MECHANISMS OF SUPPRESSION AND AMPLIFICATION OF RIBOSOMAL RNA GENES IN DROSOPHILA MELANOGASTER
KALUMUCK, KAREN ELEANOR
Doctor of Philosophy
Compensation is a mechanism by which Drosophila melanogaster can increase its ribosomal RNA repeat unit multiplicity by up to two-fold. The phenomenon occurs under defined genetic conditions, is restricted to the X chromosome nucleolus organizer region, and occurs only in somatic cells of both sexes; therefore the increase is not transmitted to progeny. Molecular analysis was used to analyze which types of repeat units were disproportionately replicated during compensation. It was found that the relative percentages of uninterrupted repeats and those containing major or minor type I insertions were the same as for the normal X chromosome. This nonselective amplification of ribosomal RNA repeat units distinguishes compensation as a mechanism distinct from independent ribosomal DNA polytenization. Five Y chromosome nucleolus organizer mutants generated by ethyl methanesulfonate mutagenesis of the B('s)Ybb('+)y('+) chromosome were previously analyzed as to ribosomal RNA repeat unit content, hemizygous phenotype and behavior in additivity tests with rDNA-deficient X chromosomes. Molecular analysis of the repeat unit organization of four of these mutants showed a direct correlation between their uninterrupted repeat unit number, phenotype, and functionality, as evidenced by the additivity tests. These constitute a mutant class in which greater numbers of uninterrupted repeats resulted in decreased severity of bobbed phenotypes. Another mutant allele, bb('1-4), failed to show additivity and was an early developmental lethal despite an uninterrupted repeat multiplicity sufficient to reach the late pupal stage. This apparent suppression of presumably functional repeats is indicative of a separate mutant class for the Y chromosome nucleolus organizer redundant locus.