| Drosophila appendages arise from the imaginal discs, small epithelial sacs that are derived from the embryonic ectoderm. In each imaginal disc, different domains along the proximo-distal (PD) axis are present as concentric regions, each with a unique genetic address. Proper establishment and growth of the distal territories can only occur in the absence of proximal factors. Two such factors, teashirt (tsh) and homothorax (hth), are initially co-expressed throughout the entire wing disc, but are later repressed in the prospective wing blade. This repression coincides with the activation of distal determinants such as the vestigial (vg) and nubbin (nub) genes. In this thesis, I analyze the establishment and maintenance of these different gene expression domains during PD axis patterning. I examine the role of the wingless ( wg) and decapentaplegic (dpp) pathways in the repression of tsh and hth, and the subsequent elaboration of the PD domains of the wing. Based on the analysis of a subset of the mutations isolated in a genetic screen I show that Wg and Dpp act independently as the primary signals for repression of tsh and hth, respectively. Further, although Dpp is required continuously for hth repression throughout development, Wg is only required for the initiation of tsh repression. Instead, the maintenance of tsh repression requires Polycomb group (PcG) mediated gene silencing, which is dispensable for hth repression. The role of PcG silencing in the maintenance of the PD axis suggested the existence of cell lineage restrictions in the wing disc in addition to the previously characterized compartments. I show that there are, in fact, significant cell lineage restrictions corresponding to the Nubbin and Teashirt domains, I propose that, early in wing development, a transient interface between these domains creates a compartment boundary such that, once formed, the Tsh and Nub lineages remain distinct for the remainder of development. Finally, I map, clone, and characterize a novel Hh pathway component, called Tamamushi, that was identified during the screen for factors involved in PD patterning. |
