Abstract
The patterns of the ribosomal DNA (rDNA) repeat units in seven Drosophila melanogaster inversional mutants have been studied. Among them, only the In(1)sc8 and its deletional derivative Df(1)mal12 female rDNAs exhibited significant reduction in the size of nearly all units, compared to the wild-type females (Canton S, Oregon R). Further investigation shows that each kind of repeat (insertion-free, insertion-containing) in the Xsc8 rDNA array is highly enriched with short (reduced to 4 kilobases) intergenic spacers (IGSs). We revealed two main types of rearrangements. Only part of the 4 kb IGSs display variable length deletions (0.2-0.6 kb) at the 5' ends, within the so-called '1900' base pair (bp) region, recognizable by restriction endonuclease AluI. The presence of additional 100-150 bp DNA in the start portion of this region has also been demonstrated. In contrast, the 3' end spacer regions, corresponding to the external transcribed spacer, do not show any changes in size. These data indicate how reductions of approximately 1.1 kb DNAs in sc8 IGSs, carrying both the rearranged and non-rearranged '1900' sequences, are achieved: the fixed decrease of a number of 240 bp AluI subrepeats, clustered in the central IGS portion, also contribute. None of the other similar inversional mutants examined has so many IGS variants. Therefore, alterations in the Xsc8 rRNA gene cluster seem not to be dependent on its inversional status.
