Practice question

Tadpole tail normally regenerates tail. When tadpoles treated with retinoic acid after tail amputation, blastema converts to limb program producing hindlimb instead of tail. RA increases positional value along proximal-distal axis activating Meis, proximal Hox genes and posteriorizes inducing limb-specific transcription factors, altering RA to FGF ratio interpretation. Tail blastema expressing different Hox code becomes competent to adopt more proximal and posterior limb identity under RA influence, representing homeotic transformation via positional respecification rather than mutation or cell death. Molecularly RA upregulates Shh, Fgf10 associated with limb initiation program replacing tail gene network.