DEVELOPMENTAL DEFECTS IN BRAIN SEGMENTATION CAUSED BY MUTATIONS OF THE HOMEOBOX GENES ORTHODENTICLE AND EMPTY SPIRACLES IN DROSOPHILA

We have studied the roles of the homeobox genes orthodenticle (otd) an d empty spiracles (ems) in embryonic brain development of Drosophila. The embryonic brain is composed of three segmental neuromeres. The otd gene is expressed predominantly in the anterior neuromere; expression of ems is restricted to the two posterior neuromeres. Mutation of otd eliminates the first (protocerebral) brain neuromere. Mutation of ems eliminates the second (deutocerebral) and third (tritocerebral) neuro meres. otd is also necessary for development of the dorsal protocerebr um of the adult brain. We conclude that these homeobox genes are requi red for the development of specific brain segments in Drosophila, and that the regionalized expression of their homologs in vertebrate brain s suggests an evolutionarily conserved program for brain development.