Molecular characterization and expression of sevenless, a gene involved in neuronal pattern formation in the Drosophila eye.

The Drosophila sevenless mutation results in lack of a single neuron (photoreceptor cell R7) in every ommatidium of the compound eye; the developmental defect occurs in the larval eye disc. We created P-element-induced alleles and used them to isolate the sev gene. An 8.2 kb transcript is expressed in the eye disc, behind the morphogenetic furrow, coincident with recruitment and differentiation of photoreceptor clusters. The transcript becomes localized at the apical surface, persists in the prepupa, and fades out at pupation. It is again detected in the adult head. In some alleles the 8.2 kb transcript is absent. In others, the transcript is expressed, in spite of the absence of cell R7. Localization of the gene product in the eye disc was obtained with antibody raised against sev protein.