Chromosomal rearrangements which affect the chromosomal integration of the ribosomal genes in Drosophila melanogaster.

Sucrose gradient analysis of DNA from detergent-pronase lysates of whole adult flies has been used to examine a variety of genotypes for the presence of ribosomal genes not integrated into the DNA of the chromosome. Such genes were found in females in which one X chromosome carries an inversion, having one of its breakpoints between the nucleolus organizer and the centromere. These inversions move the nucleolus organizer to the distal end of the X chromosome. Other inversions which do not move the nucleolus organizer, as well as a series of bobbed deficiencies, did not induce unintegrated genes. The same inversions which induce unintegrated genes in adults also produce them in the diploid brain and imaginal discs of larvae. On the other hand, in the polytene salivary glands, unintegrated genes were found in every genotype examined.

The effect of X chromosome heterozygosity on the structure of the ribosomal genes in Drosophila melanogaster.

Sucrose gradient analysis of DNA isolated from detergent-pronase lysates of adult flies has been used to look for ribosomal genes not integrated into the DNA of the chromosome in genotypes containing various combinations of inversions having breakpoints in the proximal heterochromatin of the X chromosome. Unintegrated genes are found in females heterozygous for inversions which have one breakpoint between the nucleolus organizer and the centromere. Homozygotes and males do not have unintegrated genes. The results suggest that unintegrated ribosomal genes result from an interaction between homologues having different arrangements of the proximal heterochromatin. In addition, data from a series of stocks carrying duplications of the X heterochromatin provide independent evidence for the size of the DNA on our gradients.

Ribosomal genes not integrated into chromosomal DNA in Drosophila melanogaster.

DNA obtained by a gentle lysis procedure from adult Drosophila melanogaster was analyzed by sucrose gradient sedimentation. The major portion of the DNA has an estimated weight of at least 5--10 X 10(9) daltons. All of the ribosomal genes are present in this high molecular weight DNA in adult males with one nucleolus organizer or in adult females with two nucleolus organizers as shown by hybridizing fractions of the gradient with ribosomal RNA. In female adults with one nucleolus organizer instead of the usual two, 68% of the ribosomal genes are found in high molecular weight DNA and 32% are found in DNA of smaller size (approximately 3 X 10(8) daltons). We propose that these latter genes are not integrated into the DNA of the chromosome.

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.