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Centrohelid

Centroplasthelida

Description of Centroheliozoa

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Circumscription: The centrohelids, one of several groups previously assigned to the Heliozoa that are now regarded as polyphyletic. Round bodied with stiff radiating arms (axopodia) supported internally by microtubules and with extrusomes that are quite easy to see. Cell surface naked or with organic or siliceous spines and/or scales. Some species with symbiotic algae. No flagellated stages known, may form cysts. Ultrastructural identity: Mitochondria with lamellate flat cristae. Microtubules of radiating axonemes in hexagon with triangle arrangement and arising from a multilamellate microtubule organizing centre, extrusomes with ball-and-cone organization. Radiating system of lacunae near cell surface. Many conventional. dictyosomes dispersed in cytoplasm. Siliceous elements of periplast created within silicon deposition vesicles. Synapomorphy: Platycristate heliozoa with multilamellate microtubule organizing centre faced with hemispherical structures giving rise to axonemes of microtubules arranged in hexagons and triangles. Composition: About 85 species; Family Heterophryidae (Cienkowskya, Oxnerella, Sphaerastrum, Heterophrys), Family Raphidiophryidae (Parasphaerastrum, Polyplacocystis, Raphidiophrys, Raphidocystis,) Family Acanthocystidae (Pseudoraphidocystis, Pseudoraphidiophrys, Pterocystis, Echinocystis, Choanocystis, Acanthocystis).
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Centrohelid

provided by wikipedia EN

The centrohelids or centroheliozoa are a large group of heliozoan protists.[3] They include both mobile and sessile forms, found in freshwater and marine environments, especially at some depth.

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.

Classification

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,[4] 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).[5] They are currently classified as Hacrobia, under the Plants+HC clade, although some research studies have found evidence against the monophyly of this group.[6] Centrohelids are currently divided into two orders with contrasting scale morphology and ultrastructure: Pterocystida and Acanthocystida.[1]

References

  1. ^ a b 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. PMID 22317961..mw-parser-output cite.citation{font-style:inherit}.mw-parser-output q{quotes:"""""'"'"}.mw-parser-output code.cs1-code{color:inherit;background:inherit;border:inherit;padding:inherit}.mw-parser-output .cs1-lock-free a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/6/65/Lock-green.svg/9px-Lock-green.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-lock-limited a,.mw-parser-output .cs1-lock-registration a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/d/d6/Lock-gray-alt-2.svg/9px-Lock-gray-alt-2.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-lock-subscription a{background:url("//upload.wikimedia.org/wikipedia/commons/thumb/a/aa/Lock-red-alt-2.svg/9px-Lock-red-alt-2.svg.png")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration{color:#555}.mw-parser-output .cs1-subscription span,.mw-parser-output .cs1-registration span{border-bottom:1px dotted;cursor:help}.mw-parser-output .cs1-hidden-error{display:none;font-size:100%}.mw-parser-output .cs1-visible-error{font-size:100%}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration,.mw-parser-output .cs1-format{font-size:95%}.mw-parser-output .cs1-kern-left,.mw-parser-output .cs1-kern-wl-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right,.mw-parser-output .cs1-kern-wl-right{padding-right:0.2em}
  2. ^ Kühn, A. (1926). Morphologie der Tiere in Bildern. Heft 2: Protozoen. Teil 2. Rhizopoden. Gebrüder Borntraeger: Berlin.
  3. ^ Nikolaev SI; Berney C; Fahrni JF; et al. (May 2004). "The twilight of Heliozoa and rise of Rhizaria, an emerging supergroup of amoeboid eukaryotes". Proc. Natl. Acad. Sci. U.S.A. 101 (21): 8066–8071. doi:10.1073/pnas.0308602101. PMC 419558. PMID 15148395.
  4. ^ Cavalier-Smith T, Chao EE (April 2003). "Molecular phylogeny of centrohelid heliozoa, a novel lineage of bikont eukaryotes that arose by ciliary loss". J. Mol. Evol. 56 (4): 387–396. doi:10.1007/s00239-002-2409-y. PMID 12664159.
  5. ^ Burki, F; Inagaki, Y; Bråte, J; Archibald, J.; Keeling, P.; Cavalier-Smith, T; Sakaguchi, M; Hashimoto, T; Horak, A; Kumar, S; Klaveness, D; Jakobsen, K.S; Pawlowski, J; Shalchian-Tabrizi, K (2009). "Large-scale phylogenomic analyses reveal that two enigmatic protist lineages, Telonemia and Centroheliozoa, are related to photosynthetic chromalveolates" (Free full text). Genome Biology and Evolution. 1: 231–238. doi:10.1093/gbe/evp022. PMC 2817417. PMID 20333193.
  6. ^ Zhao, Sen; Burki, Fabien; Bråte, Jon; Keeling, Patrick J.; Klaveness, Dag; Shalchian-Tabrizi, Kamran (2012). "Collodictyon—An Ancient Lineage in the Tree of Eukaryotes". Molecular Biology and Evolution. 29 (6): 1557–68. doi:10.1093/molbev/mss001. PMC 3351787. PMID 22319147.
  7. ^ "Virae, Prokarya, Protists, Fungi". Collection of genus-group names in a systematic arrangement. Retrieved 30 June 2016.

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Centrohelid: Brief Summary

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The centrohelids or centroheliozoa are a large group of heliozoan protists. They include both mobile and sessile forms, found in freshwater and marine environments, especially at some depth.

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