Choanoflagellates (Choanoflagellida) are free-living, single-celled, and colony-forming eukaryotes that are ubiquitous in aquatic environments. They are believed to be the sister group to the animals (kingdom Animalia). As their name implies, choanoflagellates (collared flagellates) have a distinctive cell morphology characterized by an ovoid or spherical cell body 3-10 µm in diameter with a single apical flagellum surrounded by a collar of 30-40 microvilli. Movement of the flagellum creates water currents that can propel free-swimming choanoflagellates through the water column and trap bacteria and detritus against the collar of microvilli where these foodstuffs are engulfed (see this animation from the University of Alberta).
Dujardin, a French biologist interested in protozoan evolution, noted the morphological similarities of choanoflagellates and sponge choanocytes and proposed the possibility of a close relationship as early as 1841 (Leadbeater and Kelly 2001). Over the past decade, this hypothesized relationship between choanoflagellates and animals has been supported by independent analyses of multiple unlinked sequences: 18S rDNA, nuclear protein-coding genes, and mitochondrial genomes (Wainright et al. 1993; Mendoza et al. 2002; Burger et al. 2003; Ruiz-Trillo et al. 2004, 2006; Steenkamp et al. 2006). Importantly, comparisons of mitochondrial genome sequences from a choanoflagellate and three sponges confirm the placement of choanoflagellates as an outgroup to Metazoa and negate the possibility that choanoflagellates evolved from metazoans (Lavrov et al. 2005). Finally, recent studies of genes expressed in choanoflagellates have revealed that choanoflagellates synthesize homologues of metazoan cell signaling and adhesion genes (King et al. 2003). Because choanoflagellates and metazoans are closely related, comparisons between the two groups promise to provide insights into the biology of their last common ancestor and the earliest events in metazoan evolution.
Choanoflagellates are found globally in marine, brackish and freshwater environments from the Arctic to the tropics, occupying both pelagic and benthic zones. Although most sampling of choanoflagellates has occurred between 0 m and 25 m, they have been recovered from as deep as 300 m in open water (Thomsen 1982) and 100 m under Antarctic ice sheets (Buck and Garrison 1988). Many species are hypothesized to be cosmopolitan on a global scale, while other species are reported to have restricted regional distributions (Thomsen et al. 1991).
(slightly modified from ChoanoWiki contributors 2011)
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- Burger, G. L. Forget, Y. Zhu, M. Gray, and B. Lang. 2003. Unique mitochondrial genome architecture in unicellular relatives of animals. Proceedings of the National Academies of Sciences (U.S.A.) 100 (3): 892-897.
- ChoanoWiki contributors, 'Choanoflagellates', ChoanoWiki, , 13 July 2011, 05:28 UTC, http://www.choano.org/choanowiki/index.php?title=Choanoflagellates&oldid=1713 [accessed 7 December 2012]
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- Lavrov, D., L. Forget, M. Kelly, and B. Lang. 2005. Mitochondrial genomes of two Demosponges provide insights into an early stage of animal evolution. Molecular Biology and Evolution 22(5): 1231-1239.
- Leadbeater, B. and M. Kelly, M. 2001. Evolution of animals choanoflagellates and sponges. Water and Atmosphere Online. 9 (2): 9-11.
- Mendoza, L. and Taylor, J. and Ajello, L. (2002) The class mesomycetozoea: a heterogeneous group of microorganisms at the animal-fungal boundary. Annual Review of Microbiology 56: 315-344.
- Ruiz-Trillo, I,, Y. Inagaki, L.A. Davis, S. Sperstad, B. Landfald, and A.J. Roger. 2004. Capsaspora owczarzaki is an independent opisthokont lineage. Current Biology 14(22): R946-947.
- Ruiz-Trillo, I., C.E. Lane, M.J. Archibald, and A.J. Roger. 2006. Insights into the evolutionary origin and genome architecture of the unicellular opisthokonts Capsaspora owczarzaki and Sphaeroforma arctica. Journal of Eukaryotic Microbiology 53(5): 379-84.
- Steenkamp, E., J. Wright, and S. Baldauf. 2006. The Protistan Origins of Animals and Fungi. Molecular Biology and Evolution 23(1): 93-106.
- Thomsen, H.A. 1982. Planktonic choanoflagellates from Disko Bugt, West Greenland, with a survey of the marine nanoplankton of the area. Meddelelser om Gronland, Bioscience 8: 3-36.
- Thomsen, H.A., K. Buck, and F. Chavez. 1991 Choanoflagellates of the central California waters: Taxonomy, morphology and species assemblages. Ophelia, 33(2): 131-164.
- Wainright, P. , G. Hinkle, M. Sogin, and S. Stickle. 1993. Monophyletic origins of the metazoa: an evolutionary link with fungi. Science, 260 (5106): 340-342.