Abstract
The Drosophila wing imaginal disc gives rise to three main regions along the proximodistal axis of the dorsal mesothoracic segment: the notum, proximal wing, and wing blade. Development of the wing blade requires the Notch and wingless signalling pathways to activate vestigial at the dorsoventral boundary. However, in the proximal wing, Wingless activates a different subset of genes, e.g., homothorax. This raises the question of how the downstream response to Wingless signalling differentiates between proximal and distal fate specification. Here, we show that a temporally dynamic response to Wingless signalling sequentially elaborates the proximodistal axis. In the second instar, Wingless activates genes involved in proximal wing development; later in the third instar, Wingless acts to direct the differentiation of the distal wing blade. The expression of a novel marker for proximal wing fate, zfh-2, is initially activated by Wingless throughout the "wing primordium," but later is repressed by the activity of Vestigial and Nubbin, which together define a more distal domain. Thus, activation of a distal developmental program is antagonistic to previously established proximal fate. In addition, Wingless is required early to establish proximal fate, but later when Wingless activates distal differentiation, development of proximal fate becomes independent of Wingless signalling. Since P-element insertions in the zfh-2 gene result in a revertable proximal wing deletion phenotype, it appears that zfh-2 activity is required for correct proximal wing development. Our data are consistent with a model in which Wingless first establishes a proximal appendage fate over notum, then the downstream response changes to direct the differentiation of a more distal fate over proximal. Thus, the proximodistal domains are patterned in sequence and show a distal dominance.
