High Erk activity in planarians:
In planaria anterior-posterior polarity, Erk MAPK signaling is enriched posteriorly where it promotes tail identity and suppresses anterior head program. High Erk activity sustains Wnt expression posteriorly maintaining beta-catenin activation, which inhibits head formation. Pharmacological activation of Erk reduces head regeneration capacity, while inhibition anteriorizes posterior blastema inducing ectopic head. Therefore high Erk activity functionally inhibits head formation at posterior wound ensuring tail specification. Interaction with Wnt-beta-catenin mediates repression, explaining why posterior Erk high environment is incompatible with head organizer formation and preserves correct polarity during whole-body regeneration and homeostasis.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 21: High Erk activity inhibits head formation in planarians.