Distinguishable functions for engrailed and Invected in anterior–posterior patterning in the Drosopila wing

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Abstract
SUBDIVISION of the limb primordia into compartments initiates pattern formation in the developing limbs1,2. Interaction between distinctly specific cells in adjacent compartments leads to localized expression of the secreted signalling molecules Wingless (Wg) or Decapentaplegic (Dpp), which in turn organize pattern and control growth of the limbs3–8. The homeobox gene engrailed has been implicated in specification of posterior cell fate 9–12, whereas the LIM/homeobox gene, apterous, specifies dorsal fate3. Removing apterous activity causes a complete transformation from dorsal to ventral fate and leads to the formation of an ectopic dorsal-ventral boundary organizer3,13. By contrast, removing engrailed activity causes incomplete morphological transformation from posterior to anterior fate in the wing10,14,15, and fails to produce an ectopic anterior–posterior organizer (reviewed in ref. 2). Complete transformation can only be effected by simultaneously eliminating activity of engrailed and its homologue invected16–18. Here we show that invected functions principally to specify posterior cell fate. Thus establishment of the anterior–posterior organizer and control of compartment identity are genetically distinguishable, and invected may perform a discrete subset of functions previously ascribed to engrailed.