A NODAL-RELATED GENE DEFINES A PHYSICAL AND FUNCTIONAL DOMAIN WITHIN THE SPEMANN ORGANIZER

A functional screen for gene products that rescue dorsal development i n ventralized Xenopus embryos has yielded Xenopus nodal-related 3 (Xnr 3), a diverged member of the TGF beta superfamily. Xnr3 is specificall y expressed in the Spemann organizer and is only expressed in the epit helial layer of the organizer immediately preceding and extending thro ugh gastrulation, Like noggin, Xnr3 can induce muscle in ventral mesod erm explants, consistent with a role in patterning the gastrula, In ot her ways, the activity of Xnr3 is different from noggin. Embryos recei ving injections of Xnr3, particularly in the animal pole, send out tub e-like extensions of tissue from the site of injection. These protrusi ons usually contain no axial mesoderm and only occasionally are positi ve for neural markers. It has previously been proposed that the epithe lial layer of the organizer initiates and coordinates the morphogeneti c movements at gastrulation. The protrusions observed may reflect an a ctivity of Xnr3 in promoting morphogenesis.