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Apusomonas sp.
Scientific classification
Domain: Eukaryota
(unranked): Unikonta?
Phylum: Apusozoa
Cavalier-Smith 1997 em.


The Apusozoa are a phylum[1] comprising several genera of flagellate eukaryotes. They are usually around 5–20 μm in size, and occur in soils and aquatic habitats, where they feed on bacteria. They are grouped together based on the presence of an organic shell or theca under the dorsal surface of the cell.

The name derives from the Ancient Greek words for footless (ἄπους) and animal (ζῷον).[2][3]

This phylum may be obsolete or paraphyletic as it appears the opisthokonts emerged within the Apusozoa, with the Apusomonadida as sister clade. The opisthokonts, Apusozoa and Breviate together form the Obazoa clade, a sister clade to the Amoebozoa.


There are three orders, often treated as separate groups: the apusomonads, ancyromonads, and hemimastigids.

Name Genera Flagella Mitochondrial cristae
apusomonads Apusomonas and Amastigomonas. Some sources further subdivide this group.[4] biflagellate tubular
ancyromonads Ancyromonas biflagellate flat
hemimastigids (also called the spironemids or Hemimastigophora) Hemimastix, Spironema, and Stereonema multiflagellate tubular

It has been suggested that Mantamonas be classified in Apusozoa.[5]


The apusomonads and ancyromonads have two flagella inserted at right angles, near the anterior of the cell. They move by gliding, with one flagellum trailing along the side and one directed to the anterior. By contrast, hemimastigids cell has multiple flagella, arranged in rows from the anterior of the cell towards the posterior.

The form of the mitochondria varies between the different orders. Among the apusomonads they have tubular cristae, the ancyromonads flat cristae, and the hemimastigids ambiguous or sacculate cristae. This characteristic was originally considered a good indicator of relationships, but is now known to vary even among close relatives.

Relationship to other eukaryotes[edit]

On molecular trees, the apusomonads and ancyromonads group together, but their relationship to other eukaryotes is uncertain.

Although it has sometimes been included in the Rhizaria, based on 18S rRNA gene sequencing, it has been concluded that the Apusozoa are not closely related to other Rhizaria.

Cavalier-Smith, in a collaborative paper with Alexandra Stechmann, postulated that Apusozoa belongs in the bikont clade.[6][7] It may be one of the most divergent bikont lineages.[8]

It has more recently been grouped with the unikonts.[5] It has been suggested that it may be related to the opisthokont group,[9] specifically the Apusmonadida could be the sister clade.

Apusozoa has been rendered paraphyletic by the cladistic inclusion of opisthokont as sister of Apusmonadida. Together they form an as yet unnamed clade which is sister to the Apusozoan Breviatea clade. Together these form the Obazoa.[10][11]

Stem groups[edit]


  1. ^ Cavalier-Smith T, Chao EE, Stechmann A, Oates B, Nikolaev S (October 2008). "Planomonadida ord. nov. (Apusozoa): ultrastructural affinity with Micronuclearia podoventralis and deep divergences within Planomonas gen. nov". Protist. 159 (4): 535–62. PMID 18723395. doi:10.1016/j.protis.2008.06.002. 
  2. ^ ἄπους. Liddell, Henry George; Scott, Robert; A Greek–English Lexicon at the Perseus Project
  3. ^ ζῷον. Liddell, Henry George; Scott, Robert; A Greek–English Lexicon at the Perseus Project
  4. ^ Cavalier-Smith T, Chao EE (May 2010). "Phylogeny and Evolution of Apusomonadida (Protozoa: Apusozoa): New Genera and Species". Protist. 161 (4): 549–76. PMID 20537943. doi:10.1016/j.protis.2010.04.002. 
  5. ^ a b Glücksman E, Snell EA, Berney C, Chao EE, Bass D, Cavalier-Smith T (September 2010). "The Novel Marine Gliding Zooflagellate Genus Mantamonas (Mantamonadida ord. n.: Apusozoa)". Protist. 162 (2): 207–221. PMID 20884290. doi:10.1016/j.protis.2010.06.004. 
  6. ^ Cavalier-Smith, T.; Stechmann, Alexandra (2003). "The root of the eukaryote tree pinpointed". Current Biology. 13 (17): R665–R666. PMID 12956967. doi:10.1016/S0960-9822(03)00602-X. 
  7. ^ Cavalier-Smith T (March 2002). "The phagotrophic origin of eukaryotes and phylogenetic classification of Protozoa". Int. J. Syst. Evol. Microbiol. 52 (Pt 2): 297–354. PMID 11931142. doi:10.1099/00207713-52-2-297. 
  8. ^ Moreira D, von der Heyden S, Bass D, López-García P, Chao E, Cavalier-Smith T (July 2007). "Global eukaryote phylogeny: Combined small- and large-subunit ribosomal DNA trees support monophyly of Rhizaria, Retaria and Excavata". Mol. Phylogenet. Evol. 44 (1): 255–66. PMID 17174576. doi:10.1016/j.ympev.2006.11.001. 
  9. ^ Cavalier-Smith T (2009). "Megaphylogeny, cell body plans, adaptive zones: causes and timing of eukaryote basal radiations". J. Eukaryot. Microbiol. 56 (1): 26–33. PMID 19340985. doi:10.1111/j.1550-7408.2008.00373.x. 
  10. ^ Brown, Matthew W.; Sharpe, Susan C.; Silberman, Jeffrey D.; Heiss, Aaron A.; Lang, B. Franz; Simpson, Alastair G. B.; Roger, Andrew J. (2013-10-22). "Phylogenomics demonstrates that breviate flagellates are related to opisthokonts and apusomonads". Proceedings of the Royal Society of London B: Biological Sciences. 280 (1769): 20131755. ISSN 0962-8452. PMC 3768317Freely accessible. PMID 23986111. doi:10.1098/rspb.2013.1755. 
  11. ^ Cavalier-Smith, Thomas; Fiore-Donno, Anna Maria; Chao, Ema; Kudryavtsev, Alexander; Berney, Cédric; Snell, Elizabeth A.; Lewis, Rhodri (2015-02-01). "Multigene phylogeny resolves deep branching of Amoebozoa". Molecular Phylogenetics and Evolution. 83: 293–304. doi:10.1016/j.ympev.2014.08.011. 
  12. ^ Phylogeny based on:

External links[edit]