The knirps and knirps-related genes organize development of the second wing vein in Drosophila

The neighboring homologous knirps (kni) and knirps-related (knrl) genes in Drosophila encode transcription factors in the steroid hormone receptor sup erfamily, During early embryogenesis, kni functions as a gap gene to contro l expression of segmentation genes within the abdominal region of the embry o. In this study, we present evidence that kni and knrl link A/P positional information in larval wing imaginal discs to morphogenesis of the second l ongitudinal wing vein (L2), We show that kni and knrl are expressed in simi lar narrow stripes corresponding to the position of the L2 primordium, The kni and knrl L2 stripes abut the anterior border of the broad central expre ssion domain of the Dpp target gene spalt major (salm). We provide evidence that radius incompletus (ri), a well-known viable mutant lacking the L2 ve in, is a regulatory mutant of the kni/knrl locus, In ri mutant wing discs, kni and knrl fail to be expressed in the L2 primordium. In addition, the po sitions of molecular breakpoints in the kni/knrl locus indicate that the ri function is provided by cis-acting sequences upstream of the kni transcrip tion unit, Epistasis tests reveal that the kni/knrl locus functions downstr eam of spalt major (salm) and upstream of genes required to initiate vein-v ersus-intervein differentiation. Mis-expression experiments suggest that kn i and knrl expressing cells inhibit neighboring cells from becoming vein ce lls. Finally, kni and knrl are likely to refine the L2 position by positive ly autoregulating their own expression and by providing negative feedback t o repress salm expression, We propose a model in which the combined activit ies of kni and knrl organize development of the L2 vein in the appropriate position.