Practice question

Two-headed planaria result from manipulation abolishing posterior polarity cues causing anteriorization of tail blastema. Posterior identity requires Erk MAPK activity and Wnt-beta-catenin activation. Inhibition of Erk via MEK inhibitors like U0126 or RNAi after transverse amputation suppresses Wnt1 expression posteriorly, derepresses Notum and anterior transcription factors, allowing head specification at posterior wound in addition to anterior. Similar phenotype arises from blocking Wnt directly. This demonstrates antagonistic relationship where high Erk suppresses head, blocking Erk removes suppression triggering head formation both ends producing bipolar two-headed regenerate during regeneration.