Figure 6. Effects of loss of function of Notch and shaggy on the stability of Armadillo.

Clones of wild type and cells mutant for sgg, Notch or Notch and sgg were induced using the FLP recombinase system and were identified by the loss of GFP (see materials and methods). (A-C) Loss of function of shaggy results in a cell autonomous elevation of the levels of Armadillo, which remains largely associated with adherens junctions (inset in C in which apical is at the top). Notice also that only clones of a certain size (> about five cells) show the elevated levels of Armadillo; this is probably due to the long perdurance of Shaggy. The epithelium looks very thick (compare to inset in F) because the loss of sgg affects the epithelial organisation of the cells (Martinez Arias unpublished). (D-F) Simultaneous loss of Notch and shaggy results in very elevated levels of Armadillo that now appear delocalised in the cytoplasm of the cell (inset in F). The clones are small. Loss of function of Notch affects cell proliferation ((de Celis and Garcia-Bellido, 1994) and see also J-L) and this effect appears to be epistatic over the increased size of loss of function of sgg. (G-I) Expression of wild type Armadillo results in a Wingless dependent stabilisation of Armadillo (see also Figure 3 and S2). When wild type Armadillo is expressed under the control of ptcGAL4, high levels of Armadillo are observed in a narrow band at the AP border where the levels of ptcGAL4 expression are highest and, in particular, at the DV boundary (arrow in H) where the levels of Wingless are highest. Clones of wild type cells (loss of GFP) do not change the instability of the ectopic Armadillo (dots indicate regions in clones where Armadillo is not accumulated) . (J-L) Loss of function of Notch results in the stabilisation of the ectopic Armadillo. Notice that in this disc, which is heterozygous for Notch (Df(1)N^81k/+) the dppGal4 driven expression of Armadillo is broader and contains more cells maintaining high levels of Armadillo than in wild type. Furthermore, within the clones of cells lacking Notch there are more cells with high levels of Armadillo than there are in wild type clones (compare with G-I).Notice that in some instances clones, that lie far away from that AP boundary (arrow head) maintain the expression of Armadillo. The clones are small because loss of function of Notch affects cell proliferation (see above). N.B. The effects of loss of function of Notch alone on the stability of endogenous Armadillo are not reproducible, In general we do not observe changes in the levels of Armadillo as a result of loss of Notch function (A. Martinez Arias unpublished), but in some experiments we observe an elevation in the levels of Armadillo. This elevation is always observed in the neighbourhood of the Dorsal Ventral boundary (A. Martinez Arias unpublished). Unfortunately this effect is not reproducible and therefore should remain anecdotal. It might reflect the existence of a very unstable pool of Armadillo that is not easy to fix and which is increased in the absence of sgg or by the ectopic expression of higher levels of wild type.