did most slime molds.
However, revised trees byCavalier-Smith and Chao in 1996[20] suggested that the remaining lobosans do form a monophyletic group, to which the Archamoebae and Mycetozoa were closely related, although the percolozoans were not. Subsequently, they emended the phylum Amoebozoa to include both the subphylum Lobosa and a new subphylumConosa, comprising the Archamoebae and theMycetozoa.[4]
Recent molecular genetic data appear to support this primary division of the Amoebozoa into Lobosa and Conosa.[9] The former, as defined by Cavalier-Smith and his collaborators, consists largely of the classic Lobosea: non-flagellated amoebae with blunt, lobose pseudopods (Amoeba,Acanthamoeba, Arcella, Difflugia etc.). The latter is made up of both amoeboid and flagellated cells, characteristically with more pointed or slightly branching subpseudopodia (Archamoebae and the Mycetozoan slime molds).
From older studies by Cavalier-Smith, Chao & Lewis 2016[21] and Silar 2016.[22] Also recent phylogeny indicates the Lobosa are paraphyletic: Conosa is sister of the Cutosea.[6][23][24]
| Amoebozoa phylogeny | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Phylum AmoebozoaLühe 1913 emend. Cavalier-Smith 1998 [Amoebobiota; EumycetozoaZopf 1884 emend Olive 1975]
Vase-shapedmicrofossils (VSMs) discovered around the world show that amoebozoans have existed since theNeoproterozoic Era. The fossil speciesMelanocyrillium hexodiadema,Palaeoarcella athanata, andHemisphaeriella ornata come from rocks 750 million years old. All three VSMs share a hemispherical shape, invaginated aperture, and regular indentations, that strongly resemble modernarcellinids, which areshell-bearing amoebozoans belonging to the classTubulinea.P. athanata in particular looks the same as the extant genusArcella.[1][26]
The recently availableAcanthamoebagenome sequence revealed severalorthologs of genes employed inmeiosis of sexualeukaryotes. These genes includedSpo11,Mre11,Rad50,Rad51,Rad52, Mnd1,Dmc1,Msh andMlh.[27] This finding suggests thatAcanthamoeba is capable of some form of meiosis and may be able to undergo sexual reproduction.
In sexually reproducing eukaryotes,homologous recombination (HR) ordinarily occurs during meiosis. The meiosis-specificrecombinase,Dmc1, is required for efficient meiotic HR, and Dmc1 is expressed inEntamoeba histolytica.[28] The purified Dmc1 fromE. histolytica formspresynaptic filaments and catalyzesATP-dependenthomologous DNA pairing and DNA strand exchange over at least several thousandbase pairs.[28] The DNA pairing and strand exchange reactions are enhanced by the eukaryotic meiosis-specific recombination accessory factor (heterodimer) Hop2-Mnd1.[28] These processes are central to meiotic recombination, suggesting thatE. histolytica undergoes meiosis.[28]
Studies ofEntamoeba invadens found that, during the conversion from thetetraploiduninucleatetrophozoite to the tetranucleate cyst,homologous recombination is enhanced.[29] Expression of genes with functions related to the major steps of meiotic recombination also increased during encystations.[29] These findings inE. invadens, combined with evidence from studies ofE. histolytica indicate the presence of meiosis in theEntamoeba. A comparative genetic analysis indicated thatmeiotic processes are present in all major amoebozoan lineages.[30]
SinceAmoebozoa diverged early from the eukaryotic family tree, these results also suggest that meiosis was present early in eukaryotic evolution.
Amoebiasis, also known as amebiasis or entamoebiasis,[31][32] is an infection caused by any of the amoebozoans of theEntamoeba group. Symptoms are most common upon infection byEntamoeba histolytica. Amoebiasis can present with no, mild, or severesymptoms. Symptoms may includeabdominal pain, milddiarrhoea,bloody diarrhea orsevere colitis withtissue death andperforation. This last complication may causeperitonitis. People affected may developanemia due to loss of blood.[33]
Invasion of the intestinal lining causesamoebic bloody diarrhea oramoebic colitis. If the parasite reaches the bloodstream it can spread through the body, most frequently ending up in the liver where it causesamoebic liver abscesses.Liver abscesses can occur without previous diarrhea. Cysts ofEntamoeba can survive for up to a month in soil or for up to 45 minutes under fingernails. It is important to differentiate between amoebiasis andbacterial colitis. The preferred diagnostic method is through faecal examination under microscope, but requires a skilled microscopist and may not be reliable when excluding infection. This method however may not be able to separate between specific types.Increased white blood cell count is present in severe cases, but not in mild ones. The most accurate test is forantibodies in the blood, but it may remain positive following treatment.[33]
Prevention of amoebiasis is by separating food and water from faeces and by propersanitation measures. There is no vaccine. There are two treatment options depending on the location of the infection. Amoebiasis in tissues is treated with eithermetronidazole,tinidazole,nitazoxanide,dehydroemetine orchloroquine, while luminal infection is treated withdiloxanide furoate oriodoquinoline. For treatment to be effective against all stages of the amoeba may require a combination of medications. Infections without symptoms do not require treatment but infected individuals can spread the parasite to others and treatment can be considered. Treatment of otherEntamoeba infections apart fromE. histolytica is not needed.[33]
Amoebiasis is present all over the world.[34] About 480 million people are infected with what appears to beE. histolytica and these result in the death of between 40,000–110,000 people every year. Most infections are now ascribed toE. dispar.E. dispar is more common in certain areas and symptomatic cases may be fewer than previously reported. The first case of amoebiasis was documented in 1875 and in 1891 the disease was described in detail, resulting in the termsamoebic dysentery andamoebic liver abscess. Further evidence from the Philippines in 1913 found that upon ingesting cysts ofE. histolytica volunteers developed the disease. It has been known since 1897 that at least one non-disease-causing species ofEntamoeba existed (Entamoeba coli), but it was first formally recognized by theWHO in 1997 thatE. histolytica was two species, despite this having first been proposed in 1925. In addition to the now-recognizedE. dispar evidence shows there are at least two other species ofEntamoeba that look the same in humans -E. moshkovskii andEntamoeba bangladeshi. The reason these species haven't been differentiated until recently is because of the reliance on appearance.[33]
.JPG)
| Tubulinea |
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| Discosea |
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| Evosea |
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| incertae sedis | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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