Fig. 7. A Snail repressor gradient helps localize Notch signaling. Embryos were collected from females that contain an hsp83-Toll10b-bcd transgene and thereby express a broad anterior-posterior Dorsal nuclear gradient. These embryos were derived from gd–/gd– females, and therefore lack the normal dorsoventral Dorsal gradient. Mutants were stained with either a snail (A,B) or sim (C) hybridization probe. (A,B) snail staining pattern in precellular (A) and cellularized (B) embryos. snail is activated by high levels of the ectopic anteroposterior Dorsal nuclear gradient in anterior regions of mutant embryos. The snail pattern is initially broad and fuzzy (A), but refines during cellularization (B) and exhibits the very sharp border seen for the normal snail pattern at the boundary between the mesoderm and mesectoderm. (C) sim expression is not detected until the onset of gastrulation. Staining is detected in cells that reside just posterior of the sharp snail expression pattern. These sim-positive cells exhibited weak, transient snail expression at earlier stages (A). (D) A model for the positioning of Notch signaling by the Snail repressor. The top and bottom circles represent cross-sections through precellular (top) and cellularized (bottom) embryos. snail is initially expressed in a broad pattern in ventral and ventrolateral regions that encompass the presumptive mesoderm and mesectoderm. At this early stage Snail might repress a number of inhibitors of Notch signaling, such as Delta and T3. At later stages, the snail expression pattern is refined and restricted to the mesoderm. Notch signaling is activated in the cells that transiently expressed the Snail repressor.
