Practice question

Gradient model proposes that graded distribution of polarity determinants directs regenerative outcome proportionally to concentration. Definitive evidence derived from RNA interference targeting Wnt pathway components. Reducing Wnt1 or beta-catenin flattens posterior gradient leading to anteriorization, increasing Wnt via APC or Axin knockdown inverts gradient causing posteriorization. Phenotype severity correlated with knockdown degree supporting concentration-dependent interpretation. Overexpression of beta-catenin and grafting experiments complement gradient notion, but initial systematic demonstration that shifting morphogen gradient systematically changes head-tail fate came from Wnt pathway RNAi manipulations combined with in situ quantification of wnt and notum expression domains showing re-establishment of graded profiles.