Apusozoa

Apusozoa
Scientific classification
Domain:	Eukaryota
(unranked):	Unikonta
Phylum:	Apusozoa Cavalier-Smith 1997 em.
Orders
Apusomonadida Ancyromonadida Hemimastigida

The Apusozoa are a phylum comprising several genera of flagellate protozoa. 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).

Classification

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.	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.

Characteristics

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

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. It may be one of the most divergent bikont lineages.

It has more recently been grouped with the unikonts. It has been suggested that it may be related to the opisthokont group.

Stem groups

Eukaryote	Bikonta Archaeplastida Chromalveolata Unikonta Varisulca Amporphea Amoebozoa Obazoa Breviatea Apusomonadida Opisthokonts Holomycota Fungi Discicristoidea Nucleariida Fonticulida Holozoa Teretosporea Corallochytrium Mesomycetozoa Filozoa Filasterea Choanoflagellatea Animalia

One view of the great kingdoms and their stem groups.