The secreted Wnt ligand Wnt8a is a strong candidate for the dorsal determinant in zebrafish. Expression of chordin at the sphere stage is shown in (A) wild type (WT), (B) an embryo injected with 500 pg of Frzb mRNA, and (C) an embryo injected with 100 pg of mRNA coding for a dominant-negative Tcf-3 (DN-Tcf3) (34). Embryos are in animal pole view with dorsal to the Right. (D) Expression of the 23 Wnt genes present in the zebrafish genome analyzed by RT-PCR at 1- to 2-cell stage (0.5 h), 1,000-cell stage (3 h), sphere stage (4 h), 40% epiboly (5 h), 60% epiboly (7 h), and at 24 hpf. Arrowheads indicate Wnt genes for which mRNAs are maternally deposited in the egg. (E–J) Whole mount in situ hybridization reveals that Wnt8a mRNAs accumulate in the Balbiani body (bb) in stage I oocytes (E) and are localized in a vegetal position in stage II (F) and stage III oocytes (G). Transcripts of Wnt8a are also observed in cleavage-stage embryos both in blastomeres and in the yolk cortical cytoplasm in vegetal position at (H) the 2-cell stage and (I) 16-cell stage. (J) Treatment with nocodazole, which depolymerizes microtubules, prevents transport of Wnt8a mRNA. Embryos are in lateral view with dorsal to the Right. Arrowheads in H–J indicate the limits of Wnt8a mRNA localization in the cortical cytoplasm.
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