DESCRIPTION OF THE PHYLUM CENTROHELOMONADA (KÜHN 1926)

EUKARYA> CHROMALVEOLATA> HACROBIAE> CENTROHELOMONADA

Centrohelomonada (sen-tro-he-lo-mo-NA-da) is made of three Greek roots that mean central (kentrokos -κεντρικός); sun (helios -ήλιος); and unit (monada -μονάδα). The reference is to a cell with spike-like rays emanating from it (formerly one of the unicells called heliozoans, sun animacules).

INTRODUCTION TO THE CENTROHELOMONADA The centrohelid heliozoans resemble other heliozoan taxa by having a round cell body with radiating axopods (text with tooltip) An axopod is a thin pseudopodium that is permanent and stiff due to the presence of an internal microtubular structure. This is characteristic of the Actinopodotista. , and stiffened cytoplasmic extensions supported by an internal microtubular framework. The centrohelids, however, have a central microtubule organizing center (MTOC) that may or may not have an axoplast (text with tooltip) An axoplast is a central granule (sometimes called centroplast) that has no internal differentiation. The axopods arise from the central granule of certain 'radiolarians'. , but is always associated with the nucleus. The most common taxa (e.g. Raphidiophrys and Acanthocystis) are covered with silicaceous scales. The cell body of Raphidiophrys has body scales that are flat and distinctive (Figure 3). Acanthocystis also has silicaceous spines (Figure 4). Many taxa are found in freshwater (e.g. Wujek 2003 and Leonov 2009). However, Cavalier-Smith and von der Heyden (2007) show several independent lineages that seem to be strictly marine. Leonov (2009) in a survey of freshwater in the Voronezh Province near Ukraine, found them in rivers, lakes, reserviors and bogs, but the highest diversity was in bogs. For some time the centrohelids seemed to have found a home in the “sarcodina” because they produced axopods (e.g. Kudo 1966; and Grell 1973). That also seemed to ally them with numerous other heliozoan-like organisms (e.g. Radiolaria, Actinophryids, Desmothoraceans, etc.) and led to the union of all axopod-bearing taxa into the phylum Actinopoda (Febvre-Chevalier 1990; and Margulis and Schwartz 1998). Smith and Patterson (1986) reviewed ultrastructural and life history details and suggested that the groups of heliozoans were only superficially alike and in need of systematic revision. Patterson (1999) separated the heliozoans into several sisterless groups, some of which were later incorporated into the Cercozoae, and the Heterokontae (=Stramenopiles). Cavalier-Smith and Chao (2003c) concluded that the centrohelids (sensu strictu) evolved from biflagellated forms, and the centroplast is just a modified centrosome. They further claimed that the centrohelids formed an independent lineage. Though other analyses confirmed that conclusion, they did show an association with the Cryptomonads (Nilolaev et al. 2004), and with the haptomonads (Cavalier-Smith 2003a, Cavalier-Smith and von der Heyden 2007). Burki et al. (2007) and Hackett et al. (2007) also show the monophyly of cryptomonads and haptophytes with a sister relationship to the Rhizaria, the Chromalveolates, or both. Analyses of Burki et al. (2009) and Okamoto et al. (2009) confirm that the centrohelids are in a monophyletic group (called Hacrobia by Okamoto et al. 2009), which also includes haptophytes and cryptophytes. The taxonomy of this group is in disarray. Febvre-Chevalier (1990) unites the centrohelids (sensu stricto) with the dimorphids on the basis that both groups have microtubule organizing centers (MTOC) in the center of the cell, sometimes surrounded by the nucleus. Although Patterson (1999) considers the two groups to be separate, we have retained them as a provisional group until further molecular/ultrastructural work can be done on them. Cavalier-Smith (1993) placed the dimorphids into the Sarcomonadea within the Filosa (Cavalier-Smith and Chao 2003) of the Supergroup Rhizaria.

