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Wikipedia / Heterolobosea

Heterolobosea or Percolozoa, commonly known as amoebomastigotes, is an enigmatic phylum of protists that includes 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

Most members of this phylum are bacterivores found in soil, fresh water and occasionally in the ocean. The only member of this phylum that is infectious to humans is Naegleria fowleri, the causative agent of the often fatal disease amoebic meningitis. The group is closely related to the Euglenozoa, and share with them the unusual characteristic of having mitochondria with discoid cristae. 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.

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.

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.

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.

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):

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:

Order Schizopyrenida

Order Acrasida

Pleurostomum flabellatum has recently been added to Heterolobosea.

Phylum Percolozoa Cavalier-Smith 1991

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

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

Order Lyromonadida Cavalier-Smith 1993

Family Gruberellidae Page & Blanton 1985

Family Paravahlkampfiidae

Family Plaesiobystridae

Family Psalteriomonadidae Cavalier-Smith 1993 [Lyromonadidae Cavalier-Smith 1993]

Class Heterolobosea Page & Blanton 1985

Order Acrasida Schröter 1886

Family Acrasidae van Tieghem 1880 ex Hartog 1906

Order Naegleriida Starobogatov 1980

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

Family Vahlkampfiidae Jollos 1917 s.s.

  • Genus Tetramitus Perty 1852 [Copromastix Aragao 1916; Learamoeba Sawyer et al. 1998, Paratetramitus Darbyshire, Page & Goodfellow 1976, Singhamoeba Sawyer, Nerad & Munson 1992]

  • Genus Vahlkampfia Chatton & LaLung-Bonnaire 1912 [Guttulidium Frenzel 1892]

Order Creneida Cavalier-Smith 2021

Family Creneidae Pánek et al. 2014

Order Percolomonadida Cavalier-Smith 1993

Family Barbeliidae Arndt 2023

Family Lulidae Hohlfeld & Arndt 2023

  • Genus Lula Arndt, Nitsche & Carduck 2021

Family Percolomonadidae Cavalier-Smith 1993 [Choanogasteraceae]

  • Genus Percolomonas Fenchel & Patterson 1986 [Choanogaster Pochmann 1959]

Family Stephanopogonidae Corliss 1961

History

The Heterolobosea were first defined by Page and Blanton in 1985 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 poorly understood genus Stephanopogon.

Cavalier-Smith later maintained the Heterolobosea as a class for amoeboid forms. He has defined Percolozoa as "Heterolobosea plus Percolatea classis nov."

References