The Ybb− chromosome has been previously shown to induce reduction of X chromosome ribosomal genes in Xbb / Ybb− or Xbb+ / Ybb− flies. These reduction events are presumed to arise as one of the two ...products of unequal sister chromatid exchanges, which result in both magnified and reduced products. Bobbed reduced chromosomes may also arise as products of other recombinative events such as intrachromatid deletions. In this report we use the Ybb− chromosome to reduce the number of ribosomal genes present on X chromosomes from two wild-type stocks under ‘non-magnifying’ conditions. We then show that the bobbed reduced X chromosomes show no detectable difference in their Southern blot rDNA patterns when compared with the parental wild-type X chromosome. This indicates that reduction events do not preferentially delete certain repeat classes, and supports previous observations that the repeat types present in the D. melanogaster X chromosome nucleolus organizer are not significantly clustered.
Summary
Y
bb−
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
Y
bb−
causes ring chromosome loss even when the rings are
bb
+
, suggesting that
Y
bb−
induces magnifying sister chromatid exchanges in
bb
+
rings. Here we show that the
Y
bb-
chromosome causes low levels of bb magnification in
bb
+
flies. We refer to the ability of
Y
bb−
to bypass the rDNA deficiency requirement for magnification as ‘constitutive’ magnification. We have magnified the ribosomal genes on the
Y
bb−
chromosome and analysed the revertant chromosomes using genetic and molecular methods. We find that magnified
Y
bb−
chromosomes also exhibit constitutive magnifier activity. Molecular analysis shows that both type 1 and type 2 intron
+
ribosomal gene repeats are associated with magnified
Y
bb−
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
Y
bb−
type 1 insertions differ from those present in the rDNA of
X
chromosomes in that they contain an
Eco
Rl site, and some may be present in tandem arrays. The constitutive magnifier activity of
Y
bb−
may reside either in the structurally unusual ribosomal gene intervening sequences associated with the chromosome, or in the locus on
Y
L
that is required for magnification to occur.
The Y super(bb-) chromosome has been previously shown to induce reduction of X chromosome ribosomal genes in X super(bb)/Y super(bb-) or X super(bb+)/Y super(bb-) flies. The authors use the Y ...super(bb-) chromosome to reduce the number of ribosomal genes present on X chromosomes from two wild-type stocks under "non-magnifying" conditions. They then show that the bobbed reduced X chromosomes show no detectable difference in their Southern blot rDNA patterns when compared with the parental wild-type X chromosome. This indicates that reduction events do not preferentially delete certain repeat classes, and supports previous observations that the repeat types present in the D. melanogaster X chromosome nucleolus organizer are not significantly clustered.
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