Practice question

Induction of supernumerary body axis requires ectopic organizer activity capable of redirecting surrounding tissue. In Hydra this occurs when Wnt3 is experimentally upregulated in gastric column via transgenesis or lithium chloride activating beta-catenin. Small cluster of Wnt3-expressing cells autoactivates downstream organizer genes including Brachyury, goosecoid, and BMP inhibitors, establishing new oral pole that evaginates tentacles and forms second hypostome. SHH homolog absent, beta-catenin is intracellular effector not secreted inducer, Pax6 controls eye specification. Thus Wnt3 ligand itself acts as primary axis-inducing morphogen initiating positive feedback loop that generates long-range patterning and complete secondary head structure during budding and regeneration.