Fig. 3.

Caudalizing factors restrict XDmrt4 expression domain. Activation of Wnt signaling pathway by injection of Wnt-3a blocks XDmrt4, XBF1, and Six1 expression at the neurula stage. Conversely, inhibition of Wnt signaling pathway by injection of β-catenin morpholino (β-cat-mo) or Xdkk1 mRNA expands posteriorly XDmrt4, XBF1, and Six1 placodal expression domains. At the tail bud stage, the same manipulations of the Wnt signaling pathway resulted in a reduction or an expansion of Xebf2 and XEmx2 expression in the nasal placode. In most cases, Xdkk1 injection leads to an expansion of these markers on both sides of the injected embryos presumably because of Xdkk1 protein diffusion. Overexpression of Fgf8 completely blocks XDmrt4, XBF1, and Six1 placodal expression at the neurula stage, as well as the nasal placode expression of Xebf2 and XEmx2 at the tail bud stage. Anterior views are shown, dorsal to the top. RNA encoding the lineage tracer β-gal was coinjected in all cases to identify the injected side (red staining, left, arrows). (Magnifications: ×8.)