Brakeless is required for lamina targeting of R1-R6 axone in the Drosophila visual system

Photoreceptors in the Drosophila eye project their axons retinotopically to targets in the optic lobe of the brain. The axons of photoreceptor cells R 1-R6 terminate in the first optic ganglion, the lamina, while R7 and R8 axo ns project through the lamina to terminate in distinct layers of the second ganglion, the medulla. Here we report the identification of the gene brake less (bks) and show that its function is required in the developing eye spe cifically for the lamina targeting of R1-R6 axons, In mosaic animals lackin g bks function in the eye, R1-R6 axons project through the lamina to termin ate in the medulla. Other aspects of visual system development appear compl etely normal: photoreceptor and lamina cell fates are correctly specified, R7 axons correctly target the medulla, and both correctly targeted R7 axons and mistargeted R1-R6 axons maintain their retinotopic order with respect to both anteroposterior and dorsoventral axes. bks encodes two unusually hy drophilic nuclear protein isoforms, one of which contains a putative C2H2 z inc finger domain. Transgenic expression of either Bks isoform is sufficien t to restore the lamina targeting of R1-R6 axons in bks mosaics, but not to retarget R7 or RS axons to the lamina, These data demonstrate the existenc e of a lamina-specific targeting mechanism for R1-R6 axons in the Drosophil a visual system, and provide the first entry point in the molecular charact erization of this process.