Single Nucleotide Polymorphism-sensitive FISH Detection of Locus-specific Ribosomal RNA Transcription in <em>Drosophila melanogaster</em>

In order to ensure sufficient ribosomal RNA (rRNA) is transcribed for ribosome function, genomes contain hundreds of tandem duplications of the sequences that encode rRNAs, composing regions called ribosomal DNA (rDNA) loci. Many organisms' genomes contain more than one rDNA locus distributed across different chromosomes, and rRNA may be transcribed from multiple rDNA loci or only a single locus. Changes in rRNA transcription sources often indicate ribosomal distress. However, the homogeneity of rRNA obstructs distinguishing if rRNA is transcribed from multiple or a single rDNA locus. Here, we describe a method that uses single nucleotide polymorphism-sensitive fluorescence in situ hybridization (SNP-FISH) to distinguish rRNA transcription between Drosophila melanogaster rDNA loci on the X and Y chromosomes. This method leverages locus-specific rRNA variants to enable easy detection of the source of rRNA transcription with single-cell resolution. This assay can be applied to any Drosophila cell type and may be adapted for use in other systems.