Centrohelid

Centrohelids
Raphidiophrys contractilis
Scientific classification
Domain:	Eukaryota
Order:	Centrohelida Kühn 1926
Families
Raphidiophryidae Acanthocystidae Heterophryidae

The centrohelids or centroheliozoa are a large group of heliozoan protists.^[1] 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,^[2] 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).^[3] They are currently classified as Hacrobia, under the Plants+HC clade.

References

1. ^ 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. http://www.pnas.org/cgi/pmidlookup?view=long&pmid=15148395. 
2. ^ 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. 
3. ^ 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 2009: 231–238. doi:10.1093/gbe/evp022. PMC 2817417. PMID 20333193. http://gbe.oxfordjournals.org/cgi/content/short/evp022v1?rss=1. 

Further reading

• Patterson DJ (October 1999). "The Diversity of Eukaryotes". Am. Nat. 154 (S4): S96–S124. doi:10.1086/303287. PMID 10527921. http://www.journals.uchicago.edu/cgi-bin/resolve?AN991002.