HEDGEHOG ACTS BY DISTINCT GRADIENT AND SIGNAL RELAY MECHANISMS TO ORGANIZE CELL-TYPE AND CELL POLARITY IN THE DROSOPHILA ABDOMEN

The epidermis of the adult Drosophila abdomen is formed by a chain of anterior (A) and posterior (P) compartments, each segment comprising o ne A and one P compartment, In the accompanying paper (Struhl ct al., 1997), we provide evidence that Hedgehog protein (Hh), being secreted from P compartment cells, organises the pattern and polarity of A comp artment cells, Here we test whether Hh acts directly or by a signal re lay mechanism, We use mutations in Protein Kinase A (PKA) or smoothene d (smo) to activate or to block Hh signal transduction in clones of A compartment cells, For cell type, a scalar property, both manipulation s cause strictly autonomous transformations: the cells affected are ex actly those and only those that are mutant. Hence, we infer that Hh ac ts directly on A compartment cells to specify the various types of cut icular structures that they differentiate, By contrast, these same man ipulations cause non-autonomous effects on cell polarity, a vectorial property, Consequently, we surmise that Hh influences cell polarity in directly, possibly by inducing other signalling factors, Finally, we p resent evidence that Hh does not polarise abdominal cells by utilising either Decapentaplegic (Dpp) or Wingless (Wg), the two morphogens thr ough which Hh acts during limb development. We conclude that, in the a bdomen, cell type and cell polarity reflect distinct outputs of Hh sig nalling and propose that these outputs are controlled by separable gra dient and signal relay mechanisms.