EARLY EVEN-SKIPPED STRIPES ACT AS MORPHOGENETIC GRADIENTS AT THE SINGLE CELL LEVEL TO ESTABLISH ENGRAILED EXPRESSION

even-skipped (eve) has been proposed to set up parasegment borders at the anterior edge of each of its seven stripes by providing a sharp expression boundary, where engrailed is activated on one side and wingless on the other. By expressing bell-shaped early eve stripes without the sharp boundary provided by narrow, late stripes, we find that the early gradient is sufficient for generating stable parasegment borders. Based on several lines of evidence, we propose that the anterior portion of each early stripe has morphogenic activity, repressing different target genes at different concentrations. These distinct repression thresholds serve to both limit and subdivide a narrow zone of paired expression. Within this zone, single cell rows express either engrailed, where runt and sloppy-paired are repressed, or wingless, where they are not. While the early eve gradient is sufficient to establish parasegmental borders without refined, late expression, late eve expression has a role in augmenting this boundary to provide for strong, continuous stripes of engrailed expression. In addition, we show that the early eve gradient is sufficient, at its posterior edge, for subdividing the ftz domain into engrailed expressing and non-expressing cells. SUMMARY