Fig. 2. FGF/ERK activity antagonizes the ability of tar*/acvr1b to induce endoderm. Expression of CB187 (A-C) and sox17 (D-F) in embryos at 80% epiboly. (A,D) Wild type. (B,C,E,F) tar*/acvr1b RNA at 200 ng/µl was injected alone or in combination with DN-FGFR1 RNA at 500 ng/µl in one animal pole blastomere at the 64-cell stage embryo. Injection of tar*/acvr1b RNA alone is able to induce ectopic CB187 (B) and sox17 (E) expression at the animal pole (arrows). Co-injection with DN-FGFR1 abolishes the ability of tar*/acvr1b to induce CB187 (C), whereas DN-FGFR1 slightly increases the ability of tar*/acvr1b to induce sox17 (F). (G) RT-PCR analysis of three whole embryos injected with tar*/acvr1b RNA at 25 ng/µl. FGF signal inhibition by treatment with 15 µM of SU5402 after the 1000-cell stage enhances cas and sox17 responses. (H,I) tar*/acvr1b RNA at 1 ng/µl was injected alone or in combination with ERK2* RNA at 50 ng/µl into one blastomere of a 16-cell stage embryo. In both cases a lineage tracer (FLDX) was co-injected and later detected by immunostaining and sox17 expression was detected. (H) The clone of cells ectopically expressing sox17 in H is congruent with the territory containing the lineage tracer. (I) The size of the clone of cells ectopically expressing sox17 is greatly decreased in the presence of ERK*. (A-F,H,I) Lateral views.