INGROWTH BY PHOTORECEPTOR AXONS INDUCES TRANSCRIPTION OF A RETROTRANSPOSON IN THE DEVELOPING DROSOPHILA BRAIN

The development of the lamina, the first optic ganglion of the fly vis ual system, depends on inductive cues from the innervating photorecept or axons. lacZ expression from a P-element insertion, A72, occurs in t he anlage of the lamina coincident with axon ingrowth from the eye ima ginal disc. Ln eyeless mutants lacking photoreceptor axons, lacZ expre ssion did not occur. The P-element was found to have inserted within t he 3' long terminal repeat (LTR) of a '17.6' type retrotransposon. The expression pattern of 17.6 transcripts in the brain in wild-type and eyeless mutants paralleled the expression of the lacZ reporter. Analys is of 17.6 cia-regulatory sequences indicates that the lamina-specific expression is due to the combined action of an enhancer element in th e LTR and a repressor element within the internal body of the retrotra nsposon. The regulation of the 17.6 retrotransposon provides a model f or the study of innervation-dependent gene expression in postsynaptic cells during neurogenesis.