In sea urchin embryos, endomesoderm specification involves β-catenin entry into the nuclei of the vegetal cells of the developing embryo. Now, on , David McClay and colleagues reveal how the embryo uses maternal information to initiate this specification by showing that maternal Wnt6 is necessary for activation of endodermal genes. They report that the addition of Wnt6 or ectopic activation of the Wnt pathway rescues endoderm specification in eggs that lack the small region of the vegetal cortex that is normally needed for the activation of the endomesoderm gene regulatory network. This part of the vegetal cortex, they report, contains a high level of Dishevelled (Dsh), a transducer of the canonical Wnt pathway. They also report that morpholino knockdown of Wnt6 in the whole embryos of two sea urchin species prevents endoderm specification but not the expression of mesoderm markers. The researchers suggest, therefore, that maternal Wnt6 plus a localised vegetal cortex molecule, possibly Dsh, are necessary for endoderm specification in sea urchin embryos.
