Regulation of twin of eyeless during Drosophila development.  The Pax-6 protein is vital for eye development in all seeing animals, from sea urchins to humans. Either of the Pax6 genes in Drosophila (twin of eyeless and eyeless) can induce a gene cascade leading to formation of entire eyes when expressed ectopically. The twin of eyeless (toy) gene in Drosophila is expressed in the anterior region of the early fly embryo. At later stages it is expressed in the brain, ventral nerve cord and (eventually) the visual primordium that gives rise to the eye-antennal imaginal discs of the larvae. These discs subsequently form the major part of the adult head, including compound eyes. We have searched for genes that are required for normal toy expression in the early embryo to elucidate initiating events of eye organogenesis. Candidate genes identified by mutation analyses were subjected to further knock-out and miss-expression tests to investigate their interactions with toy. Our results indicate that the head-specific gap gene empty spiracles can act as a repressor of Toy, while ocelliless (oc) and spalt major (salm) appear to act as positive regulators of toy gene expression.