Fig. 8.

Model for Ths and Pyr function in mesoderm spreading. Model for permissive and instructive functions of FGF8-like factors in Drosophila gastrulation. (A) During stages 6 and 7, both ths (blue) and pyr (red) are expressed in the ectoderm from the ventral midline (V) to the dorsal edge (D) of the neuroectoderm. We propose that both FGF8-like factors provide a permissive cue during collapse of the mesoderm (green) and attachment to the ectoderm (pink). (B) During dorsolateral migration of the mesoderm, expression of ths remains broad, whereas expression of pyr becomes restricted to the dorsal aspect of the neuroectodermal and ectodermal cells. We propose that at this stage, pyr generates an instructive cue and might act as a long-range signal.