Practice question

Hydra head formation relies on conserved Wnt pathway defining oral organizer. Wnt3 is transcribed at extreme tip of hypostome, most apical region, where it forms autostimulatory loop with beta-catenin and Brachyury maintaining organizer identity and driving expression of head-specific genes including BMP inhibitor and tentacle genes. Transgenic Hydra overexpressing Wnt3 generate ectopic tentacle whorls and secondary heads; knockdown prevents head regeneration causing transient foot-like transformation. HoxD13 regulates vertebrate distal limb, BMP4 ventral fate, SHH vertebrate limb posterior patterning. Therefore Wnt3 is crucial genetic trigger for Hydra head organizer, budding initiation, and axis establishment throughout life and regeneration.