Fig. 3. Expression of signaling molecules and retinal determination gene network (RDGN) members during Drosophila eye development. (A) A schematic of the eye disc that shows morphogenetic furrow progression. The eye disc undergoes waves of differentiation as the morphogenetic furrow, which is driven by the cooperative actions of the Hedgehog (HH, blue) and Decapentaplegic (DPP, green) signaling pathways, moves from the posterior to the anterior of the eye disc. The most posterior cells have differentiated into the photoreceptor cells, while anterior cells are still proliferating. Wingless (WG) expression in the dorsal- and ventral-most anterior regions of the disc prevents eye tissue formation in that region, leading to head cuticle formation. (B) A schematic of the expression pattern of RDGN members, where the eye disc is broken up into six regions based on the level of differentiation, as described by Bessa et al. (Bessa et al., 2002). There are discrepancies in the literature regarding the expression pattern of Sine oculis (SO). All reports agree on the expression shown in solid red. The red hatching indicates expression reported by antibody staining (Cheyette et al., 1994) and in situ hybridization (Seimiya and Gehring, 2000) that is not seen with antibody staining by Halder et al. (Halder et al., 1998) or in the commonly used so-lacZ enhancer trap line, which reflects expression via a specific enhancer (Punzo et al., 2002). Eyegone is not shown, as it is expressed in a stripe along the dorsoventral boundary. EY, Eyeless; EYA, Eyes absent; SO, Sine oculis; DAC, Dachshund; MF, morphogenetic furrow; PPN, preproneural region.