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Microheliella
Differential interference contrast image of a living cell of M. maris showing axopodia (arrows), tiny vacuole (V) and centrosome (C). Scale bar: 10 μm.
Scientific classification
Domain:	Eukaryota
Clade:	Diaphoretickes
Clade:	CAM
Clade:	Pancryptista
Order:	Microhelida Cavalier-Smith 2012
Family:	Microheliellidae Cavalier-Smith & Chao 2012
Genus:	Microheliella Cavalier-Smith & Chao 2012
Species:	M. maris
Binomial name
Microheliella maris Cavalier-Smith & Chao 2012

Microheliella (from Greek micro 'small' and helios 'sun') is a monotypic genus of protists containing the sole species M. maris, first described in 2012. It has a variety of unusual morphological characteristics which make its broader classification difficult. These include a centrosome with two concentric granular shells and axopodia much simpler in structure than in visually similar protists (other 'heliozoa').

Etymology

The generic name Microheliella, as well as the names of the family Microheliellidae and order Microhelida, derive from Greek micro 'small' and helios 'sun', referring to the small cell size and the authors' wishes to retain the original informal name 'microheliozoan' given to this organism prior to its formal description. The specific epithet maris refers to seawater, its natural habitat.

Taxonomy

The genus Microheliella was described from an organism discovered contaminating a culture of the amoeba Cochliopodium from the Ebro Delta, Spain, collected in 2003 by Alexey Smirnov. This organism was subsequently transferred into a pure culture through serial dilution and maintained at Oxford for nine years in a growth medium of 50% artificial seawater, with naturally occurring bacteria as its food. Initially it was given the informal name 'marine microheliozoan', and a phylogenetic analysis using 18S and 28S rRNA sequences placed it close to centrohelids. Later, the same organism was examined under electron microscopy and its Hsp90 gene was sequenced. Combining the newly obtained morphological and genetic data, it was formally described by Thomas Cavalier-Smith and Ema E. Chao in 2012 as the novel species Microheliella maris. Its type (biology) culture, CCAP 1945/1, is kept in the Culture Collection of Algae and Protozoa at Oban, in Scotland.

To accommodate this genus, the monotypic family Microheliellidae and order Microhelida were created, and classified as part of the phylum Cryptista in accordance to phylogenetic analyses. Later, phylogenomic analyses demonstrated its position as the sister clade to all Cryptista, in a clade known as Pancryptista.

Evolution

Recent phylogenomic analyses suggest the microhelida are sister to the Cryptista, forming a clade called Pancryptista, which would be sister to the Archaeplastida.

References

1. ^ Yabuki, A.; et al. (8 December 2011). "Microheliella maris (Microhelida ord. n.), an ultrastructurally highly distinctive new axopodial protist species and genus, and the unity of phylum Heliozoa". Protist. 163 (3): 356–388. doi:10.1016/j.protis.2011.10.001. PMID 22153838.
2. Cavalier-Smith, Thomas; Chao, Ema E. (2003). "Molecular phylogeny of centrohelid heliozoa, a novel lineage of bikont eukaryotes that arose by ciliary loss". Journal of Molecular Evolution. 56 (4): 387–396. doi:10.1007/s00239-002-2409-y. PMID 12664159.
3. ^ Yazaki, Euki; Yabuki, Akinori; Imaizumi, Ayaka; Kume, Keitaro; Hashimoto, Tetsuo; Inagaki, Yuji (2022). "The closest lineage of Archaeplastida is revealed by phylogenomics analyses that include Microheliella maris". Open Biol. 12: 210376. doi:10.1098/rsob.210376. PMC 9006020.
4. Yazaki, Euki; Yabuki, Akinori; Imaizumi, Ayaka; Kume, Keitaro; Hashimoto, Tetsuo; Inagaki, Yuji (2021-08-31). "Phylogenomics invokes the clade housing Cryptista, Archaeplastida, and Microheliella maris". pp. 2021.08.29.458128. bioRxiv 10.1101/2021.08.29.458128.

• Cryptista genera