Progressive patterning of sensory organs in the Drosophila leg. M. Joshi, K. Buchanan, T. Orenic. Dept Biol Sci, Univ Illinois, Chicago, IL.

   The legs of the Drosophila adult exhibit a precise organization of small sensory organs, or microchaetes (mCs), that are arranged in straight longitudinal rows. We have previously shown that this organization requires the position-specific expression of two genes, the proneural gene achaete (ac) and the transcriptional repressor Hairy (H). In pupal legs, h is expressed in two pairs of longitudinal stripes, one pair that traverses the D/V axis (D/V-h) and another pair that runs along the A/P axis (A/P-h). h patterns the leg bristle rows by regulating the periodic expression of ac. At 6 hrs. after puparium formation (APF), a stripe of ac expression on either side of each h domain defines the primordia of the leg bristle rows. In the absence of h function, expression expands into the regions normally occupied by Hairy, resulting in disorganized bristle rows in the adult. Hence, accurate establishment of the h spatial domains of expression is essential for patterning of the leg bristle rows. Periodic expression of h and ac is established progressively. The D/V-h stripes are first expressed during the 3rd larval instar, while the A/P-h stripes are not observed until 3 hrs. APF. We show here that the A/P-h stripes are spatially defined by repressive signals from the Decapentaplegic and Wingless pathways. Expression of the four h stripes establishes a prepattern that is in place before periodic expression of ac is seen by 6 hrs. APF. We find that h functions in conjunction with Notch (N)/Delta (Dl) signaling to regulate ac expression and that a prepattern of periodic Dl expression is required for proper expression of ac in pupal legs. In addition, we have characterized the cis-regulatory elements which direct ac expression in the leg mC primordia.