Fig. 5. Ubiquitous activation of FGF signaling inhibits A7.6 group gene expression. (A) The constitutively active form of FGFR (torso-FGFR) is expressed under the ZicL enhancer. Brachyury is ectopically expressed in A-line neural blastomeres, consistent with ectopic activation of the MAPK pathway. Some ectopic Ci-Bra expression is also observed in the muscle and mesenchyme. (B) Ci-Bra expression in wild-type Ciona embryo. (C-F'') Embryos injected at the 1-cell stage with torso-FGFR mRNA. Expression of A7.6 group genes is modestly reduced. To demonstrate the variable degree of reduction, embryos with no expression (C,D,F), weak expression (C',D',E,F') and wild-type expression levels (C'',D'',E',F'') are shown together with the number of embryos affected. In addition to A7.6, there are three instead of two Delta-like-positive blastomere cells (E'') consistently observed in injected embryos [in wild-type embryos, Delta-like expression in A7.8 is highly variable and only seen in 20% of the embryos (Hudson and Yasuo, 2006), suggesting that MAPK usually activates Delta-like in these cells].