Fig. 7.

Model for the timing and location of Fez suppression of BMP signaling in the sea urchin embryo. (A) BMP2/4 protein theoretically synthesized in the future oral ectoderm of unhatched blastula diffuses throughout the ectoderm including the animal plate. The highest activity of BMP2/4 is observed on the aboral side at mesenchyme blastula stage. Fez begins to be expressed in the neurogenic animal plate at the hatching blastula stage. Green and yellow show the regions where pSmad1/5/8 signal is present and absent, respectively. (B) FoxQ2 appears first in the animal plate before BMP signaling. Fez is induced by FoxQ2 and reaches levels sufficient to attenuate BMP at the outer edges of the animal plate so that BMP cannot inhibit FoxQ2 there. (C) The relationship between pSmad1/5/8 activity (green) and the size of the animal plate (magenta) with/without Fez function. In normal embryos with Fez function (upper panel), the activity of pSmad1/5/8 is dramatically decreased at the border between Fez-negative and -positive regions to a level that preserves FoxQ2 expression and the animal plate. In Fez morphants (lower panel), the gradient of pSmad1/5/8 is continuous from the aboral ectoderm to the animal pole and the abnormally high levels of pSmad1/5/8 in the periphery of the animal plate prevent expression of FoxQ2 and differentiation of the animal plate.