Fig. 5. An anterior-posterior Twist gradient specifies two dorsoventral patterning thresholds in the absence of a Dorsal gradient. Embryos were hybridized with a digoxigenin-labeled snail antisense RNA probe, and are oriented with anterior to the left and dorsal up. (A) snail is not expressed in mutant embryos derived from Toll^rm9/Toll^rm10 transheterozygous females. sim is also silent in these mutants (data not shown). (B) snail staining pattern in a mutant embryo that contain the twist-bcd transgene. snail is broadly activated in anterior regions of cellularizing embryos (compare with A). (C) The snail staining pattern sharpens in cellularized mutant embryos that express the twist-bcd transgene. Staining is lost at the anterior pole. (D) sog is expressed throughout Toll^rm9/Toll^rm10 mutant, with the exception of the termini. (E) sog staining pattern in mutant embryos that contain the twist-bcd transgene. Staining is repressed in anterior regions (compare with D) where the Snail repressor is ectopically activated (see B). (F) sim staining pattern in mutant embryos that express the twist-bcd transgene. Expression is restricted to the anterior pole, in regions where the Snail repressor is lost (C).