Unintegrated ribosomal genes in diploid and polytene tissues of Drosophila melanogaster

We have examined DNA from polytene salivary glands and diploid brains and imaginal discs of male and female larvae having one or two nucleolus organizers. DNA having an estimated molecular weight of 5 X 10(9) or greater was obtained by sucrose gradient sedimentation of gently prepared lysates. Hybridization of the gradient fractions with 3H-ribosomal RNA REVEALS THAT 42% OF THE RIBOSOMAL GENES ARE FOUND IN DNA of lower molecular weight (approximately 3 X 10(8) daltons) in the salivary glands of every genotype examined. In the brains and imaginal discs, by contrast, all of the ribosomal genes are found in the high molecular weight peak except in females with one nucleolus organizer where 42% are found in lower molecular weight DNA, as in the salivary gland. Thus unintegrated genes are not an exclusive feature of polytene tissue, but can occur in diploid tissue as well in at least one genotype.