FIGURE 1. Acanthocystis showing appearance of the cell and axopods. Image from Cavalier-Smith and von der Heyden (2007)	FIGURE 2. Raphidiophrys showing the microtubule organizing center and axopods. Image from http://www.bch.umontreal.ca/protists/raphp/appearance.html	FIGURE 3. SEM micrograph of body scales of Raphidiophrys. Image from Leonov (2009)	FIGURE 4. SEM micrograph of spicules or spines of Acanthocystis. Image from Leonov (2009)

FIGURE 5. A cladogram showing the relationship of the centrohelids (in shaded box) with the rest of the Eukaryomonadae (taxa in bold). The relationship is a simplified view of the molecular phylogeny generated by Okamoto et al. (2009) based on Hsp90.

LITERATURE CITED Burki, F., Y. Inagaki, J. Brate, J. M. Archibald, P. J. Keeling, T. Cavalier-Smith, M. Sakaguchi, T. Hashimoto, A. Horak, S. Kumar, D. Klaveness, K. S. Jakobsen, J. Pawlonski, and K. Shalchian-Tabrizi. 2009. Large-scale phylogenomic analyses reveal thet two enigmatic protist lineages, Telonemia and Centroheliozoa, are related to photosynthetic chromalveolates. Genome. Biol. Evol. 1(1): 231-238. Cavalier-Smith, T. 2003a. Protist phylogeny and the high-level classification of Protozoa. European Journal of Protistology. 39:338-348. Cavalier-Smith, T. and E. E. Chao. 2003c. Molecular phylogeny of centrohelid heliozoan, a novel lineage of bikont eukaryotes that arose by ciliary loss. Journal of Molecular Evolution. 56:387-396. Cavalier-Smith, T. and S. von der Heyden. 2007. Molecular phylogeny, scale evolution and taxonomy of centrohelid heliozoa. Molecular Phylogenetics and Evolution. 44: 1186-1203. Febvre-Chevalier, C. 1990. Heliozoa. In: Margulis, L., J. O. Corliss, M. Melkonian, and D. J. Chapman, eds. 1990. Handbook of the Protoctista; the structure, cultivation, habits and life histories of the eukaryotic microorganisms and their descendants exclusive of animals, plants and fungi. Jones and Bartlett Publishers. Boston. pp. 347-362. Grell, K. G. 1973. Protozoology. Springer-Verlag. New York. Kudo, R. R. 1966. Protozoology. 5th ed. Charles C. Thomas Publisher. Springfield. Leonov, M. M. 2009. Heliozoan fauna of waterbodies and watercourses of the Central Russian upland forest-steppe. Inland Water Biology. 2(1): 6-12. Margulis, L. and K. Schwartz. 1998. Five kingdoms, an illustrated guide to the phyla of life on earth. 3rd Edition. W. H. Freeman and Company. New York. Nikolaev, S. I., C. Berney, J. Fahrni, I. Bolivar, S. Polet, A. P. Mylnikov, V. V. Aleshin, N. B. Petrov, and J. Pawlowski. 2004. The twilight of Heliozoa and rise of Rhizaria, an emerging supergroup of amoeboid eukaryotes. Proceedings of the National Academy of Sciences. USA. 101(21): 8066-8071. Okamoto, N., C. Chantangsi, A. Horak, B. S. Leander, and P. J. Keeling. 2009. Molecular phylogeny and description of the novel katablepharid Roombia truncata gen. et sp. nov., and establishment of the Hacrobia taxon nov. PLoS One 4(9):e7080. Patterson, D. J. 1999. The diversity of eukaryotes. American Naturalist. 154 (Suppl.): S96–S124. Smith, R. M. and D. J. Patterson. 1986. Analyses of heliozoan interrelationships: an example of the potentials and limitations of ultrastructural approaches to the study of protistan phylogeny. Proceedings of the Royal Society of London. B. 227:325-266. Wujek, D. E. 2003. Freshwater Heliozoa (Protista, Heliozoa) from Indiana. Proceedings of the Indiana Academy of Science. 112(2): 169-174.

By Jack R. Holt. Last revised: 03/04/2013