Long spacers among ribosomal genes of Drosophila melanogaster

The fruit fly, Drosophila melanogaster, has 200 tandemly arranged copies of the ribosomal RNA genes (rDNA) per haploid genome. One such cluster of rRNA genes occurs on the X and one on the Y chromosome. The basic repeating unit of the rRNA gene consists of a segment coding for 18S rRNA and 28S rRNA followed by a non-transcribed spacer (Fig.1). In the X chromosome, there are two major size classes (12 and 17 kilobases) and numerous minor size classes of rDNA repeats. Most of this length heterogeneity is generated by insertions at a specific site in the 28S gene. The frequency and size patterns of these insertions in the 28S gene differ in the X and Y chromosomes. Electron microscopic analysis of rDNA-rRNA hybrids has shown that there is also length heterogeneity in the rDNA non-transcribed spacer. This heterogeneity is due in part to internal sequence repetition. We have now examined further the length heterogeneity of the rDNA spacer and have observed a class of spacers that we shall refer to as 'long spacers'. The size and frequency of these long spacers are different in the X and Y rDNA.