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Amoebozoa is a major taxonomic group containing about 2,400 described species of Amoeba , often possessing blunt, fingerlike, lobose and tubular cristae. In traditional classification schemes, Amoebozoa is usually ranked as a phylum within either the kingdom Protista or the kingdom Protozoa. In the classification favored by the International Society of Protistologists, it is retained as an unranked "supergroup" within Eukaryota. Molecular genetic analysis supports Amoebozoa as a monophyletic clade. Modern studies of eukaryotic phylogenetic trees identify it as the sister group to Opisthokonta, another major clade which contains both fungi and animals as well as several other clades comprising some 300 species of unicellular eukaryotes. Amoebozoa and Opisthokonta are sometimes grouped together in a high-level taxon, named Amorphea. Amoebozoa includes many of the best-known amoeboid organisms, such as Chaos, Entamoeba, Pelomyxa and the genus Amoeba itself. Species of Amoebozoa may be either shelled (testate) or naked, and cells may possess flagella. Free-living species are common in both salt and freshwater as well as soil, moss and leaf litter. Some live as parasites or Symbiosis 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 of slime 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 species Amoeba proteus, which may reach 800 μm in length, is often studied in schools and laboratories as a representative cell or model organism, partly because of its convenient size. Multinucleate amoebae like Chaos and Pelomyxa may be several millimetres in length, and some multicellular amoebozoa, such as the "dog vomit" slime mold Fuligo septica, can cover an area of several square meters.
While most amoebozoans are "naked," like the familiar Amoeba and Chaos, or covered with a loose coat of minute scales, like Cochliopodium and Korotnevella, members of the order Arcellinida form rigid shells, or tests, equipped with a single aperture through which the pseudopods emerge. Arcellinid tests may be secreted from organic materials, as in Arcella, or built up from collected particles cemented together, as in Difflugia.
In all amoebozoa, the primary mode of nutrition is phagocytosis, in which the cell surrounds potential food particles with its pseudopods, sealing them into 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. When food is scarce, most species can form , 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 sporangium. species living in a symbiotic relationship with microalgae of the genus Chlorella, which lives inside the cytoplasm of their host, have been found in Arcellinida and Mayorella.
The majority of Amoebozoa lack flagellum and more generally do not form microtubule-supported structures except during mitosis. However, flagella do occur among the Archamoebae, and many slime moulds produce biflagellate . The flagellum is generally anchored by a cone of microtubules, suggesting a close relationship to the . The mitochondrion in amoebozoan cells characteristically have branching tubular crista However, among the Archamoebae, which are adapted to anoxic or microaerophilic habitats, mitochondria have been lost.
Strong similarities between Amoebozoa and lead to the hypothesis that they form a distinct clade. (2025). 9780544859937, Houghton Mifflin Harcourt. ISBN 9780544859937 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 one basal body.12 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 the Diaphoretickes. More recent work places the members of Amorphea together with the malawimonas and 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 being Diphoda.
However, revised trees by Cavalier-Smith and Chao in 1996
Recent molecular genetic data appear to support this primary division of the Amoebozoa into Lobosa and Conosa. 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).
Phylum Amoebozoa Lühe 1913 emend. Cavalier-Smith 1998 Amoebobiota;
In sexually reproducing eukaryotes, homologous recombination (HR) ordinarily occurs during meiosis. The meiosis-specific recombinase, Dmc1, is required for efficient meiotic HR, and Dmc1 is expressed in Entamoeba histolytica. The purified Dmc1 from E. histolytica forms Synapsis filaments and catalyzes ATP-dependent homologous DNA pairing and DNA strand exchange over at least several thousand base pairs. 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 Entamoeba invadens found that, during the conversion from the Polyploid uninucleate trophozoite to the tetranucleate cyst, homologous recombination is enhanced. Expression of genes with functions related to the major steps of meiotic recombination also increased during encystations. These findings in E. invadens, combined with evidence from studies of E. histolytica indicate the presence of meiosis in the Entamoeba. A comparative genetic analysis indicated that meiosis are present in all major amoebozoan lineages.
Since Amoebozoa diverged early from the eukaryotic family tree, these results also suggest that meiosis was present early in eukaryotic evolution.
Invasion of the intestinal lining causes amoebic bloody diarrhea or amoebic colitis. If the parasite reaches the bloodstream it can spread through the body, most frequently ending up in the liver where it causes amoebic liver abscesses. Liver abscesses can occur without previous diarrhea. Cysts of Entamoeba can survive for up to a month in soil or for up to 45 minutes under fingernails. It is important to differentiate between amoebiasis and bacterial 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. Leukocytosis is present in severe cases, but not in mild ones. The most accurate test is for serology, but it may remain positive following treatment.
Prevention of amoebiasis is by separating food and water from faeces and by proper sanitation measures. There is no vaccine. There are two treatment options depending on the location of the infection. Amoebiasis in tissues is treated with either metronidazole, tinidazole, nitazoxanide, dehydroemetine or chloroquine, while luminal infection is treated with diloxanide furoate or iodoquinoline. 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 other Entamoeba infections apart from E. histolytica is not needed.
Amoebiasis is present all over the world. (2014). 9781118734568, John Wiley & Sons. ISBN 9781118734568 About 480 million people are infected with what appears to be E. histolytica and these result in the death of between 40,000–110,000 people every year. Most infections are now ascribed to E. 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 terms amoebic dysentery and amoebic liver abscess. Further evidence from the Philippines in 1913 found that upon ingesting cysts of E. histolytica volunteers developed the disease. It has been known since 1897 that at least one non-disease-causing species of Entamoeba existed ( Entamoeba coli), but it was first formally recognized by the WHO in 1997 that E. histolytica was two species, despite this having first been proposed in 1925. In addition to the now-recognized Entamoeba dispar evidence shows there are at least two other species of Entamoeba that look the same in humans - E. moshkovskii and Entamoeba bangladeshi. The reason these species haven't been differentiated until recently is because of the reliance on appearance. (2013). 9780702053061, Elsevier Health Sciences. ISBN 9780702053061
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