''Tetrahelia'' () is a genus of four-

ciliated The cilium, plural cilia (), is a membrane-bound organelle found on most types of eukaryotic cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike project ...

protist A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the exc ...

s belonging to the

Endohelea Endohelea is a proposed clade of eukaryotes that are related to Archaeplastida and the SAR supergroup. Classification Based on studies done by Cavalier-Smith, Chao & Lewis 2015 * Class Endohelea Cavalier-Smith 2012 ** Order Microhelida Cavalier- ...

, a group of

heterotrophic A heterotroph (; ) is an organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain, heterotrophs are primary, secondary and tertiary consumers, but ...

eukaryotes previously considered

heliozoa Heliozoa, commonly known as sun-animalcules, are microbial eukaryotes ( protists) with stiff arms ( axopodia) radiating from their spherical bodies, which are responsible for their common name. The axopodia are microtubule-supported projections f ...

. It is the only genus in the family Tetraheliidae and order Axomonadida. It is a

monotypic In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon. A monotypic species is one that does not include subspecies or smaller, infraspecific taxa. In the case of genera, the term "unispe ...

genus, containing the sole species ''Tetrahelia pterbica'', previously classified as '' Tetradimorpha''.

Description

''Tetrahelia'' are unicellular

ciliate The ciliates are a group of alveolates characterized by the presence of hair-like organelles called cilia, which are identical in structure to eukaryotic flagella, but are in general shorter and present in much larger numbers, with a differen ...

s with four standard-length centrioles that are shorter than in '' Heliomorpha'' and '' Tetradimorpha'', and axopodia generated by a globular

centrosome In cell biology, the centrosome (Latin centrum 'center' + Greek sōma 'body') (archaically cytocentre) is an organelle that serves as the main microtubule organizing center (MTOC) of the animal cell, as well as a regulator of cell-cycle prog ...

with a distinct granular shell and a

microfibril A microfibril is a very fine fibril, or fiber-like strand, consisting of glycoproteins and cellulose. It is usually, but not always, used as a general term in describing the structure of protein fiber, e.g. hair and sperm tail. Its most frequently ...

lar core. The centrioles are arranged in two pairs: each pair has two parallel centrioles, and the pairs are positioned at 30° of rotation between each other. They are linked at the base by an amorphous material that connects them to the centrosome. There are lateral

dictyosome The Golgi apparatus (), also known as the Golgi complex, Golgi body, or simply the Golgi, is an organelle found in most eukaryotic cells. Part of the endomembrane system in the cytoplasm, it packages proteins into membrane-bound vesicles ...

s on either side of the cell

nucleus Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom * Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA Nucl ...

. The axopodia have several irregularly arranged microtubules and irregularly flattened

extrusome Extrusomes are membrane-bound structures in some eukaryotes which, under certain conditions, discharge their contents outside the cell. There are a variety of different types, probably not homologous, and serving various functions. Notable extru ...

s, instead of the kinetocysts seen in '' Heliomorpha'' and '' Tetradimorpha radiata''. The cell size is larger than 60 μm, and the centrosome itself measures between 18 and 20 μm. There is a thick pseudopellicle layer beneath the

cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment ( ...

. The life cycle of ''Tetrahelia'' contains a lazily swimming, purely flagellate stage with fully retracted axopodia.

References

{{reflist, refs= {{cite journal, vauthors=Cavalier-Smith T, title=Ciliary transition zone evolution and the root of the eukaryote tree: implications for opisthokont origin and classification of kingdoms Protozoa, Plantae, and Fungi, journal=Protoplasma, volume=259, pages=487–593, date=2022, doi=10.1007/s00709-021-01665-7, pmc=9010356, pmid=34940909 {{cite journal, vauthors=Yabuki A, Chao EE, Ishida KI, Cavalier-Smith T, title=Microheliella maris (Microhelida ord. n.), an Ultrastructurally Highly Distinctive New Axopodial Protist Species and Genus, and the Unity of Phylum Heliozoa, journal=Protist, volume=163, issue=3, date=2012, pages=356–388, issn=1434-4610, doi=10.1016/j.protis.2011.10.001, pmid=22153838 Endohelean genera