Expression and amplification of the genes for ribosomal RNA in bobbed mutants ofDrosophila melanogaster

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Abstract
Summary: We have employed stocks bearing clonally derivedXchromosomes to investigate several features of the bobbed mutant syndrome, and the amplification of rDNA genes inD. melanogaster. We report that posterior macroscutellar bristle length correlates well with the rDNA content (i.e. dose ofivs–, or uninterrupted genes) in clonedXderivative strains.X/Omales andX/Xfemales with statistically indistinguishable rDNA contents have virtually identical bristle lengths. This indicates that (with respect to this phenotypic character) the rDNAs in these two genotypes are expressed equally, without apparent sexual dimorphism or dosage compensation. However, the severity of bobbed phenotype in terms of bristle morphology, turgite etching, and delayed eclosion is greater in theXbb/XNO−female than in theXbb/Omale genotype for the alleles examined. We estimate the minimum dose of functioning rRNA genes required for viability at 26 δC to be 70 genes per diploid genome. We have examined the capacity of severalXchromosomes which bear bobbed mutant alleles to compensate inX/Omales, and find that disproportionate replication of these rDNAs does not take place. In contrast, at least one of the non-compensating bobbed alleles does appear to undergo rDNA magnification.