We have studied the role the canonical Writ pathway plays in hydroid pattern formation during embryonic development and metamorphosis. Transcripts of Wnt and Tcf were asymmetrically deposited in the oocyte and subsequent developmental stages, marking the sites of first cleavage, posterior larval pole and the upcoming head of the metamorphosed polyp. To address the function of these genes, we activated downstream events of the Writ pathway by pharmacologically blocking GSK-3 beta. These treatments rendered the polar expression of Tcf ubiquitous and induced development of ectopic axes that contained head structures. These results allow concluding that Writ signaling controls axis formation and regional tissue fates along it, determining one single axis terminus from which later the mouth and hypostome develop. Our data also indicate Writ functions in axis formation and axial patterning as in higher metazoans, and thus point to an ancestral role of Writ signaling in these processes in animal evolution. (c) 2006 Elsevier Inc. All rights reserved.