WNT SIGNALING POLARIZES AN EARLY C-ELEGANS BLASTOMERE TO DISTINGUISH ENDODERM FROM MESODERM

A polarizing signal induces endoderm production by a 4-cell stage blas tomere in C. elegans called EMS. We identified 16 mutations in five ge nes, mom-1 through mom-5, required for EMS to produce endoderm. mom-1, mom-2, and mom-3 are required in the signaling cell, P-2, while mom-4 is required in EMS. P-2 signaling downregulates an HMG domain protein , POP-1, in one EMS daughter. The sequence of mom-2 predicts that it e ncodes a member of the Wnt family of secreted glycoproteins, which in other systems activate HMG domain proteins. Defective mitotic spindle orientations in mom mutant embryos indicate that Wnt signaling influen ces cytoskeletal polarity in blastomeres throughout the early embryo.