Amoebozoa is a major taxonomic group containing about 2,400 described species of amoeboid protists, often possessing blunt, fingerlike, lobose pseudopods and tubular mitochondrial 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 symbionts 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.
Morphology
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, called endoplasm, 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 determinate pseudopodia), 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.
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 vacuoles 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 cysts, 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. Mixotrophic 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 flagella 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 gametes . The flagellum is generally anchored by a cone of microtubules, suggesting a close relationship to the opisthokonts. The mitochondria in amoebozoan cells characteristically have branching tubular cristae. However, among the Archamoebae, which are adapted to anoxic or microaerophilic habitats, mitochondria have been lost.
Classification
Place of Amoebozoa in the eukaryote tree
It appears (based on molecular genetics) that the members of Amoebozoa form a sister group to animals and fungi, diverging from this lineage after it had split from the other groups, as illustrated below in a simplified diagram:
Strong similarities between Amoebozoa and Opisthokonts lead to the hypothesis that they form a distinct clade. 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. 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 malawimonids and collodictyonids 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.
Subphyla within Amoebozoa: Lobosa and Conosa Traditionally all amoebozoa with lobose pseudopods were grouped together in the class Lobosea, placed with other amoeboids in the phylum Sarcodina or Rhizopoda, but these were considered to be unnatural groups. Structural and genetic studies identified the percolozoans 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, 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 subphylum Conosa, comprising the Archamoebae and the Mycetozoa.
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).
Phylogeny and taxonomy within Amoebozoa
From older studies by Cavalier-Smith, Chao & Lewis 2016 and Silar 2016. Also recent phylogeny indicates the Lobosa are paraphyletic: Conosa is sister of the Cutosea.
Amoebozoa phylogeny
Phylum Amoebozoa Lühe 1913 emend. Cavalier-Smith 1998 [Amoebobiota; Eumycetozoa Zopf 1884 emend Olive 1975] Clade Discosea Cavalier-Smith 2004 stat. nov. Adl et al. 2018 Order ?Stereomyxida Grell 1971 Order ?Stygamoebida Smirnov & Cavalier-Smith 2011 Class Centramoebia Cavalier-Smith et al. 2016 Order Centramoebida Rogerson & Patterson 2002 em. Cavalier-Smith 2004 Order Himatismenida Page 1987 [Cochliopodiida] Order Pellitida Page 1987 [Cochliopodiida] Class Flabellinia Smirnov & Cavalier-Smith 2011 em. Kudryavtsev et al. 2014 Order Thecamoebida Schaeffer 1926 em. Smirnov & Cavalier-Smith 2011 Order Dermamoebida Cavalier-Smith 2004 em. Smirnov & Cavalier-Smith 2011 Order Vannellida Smirnov et al. 2005 Order Dactylopodida Smirnov et al. 2005 Clade Tevosa Kang et al. 2017 Clade Tubulinea Smirnov et al. 2005 stat. nov. Adl et al. 2018 Class Corycidia Kang et al. 2017 stat. nov. Adl et al. 2018 Order Trichosida Moebius 1889 Family Microcoryciidae de Saedeleer 1934 Class Echinamoebia Cavalier-Smith 2016 stat. nov. Adl et al. 2018 Order Echinamoebida Cavalier-Smith 2004 em. 2011 Class Elardia Kang et al. 2017 stat. nov. Adl et al. 2018 Subclass Leptomyxia Cavalier-Smith 2016 Order Leptomyxida Pussard & Pons 1976 em. Page 1987 Subclass Eulobosia Cavalier-Smith 2016 Order Euamoebida Lepşi 1960 em. Cavalier-Smith 2016 Order Arcellinida Kent 1880 Clade Evosea Kang et al. 2017 stat. nov. Adl et al. 2018 Clade Cutosa Cavalier-Smith 2016 stat. nov. Class Cutosea Cavalier-Smith 2016 Order Squamocutida Cavalier-Smith 2016 Subphylum Conosa Cavalier-Smith 1998 stat. nov. Infraphylum Archamoebae Cavalier-Smith 1993 stat. n. 1998 Class Archamoebea Cavalier-Smith 1983 stat. n. 2004 Family Tricholimacidae Cavalier-Smith 2013 Family Endamoebidae Calkins 1926 Order Entamoebida Cavalier-Smith 1993 Order Pelobiontida Page 1976 emend. Cavalier Smith 1987 Infraphylum Semiconosia Cavalier-Smith 2013 Class Variosea Cavalier-Smith et al. 2004 Order ?Flamellidae Cavalier-Smith 2016 Order ?Holomastigida Lauterborn 1895 [Artodiscida Cavalier-Smith 2013] Order Phalansteriida Hibberd 1983 Order Ramamoebida Cavalier-Smith 2016 Order Profiliida Kang et al. 2017 [Protosteliida Olive & Stoianovitch 1966 em. Shadwick & Spiegel 2012] Order Fractovitellida Lahr et al. 2011 em. Kang et al. 2017 Superclass Mycetozoa de Bary, 1859 ex Rostafinski, 1873 Class Dictyostelea Hawksworth et al. 1983 Order Acytosteliales Baldauf, Sheikh & Thulin 2017 Order Dictyosteliales Lister 1909 em. Olive 1970 Class Protostelea Shadwick & Spiegel et al. 2012 Order Protosteliida Shadwick & Spiegel et al. 2012 Class Ceratiomyxomycetes Hawksworth, Sutton & Ainsworth 1983 Order Protosporangiida Shadwick & Spiegel 2012 Order Ceratiomyxida Martin 1961 ex Farr & Alexopoulos Class Myxomycetes Link 1833 em. Haeckel 1866 Subclass Lucisporomycetidae Leontyev et al. 2019 Superorder Cribrarianae Leontyev 2015 Order Cribrariales Macbr. 1922 Superorder Trichianae Leontyev 2015 Order Reticulariales Leontyev 2015 Order Liceales Jahn 1928 Order Trichiales Macbride 1922 Subclass Columellomycetidae Leontyev et al. 2019 Order ?Echinosteliopsidales Shchepin et al. Superorder Echinostelianae Leontyev 2015 Order Echinosteliales Martin 1961 Superorder Stemonitanae Leontyev 2015 [Fuscisporida Cavalier-Smith 2012] Order Clastodermatales Leontyev 2015 Order Meridermatales Leontyev 2015 Order Stemonitales Macbride 1922 Order Physarales Macbride 1922
Fossil record Vase-shaped microfossils (VSMs) discovered around the world show that amoebozoans have existed since the Neoproterozoic 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 amoebozoans belonging to the class Tubulinea. P. athanata in particular looks the same as the extant genus Arcella.