Fig. 6. Sea urchin Alk4/5/7 is required for dorsal-ventral axis formation and Nodal signaling. (Aa-d) MoAlk4/5/7 disrupts dorsal ventral axis formation and blocks the response to nodal overexpression. (Ba-h) Rescue experiment to demonstrate the specificity of the Alk4/5/7 morpholino. Embryos were injected with Alk4/5/7 morpholino (b,e) or alk4/5/7 mRNA (c,f) alone, or successively injected with the Alk4/5/7 morpholino then with a synthetic alk4/5/7 mRNA containing eight mismatches over the region recognized by the morpholino (d,g,h). Whereas embryos injected with the Alk4/5/7 morpholino alone developed with a radialized phenotype indicative of inhibition of Nodal signaling, embryos co-injected with the Alk4/5/7 morpholino and the modified wild-type alk4/5/7 mRNA developed into normal pluteus larvae.