Fig. 3. The expression of Xbra is absent in the dorsal marginal region of MO-injected embryos at the gastrula stage and from the notochord region at the neurula stage. (A) The expression of Xbra in uninjected control (A), Xnr3 MO-injected (B), and Xdd1h mRNA-injected (C) embryos. Dorsal expression of Xbra was reduced at stage 11.5 (arrow) and stage 15 in the MO-injected region in Xnr3^– embryos (middle column), whereas it occurred normally at stage 11.5 in Xdd1 mRNA-injected embryos (third column). Xbra expression in Xdd1 mRNA-injected embryos was lost in the neurula stage (stage 15). (B) Xnr3-induced animal cap elongation is blocked by the expression of dominant negative dishevelled mRNA (Xdd1). Xdd1 expression alone does not cause elongation of caps. These samples were frozen and subjected to real-time PCR analysis for the expression of Xbra, MyoD, NCAM and nrp1 (Fig. 3B right hand side). All these markers continue to be induced by Xnr3 in animal caps in the presence of Xdd1. (C) The dorsal reduction of Xbra expression is specific in Xnr3 MO-injected embryos. The expression of Xbra was analyzed in dorsally and ventrally injected embryos. Xbra expression in ventrally injected embryos was slightly delayed at the injected region, but was otherwise normal. Arrowheads indicate areas where Xbra expression is missing from the blastopore ring. 20 ng of MO was injected together with NLS-lacZ RNA into two dorsal or ventral marginal two cells in 8-cell-stage embryos.