Biology Reference
In-Depth Information
genetic system (males containing one and females two X chromosomes). Dosage
compensation in
D. melanogaster
is achieved by
hypertranscription
of the single X
chromosome in males (
Marin et al. 2000, Lucchesi 2009
). As a result, males produce
equivalent
amounts of gene product as females that have two X chromosomes. By
contrast, dosage compensation in the mole cricket
Gryllotalpa fossor
is analogous
to that in mammals; one of the two X chromosomes in females is transcription-
ally
inactivated
, with the inactivation occurring randomly within each cell (
Rao and
Padmaja 1992
).
Gryllotalpa fossor
males are XO and females are XX, and one of the
two X chromosomes in female cells is late replicating and transcriptionally silent.
Males that are XY:AA are
aneuploid
for an X, which is a large fraction of the
total genome (because the Y has only a few genes on it). (AA indicates that there
are two sets of autosomes.) Aneuploidy (when the chromosomal composition in a
cell is not an exact multiple of the haploid set) is normally lethal to an organism. In
Drosophila
males,
hypertranscription
of the single X chromosome requires the func-
tions of autosomal genes,
male-specific lethal genes
+
(
msl
+
), which are under the
control of
Sxl
+
(
Table 10.1
,
Figure 10.1
) and some RNAs that are associated with
the chromatin (
Kelley and Kuroda 2000
). The MSL proteins are assembled with
the RNAs in a remodeling complex (called a
compensasome
) on
≈
100 sites on the
Figure 10.1
Model for the regulation of dosage compensation of X chromosomes in
D. melanogas-
ter
males. MLE, MSL-l, and MSL-3 proteins are produced in both sexes. The
Sxl
+
gene negatively
regulates another gene so that no functional protein, probably MSL-2, is made in females. In the
absence of this protein, the MLE, MSL-1, and MSL-3 proteins cannot associate stably as a “com-
pensasome” with the X chromosome in females. The compensasome does associate with sites on
the X chromosome in males that have the histone H4 acetylated at the lysine 16 position. As a
result, the sole X chromosome is hypertranscribed in males.