Choanoflagellate
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| Class: | Choanoflagellatea
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The choanoflagellates are a group of free-living unicellular and colonial flagellate eukaryotes considered to be the closest living relatives of the animals. As the name suggests, 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 (see figure). 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. This feeding provides a critical link the the global carbon cycle, linking trophic levels. In addition to their critical ecological roles, choanoflagellates are of particular interest to evolutionary biologists studying the origins of multicellularity in animals. As one of the closest living relatives of animals, choanoflagellates serve as a useful model for reconstructions of the last unicellular ancestor of animals.
Appearance and ecology
Choanoflagellates resemble the individual choanocyte cells of sponges:[1] each choanoflagellate has a single flagellum, surrounded by a ring of actin-filled protrusions called microvilli, forming a cylindrical or conical collar (choanos in Greek). The flagellum draws water through the collar, and bacteria and detrital particles are captured by the microvilli and ingested.[1] The flagellum also pushes free-swimming cells along, as in animal sperm — most other flagellates are pulled by their flagella. Many choanoflagellates build complex basket-shaped "houses" called lorica, from several silica strips cemented together.[citation needed]
The choanoflagellates feed on bacteria and link otherwise inacessible forms of carbon (since it is so small) to organisms higher in the trophic chain.[2] Even today they are important in the carbon cycle and microbial food web.[1]
Colonial behaviour
A number of species such as those in the genus Proterospongia form simple colonies,[1] planktonic clumps that resemble a miniature cluster of grapes in which each cell in the colony is flagellated or clusters of cells on a single stalk.[citation needed]
Abundance and diversity
There are over 125 extant species of choanoflagellates.[1] distributed 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 [e.g., Diaphanoeca grandis has been reported from North America, Europe and Australia (OBIS)], while other species are reported to have restricted regional distributions (Thomsen, et al., 1991). Co-distributed choanoflagellate species can occupy quite different microenvironments, but in general, the factors that influence the distribution and dispersion of choanoflagellates remain to be elucidated.
Comparison and relationship with other taxa
The choanocytes (also known as "collared cells") of sponges (considered the most basal metazoa) have the same basic structure as choanoflagellates. Collared cells are occasionally found in a few other animal groups, such as flatworms.[citation needed]
Genome sequencing shows that among living organisms, the choanoflagellates are most closely related to animals.[1]
The last common ancestor of animals and choanoflagellates was unicellular, perhaps forming simple colonies; in contrast, the last common ancestor of all animals was a relatively complex multicellular organism, with differentiated tissues, a definite "body plan", and complex embryonic development (including gastrulation).[1] The timing of the splitting of these lineages is difficult to constrain, but was probably in the late Precambrian, >600 million years ago.[1]
The genome of Monosiga brevicollis, with 41.6 million base pairs,[1] is similar in size to filamentous fungi and other free-living unicellular eukaryotes, but far smaller than that of typical animals.[1]
External links
- Tree of Life Webpage for Choanoflagellates
- Choanobase, the Choanoflagellate genetic library, developed and maintained by the Nicole King laboratory at the University of California, Berkeley
References
- ^ a b c d e f g h i j King, N. (2008). "The genome of the choanoflagellate Monosiga brevicollis and the origin of metazoans". Nature. 451 (7180): 783–8. doi:10.1038/nature06617.
{{cite journal}}: Unknown parameter|coauthors=ignored (|author=suggested) (help)CS1 maint: extra punctuation (link) - ^ Butterfield, N.J. (1997-04-01). "Plankton ecology and the Proterozoic-Phanerozoic transition". Paleobiology. 23 (2): 247–262. Retrieved 2007-08-19.
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 | This article includes a list of references, related reading, or external links, but its sources remain unclear because it lacks inline citations. (March 2008) |
- Snell, EA, Furlong, RF, and PWH Holland. 2001. Hsp70 sequences indicate that choanoflagellates are closely related to animals. Current Biology. 11:967-970.
- King, N., and S. B. Carroll. 2001. A receptor tyrosine kinase from choanoflagellates: molecular insights into early animal evolution. PNAS 98:15032-7.
- Lang, B. F., C. O'Kelly, T. Nerad, M. W. Gray, and G. Burger. 2002. The closest unicellular relatives of animals. Curr Biol 12:1773-8.
- Philippe, H, Snell, EA, Bapteste, E, Lopez, P, Holland, PWH, and D Casane. 2004. Phylogenomics of eukaryotes: the impact of missing data on alignments. Molecular Biology and Evolution. 21(9):123-135.