Integrative and Comparative Biology Advance Access originally published online on July 9, 2007
Integrative and Comparative Biology 2007 47(5):701-711; doi:10.1093/icb/icm028
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Implications of cnidarian gene expression patterns for the origins of bilaterality—is the glass half full or half empty?
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*ARC Centre for the Molecular Genetics of Development, Research School of Biological Sciences, Australian National University, PO Box 475, Canberra, ACT 2601, Australia; ARC Centre of Excellence in Coral Reef Studies and Comparative Genomics Centre, James Cook University, Townsville, Queensland 4811, Australia; ITZ, Ecology & Evolution, TiHo Hannover, Bünteweg 17d, D-30559 Hannover, Germany
Correspondence: 1E-mail: david.miller{at}jcu.edu.au
The past two years have seen a dramatic increase in the available data on gene sequence and gene expression for cnidarians and other "lower" Metazoa, and a flurry of recent papers has drawn on these to address the origins of bilaterality. Cnidarian homologs of many genes that play key roles in the specification of both the A/P and D/V axes of bilaterians have been characterized, and their patterns of expression determined. Some of these expression patterns are consistent with the possibility of conservation of function between Cnidaria and Bilateria, but others clearly differ. Moreover, in some cases very different interpretations have been made on the basis of the same, or similar, data. In part, these differences reflect the inevitable uncertainties associated with the depth of the divergence between cnidarians and bilaterians. In this article, we briefly summarize the cnidarian data on gene expression and organization relevant to axis formation, the varying interpretations of these data, and where they conflict. Our conclusion is that the presently available data do not allow us to unequivocally homologize the single overt axis of cnidarians with either of the bilaterian axes.
From the symposium "Key Transitions in Animal Evolution" presented at the annual meeting of the Society for Integrative and Comparitive Biology, January 3–7, 2007, at Phoenix, Arizona.