Fig. 1. ab is the L5 organizing gene. (A) A wild-type (wt) wing. The L2-L5 longitudinal veins are indicated. (B) An adult ab¹/ab¹ mutant wing. (C) ab expression in a wild-type third instar wing imaginal disc, visualized with an antisense ab probe (arrowhead indicates the ab stripe). (D) ab expression is greatly reduced in an ab¹/ab¹ mutant wing disc. (E) Double labeling of ab RNA (blue) and Dl protein (brown), shows that these genes are co-expressed in cells corresponding to the L5 primordium. Inset shows a higher magnification of the L5 primordium. The L1, and L3-L5, vein primordia are indicated. (F) rho is expressed in all longitudinal vein primordia in a wild-type third instar larval wing disc, detected with an antisense rho probe. Arrows indicate the approximate location of the L5 primordium in this and subsequent panels. (G) rho expression is lost in the L5 primordium of ab¹/ab¹ mutant wing discs. Wing discs are oriented with anterior at the top and dorsal to the left in this and subsequent panels. (H) Ubiquitous expression of ab with MS1096-GAL4 eliminates rho expression in all vein primordia. (I) Dl is expressed in the L1 and L3-L5 wing vein primordia. (J) Dl expression is lost in the L5 primordium of ab¹/ab¹ mutant wing discs. (K) Dl expression in all vein primordia is greatly reduced in discs ubiquitously expressing ab in MS1096-GAL4; UAS-ab wing discs. Weak Dl expression is visible in the ventral compartment of the disc, consistent with the lower levels of MS1096-GAL4 expression in ventral versus dorsal cells. (L) Blistered (Bs) protein is expressed at high levels in intervein cells, but is strongly downregulated in the L2-L5 vein primordia. (M) Bs downregulation in the L5 primordium is lost in ab¹/ab¹ mutant wing discs. (N) Bs expression is greatly reduced in all cells of MS1096-GAL4; UAS-ab wing discs. (O) A third instar wing imaginal disc misexpressing ab in flip-out clones (arrows) stained for ß-Gal (green) and Dl (blue). Dl is expressed in a cell-autonomous fashion within a subset of ß-Gal-expressing cells. Additional double-label experiments reveal that all cells in flip-out clones expressing ß-Gal also express Ab at high levels (O.C., unpublished). (P) Dl channel only for the disc shown in O. (Q) A third instar wing imaginal disc misexpressing ab in a flip-out clone (arrows) stained for ß-Gal (green) and the intervein marker Bs (red). Bs is downregulated in a cell-autonomous fashion within all cells of the clone. (R) Bs channel only for the disc shown in Q. (S) Ectopic veins form in a cell-autonomous fashion within small ab-expressing flip-out clones marked by being f^36a (outlined).