The Rhizaria are a species-rich supergroup of 1]eukaryotes. They vary considerably in form, but for the most part they are amoeboids with filose, reticulose, or microtubule-supported pseudopods. Many produce shells or skeletons, which may be quite complex in structure, and these make up the vast majority of protozoan fossils. Nearly all have mitochondria with tubular cristae.
There are three main groups of Rhizaria:
- Cercozoa - Various amoebae and flagellates, usually with filose pseudopods and common in soil
- Foraminifera - Am oeboids with reticulose pseudopods, common as marine benthos
- Radiolaria - Amoeboids with axopods, common as marine plankton
A few other groups may be included in the Cercozoa, but on some trees appear closer to the Foraminifera. These are the Phytomyxea and Ascetosporea, parasites of plants and animals respectively, and the peculiar amoeba Gromia. The different groups of Rhizaria are considered close relatives based mainly on genetic similarities, and have been regarded as an extension of the Cercozoa. The name Rhizaria for the expanded group was introduced by Cavalier-Smith in 2002, who also included the centrohelids and Apusozoa.
Rhizaria is part of the bikont clade, which also comprises the Archae plastida, the Chromalveolata, the Excavata, and some smaller, unresolved groups such as the Apusozoa and the Centrohelida. As bikonts, they all descend from a heterotrophic eukaryote with two flagella.
Historically, many rhizarians were considered animals, with their motility and heterotrophy as justification. However, when the five-kingdom system took prevalence over the animal-plant dichotomy, the rhizarians were put into the kingdom Protista. Then, after Woese published his three-domain system, because of the paraphyly of the kingdom Monera, taxonomists turned their attention to the eukaryote domain, and the inherent paraphyly of Protista. After much debate, which continues to this day, Rhizaria emerged as a monophyletic group.
The Infrakingdom Rhizaria is further organized into finer groupings including:
The Foraminifera, ("Hole Bearers") or forams for short, are a large group of protists with reticulating pseudopods, fine strands of cytoplasm that branch and merge to form a dynamic net. They typically produce a test, or shell, which can have either one or multiple chambers, some becoming quite elaborate in structure. These shells are made of calcium carbonate (CaCO3) or agglutinated sediment particles. About 275,000 species are recognized, both living and fossil. They are usually less than 1 mm in size, but some are much larger, and the largest recorded specimen reached 19 cm. [more]
At least 27 species and subspecies belong to the Phylum Radiozoa.
More info about the Phylum Radiozoa may be found here.
- ^ Christopher Taylor (2004). "Rhizaria". http://www.palaeos.com/Eukarya/Units/Rhizaria/Rhizaria.html.
- ^ Nikolaev SI, Berney C, Fahrni JF, et al. (May 2004). "The twilight of Hel iozoa and rise of Rhizaria, an emerging supergroup of amoeboid eukaryotes". Proc. Natl. Acad. Sci. U.S.A. 101 (21): 8066–71. doi:10.1073/pnas.0308602101. PMID 15148395. PMC: 419558. http://www.pnas. org/cgi/pmidlookup?view=long&pmid=15148395.
- ^ Moreira D, von der Heyden S, Bass D, López-García P, Chao E, Cavalier-Smith T (July 2007). "Global eukaryote phylogeny: Combined small- and large-subunit ribosomal DNA trees support monophyly of Rhizaria, Retaria and Excavata". Mol. Phylogenet. Evol. 44 (1): 255–66. doi:10.1016/j.ympev.2006.1 1.001. PMID 17174576. http://linkinghub.elsevier.com/retrieve/pii/S1055-7903(06)00433-7.
- ^ Cavalier-Smith, Thomas (2002). "The phagotrophic origin of eukaryotes and phylogenetic classification of Protozoa". International Journal of Systematic and Evolutionary Microbiology 52 (2): 297–354. ISSN 1466-5026. PMID 11931142. http://ijs.sgmjournals.org/cgi/content/abstract/52/2/297. Retrieved on 2007-06-08.
- ^ Fabien, Burki; Kamran Shalchian-Tabrizi, Marianne Minge, Åsmund Skjæveland, Sergey I. Nikolaev, Kjetill S. Jakobsen, Jan Pawlowsk i (2007). "Phylogenomics Reshuffles the Eukaryotic Supergroups". PLoS ONE 2 (8): e790-. doi:10.1371/journal.pone.0000790. http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0000790. Retrieved on 2008-01-24.
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