Practice question

Formation of functional Spemann organizer requires integration of two vegetal signals: dorsal Wnt-beta-catenin via Siamois/Twin and Nodal-related TGF-beta via Smad2. Nodal ligands Xnr1,2,5 activated by VegT phosphorylate Smad2 which enters nucleus and cooperates with Siamois/Twin on enhancers of goosecoid, chordin and Cerberus. Chromatin immunoprecipitation shows Smad2/Siamois co-binding. Either pathway alone insufficient; blocking Smad2 with Cerberus-S or Siamois with dominant-negative abolishes organizer. Therefore cooperation between Siamois, Twin and Smad2 creates robust organizer induction. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.