Wnt and Decapentaplegic cell signaling pathways act synergistically in their 
contribution to macrochaete (sense organ) patterning on the notum of Drosophila 
melanogaster. The Wingless-signaling pathway was ectopically activated by 
removing Shaggy activity (the homologue of vertebrate glycogen synthase kinase 
3) in mosaics. Proneural activity is asymmetric within the Shaggy-deficient 
clone of cells and shows a fixed "polarity" with respect to body axis, 
independent of the precise location of the clone. This asymmetric response 
indicates the existence in the epithelium of a second signal, which we suggest 
is Decapentaplegic. Ectopic expression of Decapentaplegic induces extra 
macrochaetes only in cells which also receive the Wingless signal. Activation of 
Hedgehog signaling generates a long-range signal which can promote macrochaete 
formation in the Wingless activity domain. This signal depends upon 
decapentaplegic function. Autonomous activation of the Wingless signal response 
in cells causes them to attenuate or sequester this signal. Our results suggest 
a novel patterning mechanism which determines sense organ positioning in 
Drosophila.