Optomotor-blind in the development of the Drosophila HS and VS lobula plate tangential cells.

The horizontal system and vertical system cells of the dipteran optic lobes are well understood regarding their physiology and role in visually guided behavior. Little is known, however, about their development. Drosophila optomotor-blind (omb) is required for the development of the HS/VS cells which are lacking in the adult brain of the In(1)omb[H31] regulatory mutant. We have analyzed the omb regulatory region, required for HS/VS development, for enhancers active in the central nervous system. A 1-kb fragment, ombJb, was identified 114 kb downstream of the omb transcription start site, that could drive expression in much of the presumptive embryonic optic lobe anlage. Expression in these cells is lost in In(1)omb[H31] suggesting that they contain the HS/VS precursor cell(s). We used Laser ablation in the embryonic CNS in order to localize the position of the HS/VS precursor cell(s) in this tissue. An omb-Gal4 enhancer trap line, which showed activity in the optic lobe anlage in a pattern similar to ombJb enhancer, was used to drive GFP expression, thus allowing to focus the Laser beam to the relevant area. We identified a small region in the embryonic brain from which the HS/VS cells are likely to develop. Omb encodes a transcription factor of the T-box family. Since loss of omb disrupts HS/VS cell development, we assume that HS/VS ontogeny is controlled by Omb target genes. As a first step toward their identification, we characterized the Omb DNA-binding specificity.