Formation of the Drosophila larval photoreceptor organ and its neuronal differentiation require continuous Krüppel gene activity.

The Drosophila segmentation gene Krüppel (Kr) is redeployed to play a critical role for the establishment of the larval visual system. Using reporter gene expression conducted by a specific Kr cis-acting element, we were able to trace back the origin of the larval photoreceptor organ, the Bolwig organ, to a single progenitor neuron and to examine Kr function in Bolwig organ development when Kr+ activity is absent from embryos due to specific mutations or reduced by neuron-specific and temporally restricted Kr antisense RNA expression. Our results show that Kr is required for neurons to differentiate into Bolwig organs, for fasciculation of the Bolwig nerve, and for this nerve to follow a specific pathway toward the synaptic targets in the larval brain. The transcription factor encoded by Kr is likely to regulate surface molecules necessary for neuronal cell adhesion and recognition in the developing larval visual system.