Choanoflagellate

The choanoflagellates are a group of flagellate protozoa. They are considered to be the closest living relatives of the animals, and the last unicellular ancestors of animals are thought to have resembled modern choanoflagellates.

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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 pulls water through the collar, and small food particles are captured by the microvilli and ingested. The flagellum also pushes free-swimming cells along, as in animal sperm, whereas most other flagellates are pulled by their flagella. Many choanoflagellates build complex basket-shaped "houses" called lorica, from several silica strips cemented together.

Most choanoflagellates are sessile, with a stalk opposite the flagellum. A number of species such as those in the genus Proterospongia are colonial, 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. There exist historical accounts of an extracellular matrix and non-flagellated cells in some colonies, but these accounts are unsubstantiated.^{[citation needed]}

The choanocytes (also known as "collared cells") of sponges have the same basic structure as choanoflagellates. Collared cells are occasionally found in a few other animal groups, such as flatworms. These relationships make colonial choanoflagellates a plausible candidate as representative of the ancestors of the animal kingdom.

References

• 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. Proc Natl Acad Sci U S A 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.
• James, T. Y., F. Kauff, C. L. Schoch, et al. mult. 2006. Reconstructing the early evolution of Fungi using a six-gene phylogeny. Nature 443:818-22.

• Choanobase, the Choanoflagellate genetic library, developed and maintained by the Nicole King laboratory at the University of California, Berkeley