The Apusozoa are a

paraphyletic Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...

phylum In biology, a phylum (; : phyla) is a level of classification, or taxonomic rank, that is below Kingdom (biology), kingdom and above Class (biology), class. Traditionally, in botany the term division (taxonomy), division has been used instead ...

flagellate A flagellate is a cell or organism with one or more whip-like appendages called flagella. The word ''flagellate'' also describes a particular construction (or level of organization) characteristic of many prokaryotes and eukaryotes and the ...

eukaryotes The eukaryotes ( ) constitute the domain of Eukaryota or Eukarya, organisms whose cells have a membrane-bound nucleus. All animals, plants, fungi, seaweeds, and many unicellular organisms are eukaryotes. They constitute a major group of ...

. They are usually around 5–20 μm in size, and occur in soils and aquatic habitats, where they feed on

bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...

. They are grouped together based on the presence of an organic shell or

theca In biology, a theca (: thecae) is a sheath or a covering. Botany In botany, the theca is related to plant's flower anatomy. The theca of an angiosperm consists of a pair of microsporangia that are adjacent to each other and share a common ar ...

under the dorsal surface of the cell. The name derives from the Ancient Greek words for footless () and animal (). This phylum was defined as containing the

Breviata ''Breviata anathema'' is a single-celled flagellate amoeboid eukaryote, previously studied under the name '' Mastigamoeba invertens''. The cell lacks mitochondria, much like the pelobionts to which the species was previously assigned, but has rem ...

and the

Apusomonadida The apusomonads (family Apusomonadidae) are a group of protozoan zooflagellates that glide on surfaces, and mostly consume prokaryotes. They are of particular evolutionary interest because they appear to be the sister group to the Opisthokonts ...

. However, it is viewed as paraphyletic, with the Breviata as more basal. The opisthokonts appear to have emerged as sister of the Apusomonadida. It has been suggested that the Mantamonadida be classified in Apusozoa. The

Ancyromonadida Ancyromonadida or Planomonadida is a small group of biflagellated eukaryotes found in the soil and in aquatic habitats, where they feed on bacteria.Cavalier-Smith, T. (2013)Early evolution of eukaryote feeding modes, cell structural diversity, a ...

appear to be

Varisulca Varisulca was a proposed basal Podiate taxon. It encompassed several lineages of heterotrophic protists, most notably the ancyromonads (planomonads), collodictyonids (diphylleids), rigifilids (''Rigifila'', ''Micronuclearia'') and mantamona ...

, Planomonadida, shifting them possibly more basal than the Amoebozoa, or less basal. While some classification systems have placed Hemimastigida in Apusozoa, 2018 research indicated that hemimastigotes (/

Hemimastix Spironematellidae (previously known as Spironemidae) is a family (biology), family of heterotrophic flagellates, in the group Hemimastigophora. They vary in size and shape from the ellipsoid ''Hemimastix amphikineta'' (14 × 7 μm) to the ve ...

/ Spironematellidae) are their own supra-kingdom.

Characteristics

The apusomonads 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. The form of the

mitochondria A mitochondrion () is an organelle found in the cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is us ...

varies between the different orders. Among the apusomonads they have tubular

crista A crista (; : cristae) is a fold in the inner mitochondrial membrane, inner membrane of a mitochondrion. The name is from the Latin for ''crest'' or ''plume'', and it gives the inner membrane its characteristic wrinkled shape, providing a large a ...

e, 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.

Evolution

As currently defined, Apusozoa is a

grade that includes two classes: Thecomonadea (the apusomonad flagellates) and Breviatea (the breviate amoebae), each of them

monophyletic In biological cladistics for the classification of organisms, monophyly is the condition of a taxonomic grouping being a clade – that is, a grouping of organisms which meets these criteria: # the grouping contains its own most recent co ...

and

sister group In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and ...

s to the Opisthokonta supergroup. Together, these three clades are part of the larger clade

Obazoa Obazoa is a proposed sister clade of Amoebozoa (which together form Amorphea). The term Obazoa is based on the OBA acronym for Opisthokonta, Breviatea, and Apusomonadidae, the group's three constituent clades. Determining the placement of Bre ...

, which in turn is the sister group to the

Amoebozoa Amoebozoa is a major Taxonomy (biology), taxonomic group containing about 2,400 described species of Amoeba, amoeboid protists, often possessing blunt, fingerlike, Pseudopod#Morphology, lobose pseudopods and tubular mitochondrial cristae. In trad ...

supergroup.

References

External links

Amorphea phyla Taxa named by Thomas Cavalier-Smith Obsolete eukaryote taxa {{Unikont-stub