Fig. 2. Inhibition of MAPK signal causes ectopic expression of A7.6 group genes in the anterior endoderm. (A-E) 110-cell Ciona embryos treated with 2 µM U0126 at late 32-cell stage. (A-D) A7.6 group genes are ectopically expressed in the anterior endoderm blastomeres upon MEK inhibition, whereas A7.6 expression is unaffected (numbers indicate affected embryos/total embryos). Conversely, the endoderm-specific marker TTF-1 is completely lost in the endoderm when MAPK signaling is inhibited (E). (F,G) Endogenous pattern of MAPK activation in the wild-type embryo. At the 64-cell stage (F), dpERK staining is observed in the nuclei of notochord precursors (A7.3, A7.7), all endoderm blastomeres (A7.1, A7.2, A7.5, B7.1, B7.2) and mesenchyme cells (B7.3, B7.7), but is strikingly absent from A7.6 mesoderm. This MAPK dichotomy in A7.5/A7.6 siblings persists to the 110-cell stage (G). (H) MAPK activation visualized by dpERK staining is completely lost in embryos treated with U0126.