Sequential Signalling at the Fng Boundary Figure 6. Clonal analysis of the allele Su(H)d47 with the MARCM system. Clones are labelled positively with the GFP marker. (A) Expression of Ap in Su(H)d47 mutant wing imaginal discs. The boundary between the Ap expressing and non-expressing cells (arrow) is smooth, indicating that the D/V compartment boundary has formed in the absence of Su(H) function. (B–P) Clones of Su(H) abutting the dorsal and/or ventral side of the D/V boundary. The dorsal cells are additionally labelled with Ser-lacZ. (B–D) A clone that crosses the D/V boundary causes loss of expression of Wg along the D/V boundary (arrow). (E-I) A Clone abutting the dorsal side of the D/V boundary causes a thinning, but no loss of the Wg expression domain. (J–N) In contrast, clones abutting the ventral side of the D/V boundary completely abolish expression of Wg and the vgBE (highlighted by the arrows). (O–S) A clone abutting the ventral side of the D/V boundary in an early third instar disc. Wg is interrupted in the area of the clone (arrow). doi:10.1371/journal.pone.0049007.g006 At the ectopic FB, we found that eliminating the activity of the Notch pathway in PBCs does not result in the loss of expression of Wg in ABCs. It only prevents the late symmetric phase. This indicates that establishment of a Dl/Ser feedback loop in A/P boundary cells is not essential for reaching sufficient high levels of Notch signalling to induce Wg expression during the asymmetric phase. We observed the same for the D/V boundary: Depletion of Su(H) function or over- PLOS ONE | www.plosone.org 10 November 2012 | Volume 7 | Issue 11 | e49007