The Hedgehog morphogen and gradients of cell affinity in the abdomen of Drosophila

The adult abdomen of Drosophila is a chain of anterior (A) and posterior (P ) compartments. The engrailed gene is active in all P compartments and sele cts the P state. Hedgehog enters each A compartment across both its anterio r and posterior edges; within A its concentration confers positional inform ation, The A compartments are subdivided into an anterior and a posterior d omain that each make different cell types in response to Hedgehog, We have studied the relationship between Hedgehog, engrailed and cell affinity. We made twin clones and measured the shape, size and displacement of the exper imental clone, relative to its control twin, We varied the perceived level of Hedgehog in the experimental clone and find that, if this level is diffe rent from the surround, the clone fails to grow normally, rounds up and som etimes sorts out completely, becoming separated from the epithelium. Also, clones are displaced towards cells that are more like themselves: for examp le groups of cells in the middle of the A compartment that are persuaded to differentiate as if they were at the posterior limit of A, move posteriorl y, Similarly, clones in the anterior domain of the A compartment that are f orced to differentiate as if they were at the anterior limit of A, move ant eriorly. Quantitation of these measures and the direction of displacement i ndicate that there is a U-shaped gradient of affinity in the A compartment that correlates with the U-shaped landscape of Hedgehog concentration. Sinc e affinity changes are autonomous to the clone we believe that, normally, e ach cell's affinity is a direct response to Hedgehog, By removing engrailed in clones we show that A and P cells also differ in affinity from each oth er, in a manner that appears independent of Hedgehog, Within the P compartm ent we found some evidence for a U-shaped gradient of affinity, but this ca nnot be due to Hedgehog which does not act in the P compartment.