twin of eyeless, a second Pax-6 gene of Drosophila, acts upstream of eyeless in the control of eye development

The Drosophila Pax-6 gene eyeless (ey) plays a key role in eye development. Here we show that Drosophila contains a second Pax-6 gene, twin of eyeless (toy), due to a duplication during insect evolution. Toy is more similar t o vertebrate Pax-6 proteins than Ey with regard to overall sequence conserv ation, DNA-binding function, and early expression in the embryo, toy and ey share a similar expression pattern in the developing visual system, and ta rgeted expression of Toy, like Ey, induces the formation of ectopic eyes, G enetic and biochemical evidence indicates, however, that Toy functions upst ream of ey by directly regulating the eye-specific enhancer of ey, Toy is t herefore required for initiation of ey expression in the embryo and acts th rough Ey to activate the eye developmental program.