Abstract
The organization of the 5S RNA gene cluster of Drosophila melanogaster is different in two Oregon R stocks that have been separated for a number of years. The Oregon R Yale population contains various different arrangements of the cluster. One of these is due to the insertion of a B104 element near one end of the cluster. Other arrangements lack the B104 insertion and have instead a variety of deletions originating in the vicinity of the B104 insertion site and removing from 0 to 60% of the 5S RNA genes without affecting nearby tRNA genes. In contrast, the Oregon R Heidelberg population has no B104 element in the 5S gene cluster and no heterogeneity in the arrangement of the cluster. We propose that transposable elements inserted at a genomic locus generate heterogeneity in a population at that locus due to excision of the element with and without accompanying deletions of flanking sequences. As a consequence, a fly population would accumulate a large number of deletions scattered throughout the genome in as many loci as contain transposable elements. We show further that D. melanogaster contains a large redundancy of 5S RNA genes since the 60% deletion of the cluster shows no visible phenotype when homozygous or when heterozygous against a total deletion of the entire 5S gene cluster.