Interactions between Wnt and Vg1 signalling pathways initiate primitive streak formation in the chick embryo

The posterior marginal zone (PMZ) of the chick embryo has Nieuwkoop centre- like properties: when transplanted to another part of the marginal zone, it induces a complete embryonic axis, without making a cellular contribution to the induced structures. However, when the PMZ is removed, the embryo can initiate axis formation from another part of the remaining marginal zone. Chick Vg1 can mimic the axis-inducing ability of the PMZ, but only when mis expressed somewhere within the marginal zone. We have investigated the prop erties that define the marginal zone as a distinct region. We show that the competence of the marginal zone to initiate ectopic primitive streak forma tion in response to cVg1 is dependent on Wnt activity. First, within the Wn t family, only Wnt8C is expressed in the marginal zone, in a gradient decre asing from posterior to anterior. Second, misexpression of Wnt1 in the area pellucida enables this region to form a primitive streak in response to cV g1. Third, the Wnt antagonists Crescent and Dkk-1 block the primitive strea k-inducing ability of cVg1 in the marginal zone. These findings suggest tha t Wnt activity defines the marginal zone and allows cVg1 to induce an axis. We also present data suggesting some additional complexity: first, the Vg1 and Wnt pathways appear to regulate the expression of downstream component s of each other's pathway; and second, misexpression of different Wnt antag onists suggests that different classes of Wnts may cooperate with each othe r to regulate axis formation in the normal embryo.