Complex alterations of the ribosomal gene spacers in mutant sc super(8) of Drosophila melanogaster

Extreme changes in the proportions of the rDNA intergenic spacers (IGSs) have previously been demonstrated by us in the X-chromosomal rDNA array of the Drosophila melanogaster mutant sc super(8), where nearly all IGSs were found to be reduced in size. We have cloned different structural variants of the Xsc super(8) ribosomal units and mapped their spacers by restriction endonuclease analysis. Most of the IGSs exhibited complex rearrangements within the subrepeated region at their 5' terminus. The sequence data revealed the presence of an additional 100 bp subrepeat. In addition, extended deletions/substitutions down to the 18S gene were also found. Examination of the Ysc super(8) IGS polymorphism indicates that some of the X-chromosomal IGS variants are probably the result of X-Y interchanges. The rarity of alternative recombination products implies that the overabundant deleted spacers in sc super(8) are generated by nonreciprocal recombination. Our results demonstrate nonrandom distribution of deletional breakpoints within the "1900" region in sc super(8) and show that the deletions do not truncate the internal IGS subregions at either breakpoint but eliminate them as discrete blocks.