SIAMOIS IS REQUIRED FOR FORMATION OF SPEMANNS ORGANIZER

Spemann's organizer develops in response to dorsal determinants that a ct via maternal components of the wnt pathway. The function of siamois , a wnt-inducible homeobox gene, in Spemann's organizer development wa s examined by fusion of defined transcriptional regulatory domains to the siamois homeodomain. Similar to native siamois, a VP16 activator f usion induced axis formation, indicating that siamois functions as a t ranscriptional activator in axis induction. Fusion of the engrailed re pressor generated a dominant inhibitor that blocked axis induction by Xwnt8, beta-catenin, and siamois, and repressed wnt activation of the goosecoid promoter. Dorsal injection of the engrailed-siamois fusion r esulted in complete inhibition of dorsal development and organizer gen e expression, an effect rescued by siamois, but not by Xwnt8 or beta-c atenin. Thus, as a zygotic mediator of maternal dorsal signals, siamoi s function is required for development of Spemann's organizer.