Figure 5.

Models of sex-specific mechanisms of dosage compensation. Model 1 assumes that prior to compensation, female (red line) and male (dotted black line) expression is unrelated to the eventual amount of dosage compensation. If the male reduces expression of genes to compensate, the line tilts down on the left, resulting in a pattern of positive correlation of level of expression and M:F ratio (black solid line), close to that observed. Model II assumes that prior to dosage compensation, female (red dotted line) and male (black solid line) expression is correlated with the eventual level of compensation (lower average expression in genes to be compensated). In model II, female-specific compensation, illustrated by the shift to the red solid line in the female, leads to the observed pattern in which female expression does not correlate with M:F ratio. Arrows indicate shifts required in each model to achieve the observed pattern that male gene expression is correlated with M:F ratio, but female level of expression is not.