Diagram of a typical Drosophila rRNA gene locus. Shown at the top is the tandemly repeated structure of the locus with only the 18S and 28S genes indicated. R1 and R2 elements (white boxes) are inserted into the 28S gene of 25%–50% of the units (Lathe et al. 1995; Lathe and Eickbush 1997). The expanded region shows a more detailed view of the organization of the complete rDNA unit found in all Drosophila species studied here. In addition to the 18S, 5.8S, and 28S genes, insect rDNA units also contain a 2S gene (Jordan et al. 1976). The transcribed region of the unit is indicated by the horizontal arrow at the bottom. The gene regions are shown as black boxes, the external transcribed spacer (ETS) and internal transcribed spacers (ITS1 and ITS2) as dotted boxes. The intergenic spacer region, IGS, is predominately composed of internal subrepeats (rounded boxes). The IGS of different units varies in length due to differences in the number of copies of each subrepeat. The sequence and length of these subrepeats vary dramatically between species (see Fig. 5).
