Fig. 1. FGFR is required for mediolateral convergence of the Ciona notochord. (A,B) Ci-FGFR is strongly expressed in the developing notochord plate (A, neurula stage, white circle) and in notochord after intercalation (B, tailbud stage). (C-D') Wild-type notochord cells (Bra::GFP, green; phalloidin, red) undergo progressive ML intercalation (C,C', early tailbud) to form a single cell row (D,D', late tailbud). (E,E') Noto1::dnFGFR-Venus-expressing (green) notochord cells display cell-autonomous defects in intercalation at early tailbud stage. (F,F') Noto1::dnFGFR+Bra::GFP (green) embryos display strong intercalation defects at late tailbud stage. (53/75 embryos displayed CE defects.) Images in C'-F' are magnifications from C-F, respectively. (G,H) dpErk staining of wild-type embryos at neurula (G) and late tailbud (H) stages shows absence of MAPK activation in the notochord. (I) Treatment of embryos with U0126 from late neurula stage on eliminates MAPK activation but does not affect notochord intercalation (103/103 embryos). (J) Distribution of the number of actin protrusions per internal notochord edge versus the ratio of ML- to AP-oriented protrusions. Note the difference between wild-type (blue) and dnFGFR (red) notochord indices (eight embryos each). P-values are indicated for each axis.