Choanoflagellate
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The choanoflagellates are a group of occasionally colonial flagellate protozoa. They are considered to be the closest living relatives of the animals; the last unicellular ancestors of animals are thought to have resembled modern choanoflagellates.
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[edit] Appearance and ecology
Choanoflagellates resemble the individual 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.
The chonoflagellates likely performed an important ecological role prior to the evolution of animals; since they fed on bacteria, they 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]
[edit] Colonial behaviour
Most choanoflagellates are sessile, with a stalk opposite the flagellum. A number of species such as those in the genus Proterospongia may form simple colonies,[1] usually taking the form of a cluster of cells on a single stalk, but often forming planktonic clumps that resemble a miniature cluster of grapes in which each cell in the colony is flagellated.[citation needed] Many choanoflagellates build complex basket-shaped "houses" called lorica, from several silica strips cemented together.[citation needed] This multicellularity is achieved in a very different way to the animals.[1]
[edit] Abundance and diversity
There are over 125 extant species of choanoflagellates.[1] They are distributed ...
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[edit] 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, >.[1]
The genome of choanoflagellates, 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]
[edit] 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
[edit] References
- ^ a b c d e f g h i j k King, N.; Westbrook, M.J.; Young, S.L.; Kuo, A.; Abedin, M.; Chapman, J.; Fairclough, S.; Hellsten, U.; Isogai, Y.; Letunic, I.; Others, (2008). "The genome of the choanoflagellate Monosiga brevicollis and the origin of metazoans". Nature 451 (7180): 783-8. doi:.
- ^ Butterfield, N.J. (1997-04-01). "Plankton ecology and the Proterozoic-Phanerozoic transition". Paleobiology 23 (2): 247-262.
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This article or section includes a list of references or external links, but its sources remain unclear because it lacks in-text citations. You can improve this article by introducing more precise citations. |
- 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.
