EYELESS INITIATES THE EXPRESSION OF BOTH SINE OCULIS AND EYES ABSENT DURING DROSOPHILA COMPOUND EYE DEVELOPMENT

The Drosophila Pax-6 gene eyeless acts high up in the genetic hierarch y involved in compound eye development and can direct the formation of extra eyes in ectopic locations, Here we identify sine oculis and eye s absent as two mediators of the eye-inducing activity of eyeless, We show that eyeless induces and requires the expression of both genes in dependently during extra eye development. During normal eye developmen t, eyeless is expressed earlier than and is required for the expressio n of sirte oculis and eyes absent, but not vice versa. Based on the re sults presented here and those of others, we propose a model in which eyeless induces the initial expression of both sine oculis and eyes ab sent in the eye disc. sine oculis and eyes absent then appear to parti cipate in a positive feedback loop that regulates the expression of al l three genes. In contrast to the regulatory interactions that occur i n the developing eye disc, we also show that in the embryonic head, si ne oculis acts in parallel to eyeless and twin of eyeless, a second Pa x-6 gene from Drosophila, Recent studies in vertebrate systems indicat e that the epistatic relationships among the corresponding vertebrate homologs are very similar to those observed in Drosophila.