We have studied the role the canonical Wnt 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 Wnt pathway by pharmacologically blocking GSK-3beta. These treatments rendered the polar expression of Tcf ubiquitous and induced development of ectopic axes that contained head structures. These results allow concluding that Wnt 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 Wnt functions in axis formation and axial patterning as in higher metazoans, and thus point to an ancestral role of Wnt signaling in these processes in animal evolution.

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We studied patterning mechanisms acting on the formation of new mouths during regeneration of wedgeshaped fragments of the solitary coral Fungia granulosa. Mouth formation dynamics was studied in fragments taken from different parts of the adult polyp. The corals were fragmented into central and peripheral fragments in relation to each polyp's centrally positioned mouth. The study revealed that the maintenance of the original body plan and growth axis during the process of regeneration, appears to depend on the existence of the polyp's mouth. When damage to the coral polyp includes only partial damage to the original mouth, the regeneration process is characterized by maintenance of the growth axis and body plan. By contrast, complete removal of the original mouth results in the formation of multiple mouths and is followed by a drastic change in body plan and growth axis. Some of these mouths develop later into new polyps, which eventually detach from the original coral. During the first 2-3 mo of regeneration, 'preliminary' mouths developed and regressed until stabilization was attained. The appearance of new mouths in fragments farther from the original mouth (peripheral fragments) preceded by a week the appearance of new mouths in fragments that were more proximal to the original mouth (central fragments). Additionally, more mouths formed in peripheral than in central fragments. We interpret the results in light of the developmental model suggested by Meinhardt for Hydra. The formation of new mouths in regenerating fragments of Fungia is explained in terms of a source density gradient field, auto-activation, and lateral inhibition exerted by the original central mouth. These effectors determine the position and timing of mouth(s) development.