Figure 5

Sce and Scm are required to maintain Sens repression in the absence of Svp and N . (A–B') Representative 48 h APF mosaic ommatidia in which the cells occupying the R1/R6 niche are homozygous for a particular chromosome arm (green), stained with antibodies against Sens (blue). In each ommatidium shown, the cell occupying the R7 niche is wild-type and expresses the R7-specific marker Prospero (Pros; [44]; red). Scale bar is 5 μm. (A,A') Neither Sce nor Scm mutant R1/R6s, which express Svp, misexpress Sens (dashed outline; quantified in (C)). (B,B') Loss of Sce or Scm from svp mutant R1/R6s significantly increases the proportion that misexpress Sens (dashed outline; quantified in (C)). The arrowhead in B' indicates Sens within an R8 from an adjacent ommatidium. (C) Quantification of mutant R1/R6s that express Sens at 48 hr APF. Error bars represent SEM. No Sce or Scm mutant R1/R6s express Sens (n = 318 in 5 retinas and n = 353 in 4 retinas, respectively). 25.9% ± 1.4% (n = 6 retinas) of svp mutant R1/R6s express Sens. Loss of Sce or Scm from svp mutant R1/R6s greatly increases the proportion that express Sens (77.6% ± 7.5% (n = 4 retinas) and 75.0% ± 1.9% (n = 7 retinas), respectively). (D,E) Model for the regulation of Sens by Sce and Scm in R1/R6 precursors. (D) Wild-type R1/R6s normally express Svp, which prevents expression of Sens. This Sens repression does not require Sce or Scm, since loss of Sce or Scm from otherwise wild-type R1/R6s does not result in Sens derepression. (E) R1/R6s lacking Svp stochastically adopt one of the two “inner” photoreceptor fates, R8 or R7, depending on whether they express Sens. Those that do not initially express Sens require Sce and Scm to maintain repression of Sens.