Eye imaginal discs from late larvae are divided by the morphogenetic furrow (MF, yellow arrowhead) into an anterior region (left) with asynchronously dividing cells and a posterior region (right). Discs were immunostained for the ELAV neuronal marker (red), which marks differentiating photoreceptors in the posterior region of the disc, the H3K27me3 modification (green), the H3.3K27M oncohistone (blue), or for mitotic cells (S10-phosphorylation, magenta). (A,B) In eye imaginal discs, one last wave of mitosis occurs in the posterior portion (to the right) of the disc behind the MF, as cells initiate neuronal differentiation. The GMR-GAL4 driver induces expression of the H3.3K27M oncohistone in this region of the disc. (C,D) Expression of the S phase inhibitor p21 in the posterior region of the eye disc blocks progression of the last cell cycle, indicated by the absence of mitotic cells in the posterior region of the eye disc. GMR-GAL4 induced expression of the H3.3K27M oncohistone in the posterior region of the disc is unaffected. (E,F) Adult eye morphology is unaffected by expression of either a wild-type H3.3 transgene (+) or an H3.3K27M transgene late in development by the GMR-GAL4 driver. (G,H) GMR-induced expression of the cell cycle inhibitor p21 results in moderately reduced eye size in both wild-type (G) and H3.3K27M-expressing animals (H). (I) Eye imaginal discs from wild-type larvae (+) show high H3K27me3 signal apparent in both the anterior and posterior regions of the eye disc. (J) Induced overexpression of a wild-type H3.3 histone in the posterior portion of the eye disc does not affect H3K27me3 staining. (K) Induced overexpression of H3.3K27M histone strongly reduces H3K27me3 staining in the posterior region of the eye disc. (L) Co-expression of H3.3K27M and the p21 inhibitor show high level H3K27me3 staining in both the anterior and posterior regions of the eye disc. (M) Quantification of H3K27me3 signal intensity differences between the anterior portion of eye discs and the posterior portion, where the GMR-GAL4 driver induces histone transgene expression. At least 10 discs were measured for each genotype.