An unusual Y chromosome of Drosophila simulans carrying amplified rDNA spacer without rRNA genes.
The X and Y chromosomes of Drosophila melanogaster each contain a cluster of several hundred ribosomal RNA genes (rDNA). A nontranscribed spacer region separates adjacent rRNA genes and contains tandem copies of 240 bp repeats that include the initiation site for RNA polymerase I transcription. We show here that Drosophila simulans, a sibling species of D. melanogaster, contains few, if any, rRNA genes on its Y chromosome but carries instead a large block (3,000 kb or 12,500 copies) of 240 bp nontranscribed spacer repeats. The repeats are located at the tip of the long arm of the simulans Y chromosome, in contrast to their location among rRNA genes on the short arm of the Y chromosome of D. melanogaster. The bobbed mutation in homozygous females of D. melanogaster shortens and thins the bristles, owing to a partial deletion of rRNA genes on the X chromosome. The bristles of bobbed/Y males are normal owing to the presence of a full complement of rRNA genes on the Y chromosome. Peculiarly, in bobbed/Y males of D. simulans the short bristle phenotype does not return to normal but is enhanced by the presence of the Y chromosome. We propose that the 12,500 nontranscribed spacer repeats on the Y chromosome are responsible for this biological effect by competition for a protein factor(s) essential for normal levels of rDNA transcription at the X-linked locus.
Lohe
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AR
Department of Genetics, Case Western Reserve University School of Medicine, Cleveland, Ohio 44106.
Roberts
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PA
D000818:Animals; D001483:Base Sequence; D015139:Blotting, Southern; D002874:Chromosome Mapping; D004275:DNA, Ribosomal / Q000235:genetics*; D004276:DNA, Satellite / Q000235:genetics; D004330:Drosophila / Q000235:genetics*; D005260:Female; D008297:Male; D008969:Molecular Sequence Data; D005810:Multigene Family; D012335:RNA, Ribosomal / Q000235:genetics; D012091:Repetitive Sequences, Nucleic Acid; D012689:Sequence Homology, Nucleic Acid; D014998:Y Chromosome
D004275:DNA, Ribosomal; D004276:DNA, Satellite; D012335:RNA, Ribosomal