THE ROLES OF HEDGEHOG AND ENGRAILED IN PATTERNING ADULT ABDOMINAL SEGMENTS OF DROSOPHILA

We present evidence that hedgehog (hh) protein secreted by posterior c ompartment cells plays a key role in patterning the posterior portion of the anterior compartment in adult abdominal segments, Loss of funct ion of hh in the hh(ts2) mutant causes the loss of posterior tergite c haracteristics in the anterior compartment, whereas ectopic expression driven by hs-hh or the gain-of-function allele hh(Mir) causes transfo rmation of anterior structures toward the posterior, FLP-out hh-expres sing clones in the anterior compartment induce surrounding wild-type c ells to produce posterior tergite structures, establishing that hh fun ctions non-autonomously, The effects of pulses of ectopic expression d riven by hs-hh indicate that bristle type and pigmentation are pattern ed by hh at widely different times in pupal development. We also prese nt evidence that the primary polarization of abdominal segments is sym metric, This symmetry is strikingly revealed by ectopic expression of engrailed (en), As expected, this transforms anterior compartment cell s to posterior compartment identity In addition, however, ectopic en e xpression causes an autonomous reversal of polarity in the anterior po rtion of the anterior compartment, but not the posterior portion, By d etermining the position of polarity reversal within en-expressing clon es, we were able to define a cryptic line of symmetry that lies within the pigment band of the normal tergite, This line appears to be retai ned in hh(ts2) mutants raised at the restrictive temperature, suggesti ng it is not established by hh signaling, We argue that the primary ro le of hh in controlling polarity is to cause anterior compartment cell s to reverse their interpretation of an underlying symmetric polarizat ion. Consistent with this, we find that strong ectopic expression of h h causes mirror-symmetric double posterior patterning, whereas hh loss of function can cause mirror-symmetric double anterior patterning.