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SIGNALING MOLECULES IN THE HYDRA HEAD ORGANIZER AND THE EVOLUTION OF AXIS FORMATION. B. Hobmayer, F. Rentzsch, and T.W. Holstein. Dept. of Molecular Cell Biology, Darmstadt Univ of Technology, Darmstadt, Germany.

     The molecular nature of signaling centers (organizers) plays a pivotal role in the formation of body axes in multicellular animals. Organizers secrete growth factors, which act as long range regulators in axis formation and cell differentiation. To analyze the origin and evolution of organizers, we characterized Wnt and TGF-beta signaling pathways in Hydra, a member of the primitive animal phylum Cnidaria. Molecules of the Wnt and TGF-beta signaling pathways are expressed in the Hydra head organizer. Wnt, Tcf, and Chordin are transcriptionally upregulated early during asexual bud formation and head regeneration. Wnt and Tcf expression domains also define head organizers created by de novo pattern formation in aggregates. Thus, the Hydra head organizer exhibits astonishing similarities to the Spemann-organizer in vertebrates. Our results demonstrate that Wnt and TGF-beta signaling acts in axis formation in Hydra, and support the idea that this played a key role in the evolution of axial differentiation in the earliest multicellular animals. 

     (Supported by  the Deutsche Forschungsgemeinschaft)