HEDGEHOG ORGANIZES THE PATTERN AND POLARITY OF EPIDERMAL-CELLS IN THEDROSOPHILA ABDOMEN

The abdomen of adult Drosophila, like that of other insects, is formed by a continuous epithelium spanning several segments, Each segment is subdivided into an anterior (A) and posterior (P) compartment, distin guished by activity of the selector gene engrailed (en) in P but not A compartment cells, Here we provide evidence that Hedgehog (Hh), a pro tein secreted by P compartment cells, spreads into each A compartment across the anterior and the posterior boundaries to form opposing conc entration gradients that organize cell pattern and polarity, We find t hat anteriorly and posteriorly situated cells within the A compartment respond in distinct ways to Hh: they express different combinations o f genes and form different cell types, They also form polarised struct ures that, in the anterior part, point down the Hh gradient and, in th e posterior part, point up the gradient - therefore all structures poi nt posteriorly, Finally, we show that ectopic Hh can induce cells in t he middle of each A compartment to activate en. Where this happens, A compartment cells are transformed into an ectopic P compartment and re organise pattern and polarity both within and around the transformed t issue. Many of these results are unexpected and lead us to reassess th e role of gradients and compartments in patterning insect segments.