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Fig. 1. Independent regulation of ato in the eye and other sensory organs. (A) Schematic summarizing neuronal morphogenesis in the eye primordium, which begins early in the third larval stage (L3), when eye progenitor cells initiate transcription of ato. Development of the 800 single eyes or ommatidia occurs progressively, as consecutive rows of eye progenitors from posterior to anterior (horizontal arrow) begin to differentiate. This transition is marked by a visible indentation in the epithelium called the morphogenetic furrow (MF). Thus, in the L3 eye disc, one can visualize cells at various stages of development. Anterior to the Ato-positive cells lie retinal progenitors expressing the eye-specification or RD factors Ey, So and Eya. Posterior to the ato domain, one finds differentiating neuronal clusters and accessory cells in progressively more advanced stages of development (Wolff and Ready, 1993). The enlarged diagram (upper right) shows the early (blue and dark blue) and late (yellow) phases of ato transcription. The position of ato expression relative to the Senseless and Eya domains is indicated by the green (Senseless) and red (Eya) bars. (B) Construct maps and relationship to the genomic ato region. The 3'ato^M'- ßgal (2.1 kb) and 3'ato^M"-ßgal (2 kb) constructs, which overlap by 1.5 kb but differ at both ends, drive expression in essentially the same pattern. (C-F') In situ hybridization to LacZ mRNA (except for inset in C). In all images, discs are oriented posterior to the left and dorsal up. (C,C') 3'ato^FL- ßgal mRNA reflects ato expression in the eye (C), JO (C and C inset) and CH (C'). The inset in C shows overnight staining for ß-gal activity. Arrows point to JO in the antenna (C) and femoral CH in the leg (C'). Expression in the antenna occurs at a much lower level than in the eye. (D,D') 3'ato^L- ßgal is expressed in JO (D) and femoral CH (D'). Diffuse low level expression can also be detected upon longer staining anterior to the MF and throughout the antennal disc, but not at the sites of endogenous ato expression by the MF or in the ocellar regions. (E,E') 3'ato^M'- ßgal drives expression in the eye (E), but not in antenna (E) or leg (E'). Expression is also observed in the ocellar region (arrowhead). (F,F') 3'ato^R- ßgal does not drive expression in eye, antennal or leg discs.

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