Amoebozoa

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.^[13]

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.^[14]

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.^{[citation needed]} When food is scarce, most species can form cysts, 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 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.^[15][16]

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 ^{[citation needed]}. The flagellum is generally anchored by a cone of microtubules, suggesting a close relationship to the opisthokonts. ^{[citation needed]} 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,^[17] as illustrated below in a simplified diagram:

Opimoda	Loukozoa Podiata CRuMs Amorphea Amoebozoa Obazoa Breviata Apusomonadida Opisthokonta Fungi Animalia

Strong similarities between Amoebozoa and

Subphyla within Amoebozoa: Lobosa and Conosa

Traditionally all amoebozoa with lobose pseudopods were grouped together in the class

However, revised trees by Cavalier-Smith and Chao in 1996^[19] 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.^[3]

Recent molecular genetic data appear to support this primary division of the Amoebozoa into Lobosa and Conosa.^[8] 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^[20] and Silar 2016.^[21] Also recent phylogeny indicates the Lobosa are paraphyletic: Conosa is sister of the Cutosea.^[5][22][23]

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 Ceratiomyxomycetes Hawksworth, Sutton & Ainsworth 1983
            • Order
              Protosporangiida
              Shadwick & Spiegel 2012
            • Order
              Ceratiomyxida
              Martin 1961 ex Farr & Alexopoulos
          • Class Myxomycetes Link 1833 em. Haeckel 1866 ^[24]
            • 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

• Entamoeba histolytica
• Acanthamoeba
• Balamuthia mandrillaris
• Endolimax

Meiosis

The recently available Acanthamoeba genome sequence revealed several orthologs of genes employed in meiosis of sexual eukaryotes. These genes included Spo11, Mre11, Rad50, Rad51, Rad52, Mnd1, Dmc1, Msh and Mlh.^[26] This finding suggests that Acanthamoeba is capable of some form of meiosis and may be able to undergo sexual reproduction.

In sexually reproducing eukaryotes,

• 
  Amoeba proteus (Lobosa: Tubulinea)
• 
  Arcella sp. test (Lobosa: Tubulinea)
• 
  Acanthamoeba sp. (Lobosa: Discosea)
• 
  Thecamoeba sp. (Lobosa: Discosea)
• 
  Entamoeba histolytica trophozoite (Conosa: Archamoebae)
• 
  Pelomyxa palustris

  (Conosa: Archamoebae)
• 
  Stemonitis sp. (Conosa: Myxogastria)
• 
  Dictyostelia

  )