Fig. 5. Col-mediated upregulation of vn transcription in the AP organiser is required but not sufficient for L4 vein formation. (A) kn¹/kn¹ and (B) kn¹/kn¹; vn^M2/+ adult wings. In kn¹/kn¹; vn^M2/+ wings, a large region of L4 vein is missing (white arrow). (C,D) vn transcription is strongly downregulated in the L3-L4 intervein primordium in col¹ (D) relative to wild type (C) third instar wing discs, except in cells close to the presumptive hinge; (E-H) col¹ and UAS-Vn/dpp-Gal4; col¹ adult wings (E,F) and corresponding larval discs (G,H). In the absence of col, vn expression in the AP organiser does not rescue formation of L4 vein, while vein L3m is widened (F). Double in situ hybridisation for rho (blue) and col (pink) transcripts in wing discs shows that rho expression is not activated in posterior L4 vein cells but is upregulated in cells corresponding to L3m vein or the posterior margin (H). The broken black line indicates the position of the AP boundary. (I) col¹ mutant clones in the wing, marked by shavenoid (sha). L4 vein forms posterior to mutant clone spanning three to four rows of cells along the AP border (blue line); L4 vein is missing and L3 is wider and shifted posteriorly when the col¹ clone fills the entire L3-L4 region (red line). (J) UAS-TkvDN/ptc-Gal4 adult wings showing that sequestering Dpp by overexpression of TkvDN in the AP organiser results in wings smaller than wild type and specific loss of the L4 vein.