Location of the 18/28S ribosomal RNA genes in two Hawaiian Drosophila species by monoclonal immunological identification of RNA.DNA hybrids in situ.

Using both heterologous rabbit antisera and mouse monoclonal antibody to RNA.DNA hybrids, we have mapped the in situ hybridization locus of the 18/28S ribosomal RNA fraction to a single large band on polytene autosome 3 in Drosophila heteroneura and Drosophila silvestris. This portion of the chromosome is not physically connected with the nucleolus at the end of larval salivary gland development. In mature larvae, little or no hybridization with the material in the nucleolus can be detected. In younger larvae, hybridization of the ribosomal RNA probe to the nucleolus itself can be observed. The chromosome 3 locus is the only band in the polytene genome that shows variation in size and intensity of staining between populations and species. The interband chromosome regions that are immediately distal or proximal to the 18/28S rRNA locus have been involved in a disproportionately large number of natural inversion breaks observed in the euchromatic portion of the polytene chromosome. In 104 species of Hawaiian Drosophila in which chromosome 3 polytene sequences have been determined, 15 breaks occur in these two regions. On a random basis, only one such break is expected. We propose that this locus may be flanked by substantial heterochromatic blocks which are not represented in the salivary gland chromosome.