Drosophila wing development in the absence of dorsal identity

The developing wing disc of Drosophila is divided into distinct lineage-res tricted compartments along both the anterior/posterior (A/P) and dorsal/ven tral (D/V) axes. At compartment boundaries, morphogenic signals pattern the disc epithelium and direct appropriate outgrowth and differentiation of ad ult wing structures, The mechanisms by which affinity boundaries are establ ished and maintained, however, are not completely understood. Compartment-s pecific adhesive differences and inter-compartment signaling have both been implicated in this process. The selector gene apterous (ap) is expressed i n dorsal cells of the wing disc and is essential for D/V compartmentalizati on, wing margin formation, wing outgrowth and dorsal-specific wing structur es, To better understand the mechanisms of Ap function and compartment form ation, we have rescued aspects of the ap mutant phenotype with genes known to be downstream of Ap, We show that Fringe (Fng), a secreted protein invol ved in modulation of Notch signaling, is sufficient to rescue D/V compartme ntalization, margin formation and wing outgrowth when appropriately express ed in an ap mutant background. When Fng and alpha (PS1), a dorsally express ed integrin subunit, are co-expressed, a nearly normal-looking wing is gene rated. However, these wings are entirely of ventral identity. Our results d emonstrate that a number of wing development features, including D/V compar tmentalization and wing vein formation, can occur independently of dorsal i dentity and that intercompartmental signaling, refined by Fng, plays the cr ucial role in maintaining the D/V affinity boundary, In addition, it is cle ar that key functions of the ap selector gene are mediated by only a small number of downstream effecters.