Fig. 5. babo mutants display cell cycle progression defects. S-phase cells (arrows) within the developing optic lobes of a wandering third-instar yw Drosophila larva (A) or babo^32/52 mutant (B) or Drosophila Smad2 (dSmad2^mb388) (C) mutant were labeled with BrdU (green). M-phase cells in optic lobes of wandering third-instar larvae of wild type yw (D) and babo^32/52 mutant (E) and Smad2^mb388 mutant (F) revealed by staining with phosphorylated histone H3 antibody (green, p-H3). Differentiated neurons are labeled by anti-Elav staining (A-F, magenta or red). Photoreceptor axons in red are labeled by 24B10 antibody (D,E). (G) Comparison of the ratio of M-phase cells versus S-phase cells in the optic lobes of yw, babo^32/52 mutant and Smad2^mb388 mutants. BrdU-positive cells or p-H3 positive cells were counted from three individual and distinct optic sections at roughly the same plane of each optic lobe of the three genotypes. The ratio of p-H3 positive cells versus BrdU-positive cells was calculated for each genotype and compared. Note that both babo^32/52 and Smad2^mb388 mutants showed a much reduced ratio of p-H3 positive cells to BrdU-positive cells compared with that of yw control. (H) Normal expression level of Cyclin A protein is seen in both IPC and OPC of an optic lobe of a yw third-instar larva. (I) High level Cyclin A protein is present at the optic center of a babo^32/52 mutant third-instar larva in both IPC and OPC stained and photographed with the same setting as wild type. (J-L) Elevated Cyclin A protein (red, arrowheads in K) is detected in a GFP-negative babo⁵² mutant clone (arrowheads in J) induced by heat shock from the optic lobe of babo^52/+ heterozygote larva brain. (L) Merged image of J and K. (M) Heterozygosity for Cyclin A rescues babo^32/52 photoreceptor axon targeting and lamina neuron phenotypes. (N) Anti-active Caspase-3 antibody staining (red) of babo⁵² mutant GFP-negative clones (arrowheads) in an otherwise babo⁵² heterozygous GFP-positive developing larval brain lobe. Caspase-3-positive apoptotic cells are indicated by arrows. (O,P) Apoptotic cells (arrows) identified by anti-active Caspase-3 antibody staining (red) of a yw wandering third-instar brain lobe and eye disc (O) compared to a babo^26/32 mutant brain (P). Note that there is no overall increase in the number of apoptoic cells in the babo mutant tissue. br, brain lobe; ed, eye disc.