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Fig. 2 Wnt8a acts as the initial dorsal determinant in zebrafish. (A–C) Expression of chordin, analyzed by in situ hybridization, at the sphere stage in a wild-type (WT) embryo (A), in an embryo overexpressing Wnt8a (100 pg Wnt8a mRNA) (B), and in an embryo overexpressing a dominant-negative form of Wnt8a (500 pg of mRNA coding for DN-Wnt8a ORF2) (C). Embryos are in animal pole view, dorsal to the Right. (D) Nocodazole treatment generates embryos displaying various ventralization phenotypes ranging from weak (lacking notochord), strong (additionally missing the head), and radialized (lacking any dorsal territories). Injection of 25 pg of Wnt8a mRNA into one marginal blastomere at the 64-cell stage efficiently rescues radialized embryos and strongly reduces the frequency of strongly ventralized embryos; injections of 400 pg of Wnt11 mRNA or of a mix of 400 pg of Wnt11 mRNA and 400 pg of Wnt5a mRNA fails to rescue these phenotypes as shown in the graph. Embryos are presented in lateral view with anterior to the Left. Means and error bars (SD) were obtained from multiple experiments (three to five) with at least 50 embryos for each experimental condition. Comparisons between experiments were undertaken using unpaired Student′s t tests. P value of <0.05 was considered statistically significant.