Heterolobosea

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Heterolobosea or Percolozoa, commonly known as amoebomastigotes,<ref name="K&D">Template:Cite book</ref> is a phylum of protists including many amoeboflagellates. Naegleria fowleri, the causative agent of the often fatal disease amoebic meningitis, is the only member of this phylum infectious to humans. Typically, their life cycle alternates between flagellate and amoeboid stages.

Characteristics

File:2023 Heterolobosean-cropped.svg
Cellular features of the amoeboid stage

Most Percolozoa are found as bacterivores in soil, fresh water and occasionally in the ocean.<ref>Template:Citation</ref> The only member of this group that is infectious to humans is Naegleria fowleri, the causative agent of the often fatal disease amoebic meningitis.<ref name=":0">Template:Citation</ref> The group is closely related to the Euglenozoa, and share with them the unusual characteristic of having mitochondria with discoid cristae.<ref name="pmid14657102" /> The presence of a ventral feeding groove in the flagellate stage, as well as other features, suggests that they are part of the Excavata group.<ref name="CavSmith 2021" />

The amoeboid stage is roughly cylindrical, typically around 20–40 μm in length. They are traditionally considered lobose amoebae, but are not related to the others, and unlike them, do not form true lobose pseudopods. Instead, they advance by eruptive waves, where hemispherical bulges appear from the front margin of the cell, which is clear. The flagellate stage is slightly smaller, with two or four anterior flagella anterior to the feeding groove.<ref name=":0" />

Usually, the amoeboid form is taken when food is plentiful, and the flagellate form is used for rapid locomotion. However, not all members are able to assume both forms. The genera Percolomonas, Lyromonas, and Psalteriomonas are known only as flagellates, while Vahlkampfia, Pseudovahlkampfia, and most acrasids do not have flagellate stages. As mentioned above, under unfavourable conditions, the acrasids aggregate to form sporangia. These are superficially similar to the sporangia of the dictyostelids, but the amoebae only aggregate as individuals or in small groups and do not die to form the stalk.<ref name=":0" />

Phylogeny

The first broadly sampled comprehensive phylogenomic (phylotranscriptomic) analysis of the group (from 2025) confirmed the monophyly of Heterolobosea and provided a robustly supported backbone of the phylogeny resulting in the revision of the classification of Heterolobosea to the family level.<ref name="Pánek-2025"/>

The basal split of the cladogram has been confirmed between the subphyla Pharyngomonada (monotypic, with a sole family Pharyngomonadidae) and Tetramitia. In Tetramitia, two main clades (new classes) were identified: Selenaionea, consisting of two orders Neovahlkampfiida and Selenaionida in previously unsuspected but fully supported sister relationship, and Eutetramitea, consisting of orders Creneida, Lyromonadida, Naegleriida (new, including acrasid slime molds), Percolomonadida and Pseudociliatida (represented by the single genus Stephanopogon; not included in the new analysis but analyses based on 18S rRNA gene sequences have repeatedly shown its close relationship to Percolomonadida with high statistical support):<ref name="Pánek-2025"/> Template:Clade

Taxonomy

These are collectively referred to as schizopyrenids, amoeboflagellates, or vahlkampfids. They also include the acrasids, a group of social amoebae that aggregate to form sporangia. The entire group is usually called the Heterolobosea, but this may be restricted to members with amoeboid stages.

One Heterolobosea classification system is:<ref name="urlHeterolobosea">Template:Cite web</ref>

Pleurostomum flabellatum has recently been added to Heterolobosea.<ref name="pmid17576098">Template:Cite journal</ref>

Phylum Percolozoa Cavalier-Smith 1991<ref>Template:Cite journal</ref>

  • Subphylum Pharyngomonada Cavalier-Smith 1991
    • Class Pharyngomonadea Cavalier-Smith 2008 [Macropharyngomonadidea]
      • Order Pharyngomonadida Cavalier-Smith 2008 [Macropharyngomonadida]
        • Family Pharyngomonadidae Cavalier-Smith 2008 [Macropharyngomonadidae Cavalier-Smith 2008]
  • Subphylum Tetramitia Cavalier-Smith 1993 em. Cavalier-Smith 2008
    • Genus ?Costiopsis Senn 1900
    • Genus ?Hoehnemastix Skvortzov 1974
    • Genus ?Planiosculum Szabados 1948
    • Genus ?Protomyxomyces Cunningham 1881
    • Genus ?Protonaegleria Michel & Raether 1985
    • Genus ?Pseudovahlkampfia Sawyer 1980
    • Genus ?Schizamoeba Davis 1926
    • Genus ?Tetramastigamoeba Singh & Hanumaiah 1977
    • Genus ?Trimastigamoeba Whitmore 1911
    • Genus ?Wasielewskia Hartmann & Schuessler 1913
    • Family Euhyperamoebidae Goodkov & Seravin 1984 [Hyperamoebidae Goodkov, Seravin & Railkin 1982]
    • Class Lunosea Cavalier-Smith 2021
      • Order Selenaionida Hanousková, Táborský & Čepička 2018
        • Family Selenaionidae Hanousková, Táborský & Čepička 2018
          • Genus Selenaion Park, De Jonckheere & Simpson 2012
          • Genus Dactylomonas Hanousková, Táborský & Čepička 2018
    • Class Neovahlkampfiea Cavalier-Smith 2021
      • Order Neovahlkampfiida Cavalier-Smith 2021
        • Family Neovahlkampfiidae Hanousková, Táborský & Čepička 2018
    • Class Lyromonadea Cavalier-Smith 1993
    • Class Heterolobosea Page & Blanton 1985
      • Order Acrasida Schröter 1886
      • Order Naegleriida Starobogatov 1980
        • Genus Marinamoeba De Jonckheere et al. 2009
        • Family Tulamoebidae Kirby et al. 2015
        • Family Naegleriidae Kudo 1954 [Schizopyrenidae Singh 1951 ex Singh 1952; Bistadiidae Doflein 1916]
          • Genus Naegleria Aléxéieff 1912 [Adelphamoeba Napolitano, Wall & Ganz 1970; Didascalus Singh 1952; Schizopyrenus Singh 1951 ex Singh 1952]
          • Genus Willaertia de Jonckheere et al. 1984
      • Order Tetramitida Doweld 2001
      • Order Creneida Cavalier-Smith 2021
        • Family Creneidae Pánek et al. 2014
      • Order Percolomonadida Cavalier-Smith 1993

History

The Heterolobosea were first defined by Page and Blanton in 1985<ref> Template:Cite journal </ref> as a class of amoebae, and so only included those forms with amoeboid stages. Cavalier-Smith created the phylum Percolozoa for the extended group, together with the enigmatic flagellate Stephanopogon.<ref>Template:Cite book </ref>

Cavalier-Smith maintained the Heterolobosea as a class for amoeboid forms. He has defined Percolozoa as "Heterolobosea plus Percolatea classis nov."<ref name="pmid14657102">Template:Cite journal</ref>

References

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