Restriction of mesendoderm to a single blastomere by the combined action of SKN-1 and a GSK-3 beta homolog is mediated by MED-1 and-2 in C-elegans

The endoderm and much of the mesoderm arise from the EMS cell in the four-c ell C. elegans embryo. We report that the MED-1 and -2 GATA factors specify the entire fate of EMS, which otherwise produces two C-like mesectodermal progenitors. The meds are direct-targets of the maternal SKN-1 transcriptio n factor; however, their forced expression can direct SKN-1-independent rep rogramming of non-EMS cells into mesendodermal progenitors. We find that SG G-1/GSK-3 beta kinase acts both as a Wnt-dependent activator of endoderm in EMS and an apparently Wnt-independent repressor of the meds in the C linea ge, indicating a dual role for this kinase in mesendoderm development. Our results suggest that a broad tissue territory, mesendoderm, in vertebrates has been confined to a single cell in nematodes through a common gene regul atory network.