Fig. 6. Activin and Activin-related (daw) genes are expressed in developing larval optic lobes and required for normal optic lobe development and correct photoreceptor axon targeting in Drosophila. (A,B) In situ hybridization of wild-type yw mid-third-instar larval brain lobes with antisense probes of activin-β (A) and daw (B). The transcripts of actβ are abundantly expressed in both larval optic lobe and the central brain lobe, whereas daw gene is also expressed in the larval optic lobe (arrow) in addition to its known expression in glial cells. (C) A daw promoter-enhancer Gal4 transgene drives the expression of a uasGFP reporter in glia cells (green and yellow) in the developing larval brain lobes. Glial cells were stained by an anti-Repo antibody (red). (D) The majority (95%) of homozygous daw^ex32 wandering third-instar larvae do not show abnormal optic lobe development or photoreceptor axon targeting defects. (E) A minority (5%) of daw³² homozygotes show optic lobe and axon targeting defects reminiscent of babo mutants. (F) Optic lobes developed normally and photoreceptor axons target correctly in actβ^ed80 mutant larvae. (G) Double mutants of the genotype daw^ex11:actβ^ed80 exhibit altered R7 and R8 growth cone bundling and morphology. (H) The daw^ex32;actβ^ed80 double mutants showed significantly enhanced penetrance of the strong optic lobe phenotype. Central bl, central brain lobe.