Tetrahelia

Tetrahelia (from Latin tetra- 'four' and helio- 'sun') is a genus of four-ciliated protists belonging to the Endohelea, a group of heterotrophic heliozoa.^[2] It is the only genus in the family Tetraheliidae, order Axomonadida, class Endohelea and subphylum Endohelia. It is a monotypic genus, containing the sole species Tetrahelia pterbica, previously classified as Tetradimorpha pterbica.^[1]

Tetrahelia
Scientific classification
Domain:	Eukaryota
Clade:	Pancryptista
Phylum:	Cryptista
Subphylum:	Endohelia Cavalier-Smith 2021
Class:	Endohelea Cavalier-Smith in Yabuki et al. 2012 emend. Cavalier-Smith 2021
Order:	Axomonadida Cavalier-Smith in Yabuki et al. 2012 emend. 2022
Family:	Tetraheliidae Cavalier-Smith 2022
Genus:	Tetrahelia Cavalier-Smith 2022
Species:	T. pterbica
Binomial name
Tetrahelia pterbica (Mikrjukov & Patterson 2000) Cavalier-Smith 2022[1]
Synonyms
Tetradimorpha pterbica Mikrjukov & Patterson in Mikrjukov 2000

Description

Tetrahelia are unicellular ciliates with four standard-length centrioles that are shorter than in Heliomorpha and Tetradimorpha, and axopodia generated by a globular centrosome with a distinct granular shell and a microfibrillar 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 dictyosomes on either side of the cell nucleus. The axopodia have several irregularly arranged microtubules and irregularly flattened extrusomes, 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.^[1]

The life cycle of Tetrahelia contains a lazily swimming, purely flagellate stage with fully retracted axopodia.^[1]

Classification

In 2021 Cavalier-Smith treated Axomonadida as a member of Endohelea, with also Microhelida (Microheliella). In 2022, Yazaki et al. excluded Microheliella from Endohelea and made it Cryptista's sister taxon.^[1][3]