The rDNA loci of Drosophila species. Each rDNA transcription unit (diagramed in detail) consists of the 18S, 5.8S, 2S and 28S genes, the external transcribed spacer (ETS) and internal transcribed spacers (ITS1, ITS2a and ITS2). The location of the R1 and R2 insertion sites are indicated with arrowheads. Transcription units are separated by an internally repetitive intergenic spacer (IGS). The rDNA loci are usually, but not always, located on the X and Y chromosomes and typically contain hundreds of copies of the rDNA unit arranged in tandem arrays.
Stage and Eickbush Genome Biology 2009 10:R49 doi:10.1186/gb-2009-10-5-r49