Specification of Neural and Imaginal Identities in the Embryonic Limb Primordium of Drosophila melanogaster. R.A. Bolinger , G. Panganiban. Anatomy, University of Wisconsin, Madison, WI.

   In Drosophila, adult appendages arise from ectodermally derived structures called imaginal discs. The thoracic imaginal discs, in turn, arise in the ventrolateral ectoderm of the embryo from clusters of cells that express the homeodomain transcription factor, Distal-less (Dll). These cells give rise to three structures in the second thoracic segment. These are: the wing and leg discs and a larval sensory structure called the Keilin's organ. While wing disc cells migrate dorsally out of the primordium, leg disc and presumptive Keilin's organ cells remain intimately associated. Both leg and Keilin's organ precursors continue to express Dll throughout embryogenesis. At stage 11, prior to the establishment of distinct subdomains, all cells in the primordium express the Hox cofactors Homothorax (Hth) and nuclear Extradenticle (n-Exd). By stage 13, discrete subdomains are clearly seen, and Hth and n-Exd are restricted to imaginal cells that also express the zinc-finger transcription factor, Escargot (Esg). Presumptive Keilin's organ cells can be detected at this stage via their expression of two neural markers, the homeodomain transcription factor Cut (Ct) and a PNS specific RNA binding protein, Couch potato (Cpo). The presence of sensory organ precursors as a subset of the primordium suggests that the choice to become either neural or imaginal may represent a binary cell fate decision akin to that known to occur between neural and ectodermal fates. Here we present the results of our investigation into how Keilin's organ precursors are specified within the embryonic primordium. We demonstrate that the ventral half of the primordium arises in the neuroectoderm and that members of the Achaete-Scute Complex (AS-C) are expressed in part of the primordium before the onset of expression of either imaginal or Keilin's organ markers. Both Dll and AS-C members are required to specify Keilin's organ identity, while Cut is required for differentiation of this structure.