On page 1335, Fujii et al. report that p38 MAP kinase is activated asymmetrically in the zebrafish embryo. The p38 is activated and, based on the effects of a dominant-negative construct, necessary for cleavage only on the future dorsal side, even though normal early embryos display symmetric cleavage.
When p38 is inhibited, the affected blastomeres still replicate their DNA, and still express the dorsal side-specific gene, dharma. Dharma expression is eliminated by treatments (UV irradiation and yolk-mass removal) that disrupt transport of dorsal determinants to the future dorsal side via a microtubule-transport pathway in the embryo; the same treatments interfere with p38 activation. The p38 activation occurs far earlier, however, so it may be triggered by lower concentrations of the same dorsal determinant, or by another determinant altogether.
After the UV treatment, what should be the dorsal...
