Centrohelea Kuhn 1926 stat. n. Cavalier-Smith 1993
The centrohelids or centroheliozoa are a large group of heliozoanprotists.[4] They include both mobile and sessile forms, found in freshwater and marine environments, especially at some depth.[clarification needed]
Characteristics
Individuals are unicellular and spherical, usually around 30–80 μm in diameter, and covered with long radial axopods, narrow cellular projections that capture food and allow mobile forms to move about.
A few genera have no cell covering, but most have a gelatinous coat holding scales and spines, produced in special deposition vesicles. These may be organic or siliceous and come in various shapes and sizes. For instance, in Raphidiophrys the coat extends along the bases of the axopods, covering them with curved spicules that give them a pine-treeish look, and in Raphidiocystis there are both short cup-shaped spicules and long tubular spicules that are only a little shorter than the axopods. Some other common genera include Heterophrys, Actinocystis, and Oxnerella.
The axopods of centrohelids are supported by microtubules in a triangular-hexagonal array, which arise from a tripartite granule called the centroplast at the center of the cell. Axopods with a similar array occur in gymnosphaerids, which have traditionally been considered centrohelids (though sometimes in a separate order from the others). This was questioned when it was found they have mitochondria with tubular cristae, as do other heliozoa, while in centrohelids the cristae are flat. Although this is no longer considered a very reliable character, on balance gymnosphaerids seem to be a separate group.
The evolutionary position of the centrohelids is not clear. Structural comparisons with other groups are difficult, in part because no flagella occur among centrohelids, and genetic studies have been more or less inconclusive. Cavalier-Smith has suggested they may be related to the Rhizaria,[6] but for the most part they are left with uncertain relations to other groups. A 2009 paper suggests that they may be related to the cryptophytes and haptophytes (see Cryptomonads-haptophytes assemblage).[7] They are currently classified as Hacrobia, under the Plants+HC clade, although some research studies have found evidence against the monophyly of this group.[8]
Centrohelids were previously divided into two orders with contrasting scale morphology and ultrastructure: Pterocystida and Acanthocystida.[9] Posterior molecular studies of 2018 have rearranged the classification of centrohelids into two taxa: Pterocystida and Panacanthocystida, which includes both Acanthocystida and the genus Yogsothoth.[2][1]
Classification
The modern classification of centrohelids, as of 2019:[2][1]
CentroplasthelidaFebvre‐Chevalier & Febvre 1984 [=Centrohelea Kühn 1926 sensu Cavalier‐Smith in Yabuki et al. 2012; Centroheliozoa Dürrschmidt & Patterson 1987]
PterocystidaCavalier‐Smith and von der Heyden 2007, emend. Shɨshkin and Zlatogursky 2018
Pterocystis Siemensma & Roijackers 1988 non Lohmann 1904
Raineriophrys Mikrjukov 2001 [Rainierophrys (sic); Raineria Mikrjukov 1999 non Osswald 1928 non de Notaris 1838; Echinocystis Mikrjukov1997 non Haeckel 1896 non Bhatia & Chatterjee 1925 non Torrey & Gray 1840 non Gregory 1897]
^Heteroraphidiophrys, mentioned by Mikrjukov in 2002, was never formally introduced and needs to be avoided; the organism designated needs to be re‐isolated, carefully studied and provided with formal description.[1]
References
^ abcdeAdl SM, Bass D, Lane CE, Lukeš J, Schoch CL, Smirnov A, Agatha S, Berney C, Brown MW, Burki F, Cárdenas P, Čepička I, Chistyakova L, del Campo J, Dunthorn M, Edvardsen B, Eglit Y, Guillou L, Hampl V, Heiss AA, Hoppenrath M, James TY, Karnkowska A, Karpov S, Kim E, Kolisko M, Kudryavtsev A, Lahr DJG, Lara E, Le Gall L, Lynn DH, Mann DG, Massana R, Mitchell EAD, Morrow C, Park JS, Pawlowski JW, Powell MJ, Richter DJ, Rueckert S, Shadwick L, Shimano S, Spiegel FW, Torruella G, Youssef N, Zlatogursky V, Zhang Q (2019). "Revisions to the Classification, Nomenclature, and Diversity of Eukaryotes". Journal of Eukaryotic Microbiology. 66 (1): 4–119. doi:10.1111/jeu.12691. PMC6492006. PMID30257078.
^ abcShɨshkin, Yegor; Drachko, Daria; Klimov, Vladimir I.; Zlatogursky, Vasily V. (November 2018). "Yogsothoth knorrus gen. n., sp. n. and Y. carteri sp. n. (Yogsothothidae fam. n., Haptista, Centroplasthelida), with notes on evolution and systematics of centrohelids". Protist. 169 (5): 682–696. doi:10.1016/j.protis.2018.06.003.
^Kühn, A. (1926). Morphologie der Tiere in Bildern. Heft 2: Protozoen. Teil 2. Rhizopoden. Gebrüder Borntraeger: Berlin.
^Cavalier-Smith, Thomas; Chao, Ema E. (2012). "Oxnerella micra sp. n. (Oxnerellidae fam. n.), a Tiny Naked Centrohelid, and the Diversity and Evolution of Heliozoa". Protist. 163 (4): 574–601. doi:10.1016/j.protis.2011.12.005. PMID22317961.
