Practice question

Posterior tail identity depends on elevated Wnt/beta-catenin transcriptional activity. Beta-catenin accumulates in nuclei of posterior-facing blastema binding TCF and coactivator Teashirt activating genes such as wnt1, fzd4, hoxD paralogs that specify tail tissues including posterior gut branches and copulatory apparatus. Experimental depletion of beta-catenin converts tails into heads, while forced stabilization via APC RNAi or GSK3 beta inhibition transforms heads into tails. Notum inhibits Wnt favoring head, Erk promotes Notum expression anteriorly, HoxD13 regulates vertebrate limb. Therefore molecule directly regulating transcription of tail program is beta-catenin acting as binary fate switch integrating Wnt gradient concentration.