title: Constitutive magnification by the Ybb- chromosome of Drosophila melanogaster.
abstract: Ybb- is an rDNA-deficient chromosome of Drosophila that has often been used in magnification experiments to induce high-frequency reversion of bobbed (bb) chromosomes. We observed previously that Ybb- causes ring chromosome loss even when the rings are bb+, suggesting that Ybb- induces magnifying sister chromatid exchanges in bb+ rings. Here we show that the Ybb- chromosome causes low levels of bb magnification in bb+ flies. We refer to the ability of Ybb- to bypass the rDNA deficiency requirement for magnification as 'constitutive' magnification. We have magnified the ribosomal genes on the Ybb- chromosome and analysed the revertant chromosomes using genetic and molecular methods. We find that magnified Ybb- chromosomes also exhibit constitutive magnifier activity. Molecular analysis shows that both type 1 and type 2 intron+ ribosomal gene repeats are associated with magnified Ybb- chromosomes. Type 2 introns have been described previously in the rDNA of both X and Y chromosomes. However, type 1 intervening sequences are thought to be present only in X, but not Y, ribosomal genes. Some of the Ybb- type 1 insertions differ from those present in the rDNA of X chromosomes in that they contain an EcoRI site, and some may be present in tandem arrays. The constitutive magnifier activity of Ybb- may reside either in the structurally unusual ribosomal gene intervening sequences associated with the chromosome, or in the locus on YL that is required for magnification to occur.
authors.affiliation: Department of Microbiology, Duke University Medical Center, Durham, North Carolina 27710.
authors.forename: D J
authors.initials: DJ
authors.lastname: Komma
authors.forename: S J
authors.initials: SJ
authors.lastname: Glass
authors.forename: S A
authors.initials: SA
authors.lastname: Endow
mesh_terms: D000818:Animals; D004275:DNA, Ribosomal / Q000235:genetics*; D004331:Drosophila melanogaster / Q000235:genetics*; D005260:Female; D007438:Introns; D008297:Male; D014998:Y Chromosome
chemical_list: D004275:DNA, Ribosomal