Disproportionately-replicated, nonfunctional rDNA in compound chromosomes of Drosophila melanogaster

The multiplicity of the genes for ribosomal RNA varies in several ways. The series of bb mutants are an example of mutant phenotypes associated with deficiencies of the rDNA. In certain genotypes the amount of rDNA present can disproportionately replicate and in some cases become incorporated into the chromosome and thus inherited. In other cases there is no stabilization of the increased multiplicity of rRna genes. In still other cases the multiplicity of rDNA does not change. We describe cases of increased, but non-functional, multiplicity of rDNA in compound chromosomes routinely used in analyses of these phenomena and discuss the associated problems.