1. Sars International Centre for Marine Molecular Biology, University of Bergen, Thormoehlensgt. 55, 5008 Bergen, Norway
2. Laboratoire de Paléontologie, Phylogénie et Paléobiologie, Institut des Sciences de l'Evolution, UMR5554-CNRS, Université Montpellier II, Place Eugène Bataillon, 34095 Montpellier Cedex 05, France
3. American Hospital of Paris, 63, boulevard Victor Hugo, BP 109, 92202 Neuilly-sur-Seine Cedex, France
4. Children's Hospital and Research Center at Oakland, Oakland, California 94609, USA
5. Department of Biological Chemistry, University of California, Irvine, 240D Medical Sciences I, Irvine, California 92697-1700, USA

Correspondence to: D. Chourrout¹U. Technau¹ Correspondence and requests for materials should be addressed to D.C. (Email: Daniel.chourrout@sars.uib.no) and U.T. (Email: Ulrich.technau@sars.uib.no). Sequences have been deposited in GenBank with accession numbers DQ500742–DQ500879

Abstract

Bilaterian animals have a Hox gene cluster essential for patterning the main body axis, and a ParaHox gene cluster. Comparison of Hox and ParaHox genes has led workers to postulate that both clusters originated from the duplication of an ancient cluster named ProtoHox, which contained up to four genes with at least the precursors of anterior and posterior Hox/ParaHox genes^{1, 2, 3}. However, the way in which genes diversified within the ProtoHox, Hox and ParaHox clusters remains unclear because no systematic study of non-bilaterian animals exists. Here we characterize the full Hox/ParaHox gene complements and genomic organization in two cnidarian species (Nematostella vectensis and Hydra magnipapillata), and suggest a ProtoHox cluster simpler than originally thought on the basis of three arguments. First, both species possess bilaterian-like anterior Hox genes, but their non-anterior genes do not appear as counterparts of either bilaterian central or posterior genes; second, two clustered ParaHox genes, Gsx and a gene related to Xlox and Cdx, are found in Nematostella vectensis; and third, we do not find clear phylogenetic support for a common origin of bilaterian Cdx and posterior genes, which might therefore have appeared after the ProtoHox cluster duplication. Consequently, the ProtoHox cluster might have consisted of only two anterior genes. Non-anterior genes could have appeared independently in the Hox and ParaHox clusters, possibly after the separation of bilaterians and cnidarians.

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