Long spacers among ribosomal genes of Drosophila melanogaster

Abstract

The fruit fly, Drosophila melanogaster, has 200 tandemly arranged copies of the ribosomal RNA genes (rDNA) per haploid genome^1,2. 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 spacer³ (Fig. 1). In the X chromosome, there are two major size classes (12 and 17 kilobases) and numerous minor size classes of rDNA repeats^3–7. Most of this length heterogeneity is generated by insertions at a specific site in the 28S gene^3–7. The frequency and size patterns of these insertions in the 28S gene differ in the X and Y chromosomes^4,7,8. Electron microscopic analysis of rDNA–rRNA hybrids has shown that there is also length heterogeneity in the rDNA non-transcribed spacer^4,6,7. This heterogeneity is due in part to internal sequence repetition^7,9. 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.