Bilateral Versus Radial Symmetry During Evolution

Science  28 May 2004:
Vol. 304, Issue 5675, pp. 1207
DOI: 10.1126/science.304.5675.1207k

It is thought that bilaterians, which display symmetry along an anterior-posterior axis and a dorsal-ventral axis, may have evolved from an organism similar to modern cnidarians, the latter of which includes sea anemones, corals, hydras, and jellyfish. Cnidarians are generally believed to be radially symmetrical animals, but some cnidarians, such as the sea anemone Nematostella, display bilateral symmetry with an oral-aboral axis and a directive axis, which is orthogonal to the oral-aboral axis. To determine whether bilateral symmetry in cnidarians and bilaterians are homologous or convergent, Finnerty et al. (p. 1335, published online 6 May 2004; see the Perspective by Holland) examined the expression patterns of genes involved in establishing the axes of the sea anemone. They conclude that bilateral symmetry evolved prior to the divergence of bilaterians and cnidarians.

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