Fig. 2.

Inhibitor treatment disrupts atrial primordium formation. Early treatment with U0126 or SU5402 disrupts atrial primordium formation. (A) Lateral view, anterior is right. An untreated animal a few hours after hatching. Arrow indicates single, right-side atrial siphon primordium (asp). Cartoon at bottom right shows area under view. (B) Larva which has undergone early treatment with 10 μM U0126. Atrial siphon primordia are absent. Acetylcholinesterase-stained untreated (C) and treated (D) larvae, about 5 hrs after hatching, dorsal view. Untreated control shows left and right side primordia (arrows), whereas in the treated larva the atrial primordia are absent, although the oral siphon primordium is present (arrow). Dll-A in situ hybridization of pre-hatching stage Ciona savignyi tailbud embryos (dorsal view, anterior to left) shows message is detected in control embryo (E) in patches of ectoderm which precede formation of visible atrial primordia, whereas in drug-treated tailbud embryos this expression is absent (F). Likewise, an antibody which recognized diphosphorylated ERK, the activated form of MEK, shows staining in bilaterally paired patches of ectoderm before the primordia are detectable (G), but in U0126-treated animals is not detected (H). (I) Post-hatching-stage larvae show ERK continues to stain atrial primordia (short arrows) as well as the oral siphon primordium (long arrow). (J) After invagination, faint signal is still detectable in the atrial ectoderm (short arrows, dotted lines) and is also observed around the oral siphon primordia (down arrow) and within the sensory vesicle (up arrow). Signal is detectable at the extreme rostral extent of the animal and in the forming palps, but is not highlighted here.