Ribeirao Preto, SP 14040-901, Brazil. 2Museu de Zoologia, Universidade de Säo Paulo, Avenida Nazare 481, Sao Paulo, SP 04263-000, Brazil. 3Departamento de Zoologia, Instituto de Biociencias, Universidade de Sao Paulo, Sao Paulo, SP 05508-900, Brazil. 4School of Life Sciences, Post Office Box 874501,Arizona State University, Tempe, AZ 85287-4501, USA. 5Department of Wildlife & Fisheries Sciences, Texas A&M University, College Station,TX 77843-2258, USA. 6Shark Research Center, South African Museum, 25 Queen Victoria Street, Post Office Box 61, Cape Town, 8000, South Africa. 7Department of Ichthyology, American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024-5192, USA. 8Department of Zoology, The Natural History Museum, Cromwell Road, London, SW7 5BD, UK. 9Department of Ichthyology, The Academy of Natural Sciences, 1900 Benjamin Franklin Parkway, Philadelphia, PA 19103, USA. 10Division of Fishes, National Museum of Natural History, Post Office Box 37012, Washington, DC 20013-7012, USA. 11School of Botany, University of Melbourne, Parkville,Victoria, 3052,Australia. References

1. A.V. Suarez, N.T.Tsutsui,Bioscience 54,1 (2004).

2. R. E. Gropp, Systematics Biodiversity 1,285 (2004).

3. A. C. Revkin,"Biologists sought a treaty; now they fault it," N.Y. Times, 7 May 2002, p. F1.

4. P. Rebelo, "Brazilian officials destroy rare fish specimens," SciDev.Net (www.scidev.net), 25 Aug. 2004.

5. See www.fapesp.br (e.g., Programa Biota/FAPESP).

6. M. L. J. Stiassny, M. C. C. de Pinna, in Systematics and Conservation Evaluation, P. L. Forey, C. J. Humphries, R. I.Vane-Wright, Eds. (Clarendon, Oxford, UK, 1994), pp. 235-249.

7. D.Tautz, P. Arctander, A. Minelli, R. H.Thomas, A. P. Vogler, TrendsEcol. Evol. 18, 70 (2003).

8. J. Mallat, K.Willmott, TrendsEcol.Evol. 18,57 (2003).

9. E. O. Wilson, in Assembling the Tree of Life, J. Cracraft, M. J. Donoghue, Eds. (Oxford Univ. Press, Oxford, 2004), pp. 539-544.

12. R. Scotland, C. Hughes, D. Bailey, A. Wortley, Systematics Biodiversity 1,139 (2003).

A Clue to the Origin of the Bilateria?

In their Report "Origins of bilateral symmetry: Hox and Dpp expression in a sea anemone" (28 May 2004, p. 1335), J. R. Finnerty et al. deduce from expression patterns of Hox genes during development of the planula larva (their fig. 3) that the oral-aboral axis in the adult sea anemone Nematostella vectensis is homologous to the anterior-posterior axis (A-P axis) of adult arthropods and craniates. In his accompanying Perspective, "The ups and downs of a sea anemone" (p. 1255), P. Holland points out that a homolog of an "anteriorly" expressed Hox gene in the anthozoan Nematostella is expressed posteriorly in the planula of the hydrozoan Podocoryne (1). This conflicting evidence may be resolved by assuming a "developmental reversal of spatial polarity" during hydro-zoan metamorphosis (Finnerty et al. SOM).

Concerning this discrepancy, we want to point out that in acoel flatworms, which are regarded by some to represent the most basal extant bilaterians (2-4), the anterior pole of the A-P axis in the developing brain is separate from the developing mouth (5), as is the case in most bilaterians (6). The developing anterior pole and the A-P axis of the acoel Convoluta pulchra can be deduced from the formation of the primary muscle grid consisting of circular and longitudinal fibers (7, 8). The mouth becomes visible after the primary muscle grid is established on the ventral side in the posterior third of the animal. The separate spatial origin of the mouth and anterior pole of the A-P axis is consistent with the planula concept for the origin of the Bilateria, in which triploblasts are derived from larval diploblasts (9, 10), but is in conflict with the co-localization of the mouth and anterior pole as indicated by Finnerty et al.

However, the new Hox gene information from Nematostella brings into focus the opposing hypotheses, in which triploblasts are either derived from larval or from adult diploblasts (11-13). Spatial expression of homolog Hox/ParaHox genes (14) in embryos of acoel flatworms and similar data on basal scalidophorans (e.g., priapulids) with the brain encircling the mouth may bring us closer to solving the puzzle ofthe origin ofthe Bilateria, summarized by Hyman in the closing sentence ofher famous "Retrospect": "Anything said on these questions lies in the realm of fantasy" [(15), p. 754].

Reinhard M. Rieger,Peter Ladurner, Bert Hobmayer

Department of Zoology and Limnology, University of Innsbruck, Technikerstrasse 25, Innsbruck A-6020,Austria. E-mail: [email protected] References

2. I. Ruiz-Trillo et al., Proc. Natl.Acad. Sci. U.S.A. 99,11246 (2002).

3. U.Jondelius et al., Zool.Scripta 31,201 (2002).

4. M.J.Telford et al., Proc. R. Soc. London B 270,1077 (2003).

6. C. Nielsen, Animal Evolution: Interrelationships of the Living Phyla (Oxford Univ. Press, New York, 2001).

7. P. Ladurner, R. Rieger, Dev. Biol. 222,359 (2000).

8. R. Rieger, P. Ladurner, Belg.J.Zool. Suppl. 1,27 (2001).

9. L. H. Hyman, The Invertebrates, Vol. II, Platyhelminthes andRhynchocoela (McGraw-Hill, New York, 1959).

10. R. Rieger, P. Ladurner,Belg.J.Zool. Suppl. 1,27 (2001).

11. J. M.Turbeville, E. E. Ruppert,Zoomorphology 103,103 (1983).

15. L. H. Hyman, The Invertebrates, Vol. V, Smaller Coelomate Groups (McGraw-Hill, New York, 1959).


Rieger etal. focus on two questions that are not directly addressed by the data in our Report: first, the evolutionary relationship of the mouth to the anterior-posterior axis, and second, the derivation of bilateri-ans from either a larval or an adult diploblastic ancestor.

On the basis of broad similarities in Hox and TGF-p expression between Nemato-stella and bilaterian metazoans, we argued that the bilateral symmetry exhibited by anthozoan cnidarians (corals, anemones, and their allies) is homologous to the bilateral symmetry exhibited by bilaterians. Our hypothesis implies a direct correspondence between the oral-aboral axis of cnidarians and the anterior-posterior axis of bilaterians. Furthermore, because Nematostella homologs of "anterior" Hox and ParaHox genes are expressed near the mouth, we suggest a correspondence between the oral end of cnidarians and the anterior end of bilaterians (1).

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