THE SPALT GENE LINKS THE A P COMPARTMENT BOUNDARY TO A LINEAR ADULT STRUCTURE IN THE DROSOPHILA WING/

During Drosophila embryogenesis, each segment is subdivided into an an terior and a posterior compartment through the action of the engrailed gene. Compartmental boundaries bisect imaginal disc primordia which g ive rise to adult appendages. In early larval development, a short-ran ge Hedgehog signal originating from the posterior compartment of the i maginal wing disc activates expression of genes including decapentaple gic (dpp) in a stripe running along the anterior-posterior compartment boundary. Secreted Dpp emanating from the A/P boundary of wing discs then acts as a secondary signal to organize the wing over large distan ces. The transcription factor encoded by spalt major (salm) gene, whic h is expressed in a broad wedge centered over the dpp stripe, is one t arget of Dpp signaling. In this manuscript, we show that the anterior edge of the salm expression domain abuts a narrow stripe of rhomboid ( rho)-expressing cells corresponding to the L2 longitudinal vein primor dium. hh mis-expression along the anterior wing margin induces a surro unding domain of salm expression, the anterior edge of which abuts a d isplaced rho L2 stripe. salm plays a key role in defining the position of the L2 vein since loss of salm function in mosaic patches induces the formation of ectopic L2 branches, which comprise salm(-) cells run ning along clone borders where salm(-) cells confront salm(+) cells. T hese data suggest that salm determines the position of the L2 vein pri mordium by activating rho expression in neighboring cells through a lo cally non-autonomous mechanism. rho then functions to initiate and mai ntain vein differentiation. We discuss how these data provide the fina l link connecting the formation of a linear adult structure to the est ablishment of a boundary by the maternal Bicoid morphogen gradient in the blastoderm embryo.