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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.
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