The ribosomal DNA of Drosophila melanogaster is organized differently from that of Drosophila hydei

Abstract

On the X chromosome of Drosophila melanogaster there is a single tandem array of 240 ribosomal RNA genes. The majority of these contain an insertion, known as type I, in the 28 S coding region. Previous genetic and electron microscopic studies indicated that genes bearing the type I insertion ( ins +) are interspersed at random with those lacking it ( ins −). In contrast, Renkawitz-Pohl et al. (1981) have analyzed the restriction pattern of X chromosomal ribosomal DNA in Drosophila hydei and demonstrated that in this case ins + genes are segregated from ins −. This suggests either that the rDNA is organized differently in these two species or that the restriction enzyme technique reveals significant clustering not detected by previous methods. By using an appropriate restriction enzyme, we demonstrate that ins + and ins − genes are intermingled at random in D. melanogaster. These experiments also indicate that genes containing the short form of the insertion are flanked by a larger spacer upstream than downstream.