Fig. 3. Twist is required for Dorsal gradient thresholds. Embryos were hybridized with a digoxigenin-labeled snail antisense RNA probe. They are oriented with anterior to the left. (A,B) Lateral (A) and ventral (B) views of wild-type embryos. snail staining is restricted to the ventral mesoderm. (C,D) Lateral (C) and ventral (D) views of twist^–/twist^– mutant embryos. The snail staining pattern is diminished and exhibits periodic repression along the anterior-posterior axis. (E,F) Wild-type (E) and mutant embryos (F) expressing a Toll^10b transgene [P(hspToll^10bbcd)], which produces a broad ectopic Dorsal nuclear gradient along the anterior-posterior axis. In a wild-type background the transgene produces uniformly intense activation of snail in the anterior third of the embryo, in addition to the endogenous pattern in the ventral mesoderm (E). There is a gap in staining where the ectopic and endogenous snail patterns intersect (arrow). There is a severe reduction in both the endogenous and ectopic snail expression patterns in twist^–/twist^– mutant embryos (F).

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