Nucleolus organizer regions in Drosophila species of the repleta group

Abstract

The locations of nucleolus organizer regions in the genomes of D. neohydei, D. eohydei and D. repleta were studied by in situ hybridization. All three species carry one nucleolus organizer in the X heterochromatin. The Y chromosomes of D. neohydei and D. eohydei carry two nucleolus organizers in terminal positions comparable to those of D. hydei. The Y chromosome of D. repleta appears also to carry two nucleolus organizer regions, but this is difficult to establish because of the small size of this chromosome. In situ hybridization of ribosomal RNA with salivary gland chromosomes revealed the presence of additional ribosomal DNA sequence clusters in autosomes 2, 3 and 4 of D. neohydei. They form no nucleolus in salivary glands nor are they associated with the nucleolus originating from the sex chromosomal nucleolus organizers. After in vitro pulse-labelling salivary glands with radioactive RNA precursors, no autoradiographic label is associated with the autosomal ribosomal RNA genes. Transcript in situ hybridization gives no label in these autosomal loci. We conclude therefore that the autosomal rDNA sequences are inactive in salivary glands. In situ hybridization with clones of a major part of the 28S intervening sequence (IVS) of D. hydei to polytene chromosomes of D. neohydei results in intense hybridization of the 28S IVS fragment to the autosomal ribosomal DNA sequences. It is therefore suspected that most if not all of the rDNA copies in the autosomes contain an IVS. The inactivity of these ribosomal genes might be the consequence of the presence of intervening sequences in the 28S gene. These observation supply additional evidence in favour of a transcriptional inactivity of ribosomal genes with an IVS in Drosophila. Additional loci with IVS sequences are found in different positions of the genome which do not hybridize with rDNA sequences without IVS.