Figure 1.

Comparison of three modes of dosage compensation. (a) Dosage compensation is required to balance gene expression between sexes and between autosomes and sex chromosomes. In mammals (top), one of the two X chromosomes is inactivated in females (blue color for silencing) to balance expression, but X-chromosome upregulation is also required in both males and females to balance expression from diploid autosomes (red color for upregulation). In fruitflies (middle), the two-fold upregulation of the male X chromosome is sufficient to balance expression between sexes and relative to autosomes. In nematode hermaphrodites (XX) (bottom), downregulation of both copies of the X chromosome is superimposed upon upregulation of X chromosomes in both sexes, to balance autosomal expression levels. (b) Distinct complexes (DCC) are involved in dosage compensation in different species, but similarities have been proposed in the targeting and spreading of the various DCCs along the target chromosomes. The models propose that binding occurs at distinct nucleation sites (top) and spreads to additional target sites (bottom), which may be distant in the linear sequence of the chromosome, in addition to local spreading (dashed arrows) of the deposited epigenetic marks around primary target sites. This scheme is meant to highlight common principles, although of course there are species-specific differences, as detailed in the main text.