Fig. 1

Maternal Smad2 is necessary for mesendoderm specification by Nodal signaling.

(A) Illustration of the Smad2 protein showing the position of the ENU-induced non-sense mutation. (B) Western blot against Smad2/3 on 24 hpf embryos of different genotypes for smad2. MZ, maternal-zygotic homozygotes, Z^-/-, zygotic homozygotes, Z^+/-, zygotic heterozygotes. The pool of maternally contributed Smad2 protein persists for at least 24 hr in zygotic homozygous embryos while it is depleted in MZsmad2 mutants. (C-J) Phenotypic analysis of 36 hpf zebrafish embryos. (C) Wild-type embryo. (D) MZoep embryo: maternal-zygotic mutant for one-eyed pinhead (oep), a cell surface protein required for Nodal signaling (Gritsman et al., 1999). (E) MZsmad2 embryo. Msmad2 mutants display a very similar phenotype (not shown). (F) MZsmad2 embryo rescued with 20 pg of smad2 mRNA. (G-H) MZsmad2 embryo rescued with 50 pg of gfp-smad2 mRNA (brightfield (G), epifluorescence (H)). smad2 mRNA appears to be more effective in rescuing the prechordal plate defects in MZsmad2 mutants as compared to gfp-smad2 mRNA. (I) MZsmad2 embryo injected with 5 pg mRNA for the zebrafish Nodal homolog squint. (J) MZsmad2 embryo injected with 5 pg mRNA for activin. Note that while Activin can activate the Nodal pathway in the absence of oep (Gritsman et al., 1999; Cheng et al., 2003), neither Squint nor Activin can activate the pathway in the absence of Smad2.