Direct Flight Muscles in Drosophila Develop from Cells with Characteristics of Founders and Depend on DWnt-2 for Their Correct Patterning

The direct flight muscles (DFMs) of Drosophila allow for the fine control of wing position necessary for flight. In DWnt-2 mutant flies, certain DFMs are either missing or fail to attach to the correct epithelial sites. Using a temperature-sensitive allele, we show that DWnt-2 activity is required only during pupation for correct DFM patterning. DWnt-2 is expressed in the epithelium of the wing hinge primordium during pupation. This expression is in the vicinity of the developing DFMs, as revealed by expression of the muscle founder cell-specific gene dumbfounded in DFM precursors. The observation that a gene necessary for embryonic founder cell function is expressed in the DFM precursors suggests that these cells may have a similar founder cell role. Although the expression pattern of DWnt-2 suggests that it could influence epithelial cells to differentiate into attachment sites for muscle, the expression of stripe, a transcription factor necessary for epithelial cells to adopt an attachment cell fate, is unaltered in the mutant. Ectopic expression of DWnt-2 in the wing hinge during pupation can also create defects in muscle patterning without alterations in stripe expression. We conclude that DWnt-2 promotes the correct patterning of DFMs through a mechanism that is independent of the attachment site differentiation initiated by stripe.