ONE-STEP AND STEPWISE MAGNIFICATION OF A BOBBED LETHAL CHROMOSOME IN DROSOPHILA MELANOGASTER

Sharyn A. Endow ¹ and Donald J. Komma ²

¹ Department of Microbiology, Duke University Medical Center, Durham, North Carolina 27710
² Department of Immunology, Duke University Medical Center, Durham, North Carolina 27710

Bobbed lethal (bb^l) chromosomes carry too few ribosomal genes for homozygous flies to be viable. Reversion of bb^l chromosomes to bb or nearly bb ⁺ occurs under magnifying conditions at a low frequency in a single generation. These reversions occur too rapidly to be accounted for by single unequal sister chromatid exchanges and seem unlikely to be due to multiple sister strand exchanges within a given cell lineage. Analysis of several one-step revertants indicates that they are X-Y recombinant chromosomes which probably arise from X-Y recombination at bb. The addition of ribosomal genes from the Y chromosome to the bb^l chromosome explains the more rapid reversion of the bb^l chromosome than is permitted by single events of unequal sister chromatid exchange. Analysis of stepwise bb^l magnified chromosomes, which were selected over a period of 4–9 magnifying generations, shows ribosomal gene patterns that are closely similar to each other. Similarity in rDNA pattern among stepwise magnified products of the same parental chromosome is consistent with reversion by a mechanism of unequal sister strand exchange.