However, as Rieger et al. point out, a different relationship is suggested by the expression of an anterior Hox gene in the hydrozoan jellyfish Podocoryne: cnoxl-Pc is expressed in a region of the planula larva corresponding to the future aboral end of the adult polyp (2). The evolutionary significance of this finding is unclear because (i) the expression patterns of Hox and Hox-related genes vary among hydrozoans (1, 2), and (ii), in one instance, a Hox-related gene undergoes an axial reversal during the course of development in Podocoryne itself (3). This variability within and between hydrozoan cnidarians makes it difficult to reconstruct the spatial expression of Hox-related genes in the ancestral hydrozoan. It is therefore impossible to reliably extrapolate these hydrozoan data to the ancestral cnidarian or the cnidarian-bilaterian ancestor. In contrast, orthologous genes in Nematostella and the coral Acropora, two anthozoan cnidarians, tend to exhibit highly similar and presumably conserved patterns of spatial expression [our Report, (4)].

We argue that the mouth of most bilateri-ans is formed "in a region" close to the anterior end of the adult body plan. On the basis of paleontological evidence, an anterior terminal mouth is likely to be the ancestral condition for all ecdysozoans (5), and a compos ite circum-oral "brain" is a feature of virtually all invertebrate animals (6). With regard to the position of the mouth in acoels, the development of the primary muscle grid may or may not be associated with the position of the anterior pole. The only way to make such a statement about the embryological origin of any structure is to perform a detailed fate map, but such a fate map has been performed only for a single species of acoelomorph (7).

Finally, it is not clear if our data can distinguish whether triploblastic bilaterians arose from larval or adult diploblasts. Diploblasts (cnidarians and ctenophores) do not generate feeding larvae and the adult mouth arises only once, at the animal pole (also the site of first cleavage). In cnidarians, the mouth arises at the posterior pole of the swimming stage (planula). If Nematostella reflects the ancestral condition for the cnidarian-bilaterian ancestor, then most descendant organisms modified an axial system in which the mouth forms at the anterior pole.

MARK Q. MARTINDALE1 AND JOHN R. FINNERTY2 1Kewalo Marine Laboratory, Pacific Biomedical Research Center, University of Hawaii, 41 Ahui Street, Honolulu, HI 96813, USA. 2Department of Biology, Boston University, 5 Cummington Street, Boston, MA 02215, USA.


1. J. R. Finnerty, D. Paulson, P. Burton, K. Pang, M. Q. Martindale, Evol.Dev. 5,331 (2003).

2. N. Yanze, J. Spring, C. Schmidli, V. Schmid, Dev. Biol. 236,89 (2001).

3. L. M. Masuda-Nakagawa, H. Groger, B. L. Aerne, V. Schmid, Dev. GenesEvol. 210,151 (2000).

4. D. C. Hayward et al., Proc. Natl. Acad. Sci. U.S.A. 99, 8106 (2002).

5. G. E. Budd, S. Jensen, Biol. Rev. 75,253 (2000).

6. E. E. Ruppert, R. S. Fox, R. D. Barnes, Invertebrate Zoology. A Functional Evolutionary Approach (Brooks/Cole-Thompson Learning, Belmont, CA, ed. 7, 2004).

7. J. Q. Henry, M. Q. Martindale, B. C. Boyer, Dev. Biol. 220, 285 (2000).


Essays: "The scientific consensus on climate change" by N. Oreskes (3 Dec. 2004, p. 1686).The final sentence of the fifth paragraph should read "That hypothesis was tested by analyzing 928 abstracts, published in refereed scientific journals between 1993 and 2003, and listed in the ISI database with the keywords 'global climate change' (9)." The keywords used were "global climate change," not "climate change."

News of the Week: "Science agencies caught in postelection spending squeeze" (3 Dec. 2004, p.1662).The article contains an incorrect reference to Michael Marx's institutional affiliation. He is a professor of physics at Stony Brook University in New York.

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