THE GENITAL DISC OF DROSOPHILA-MELANOGASTER .1. SEGMENTAL AND COMPARTMENTAL ORGANIZATION

The genital disc of Drosophila, which gives rise to the genitalia and analia of adult flies, is formed by cells from different embryonic seg ments. To study the organization of this disc, the expressions of segm ent polarity and homeotic genes were investigated. The organization of the embryonic genital primordium and the requirement of the engrailed and invected genes in the adult terminalia were also analysed. The re sults show that the three primordia, the female and male genitalia plu s the analia, are composed of an anterior and a posterior compartment. In some aspects, each of the three primordia resemble other discs: th e expression of genes such as wingless and decapentaplegic in each ant erior compartment is similar to that seen in leg discs, and the absenc e of engrailed and invected cause duplications of anterior regions. as occurs in wing discs. The absence of lineage restrictions in some reg ions of the terminalia and the expression of segment polarity genes in the embryonic genital disc suggest that this model of compartmental o rganization evolves, at least in part, as the disc grows. The expressi on of homeotic genes suggests a parasegmental organization of the geni tal disc, although these genes may also change their expression patter ns during larval development.