Figure 1. The evolution of dosage compensation on Z and X chromosomes.

Expected gene expression is shown for males (blue) and females (red) under a very low (ρ=0.1, dotted line), intermediate (ρ=0.5, dashed line) and strong inter-sexual correlation (ρ=0.8, solid line)—(see equations (13) and (14) for dynamics). a,b show equal selection in males and females (S_m=S_f=0.5); c,d show stronger selection in males (S_m=1, S_f=0.1). Evolutionary time refers to the number of generations, ignoring the time taken by successive mutations to fix. Expression is scaled according to the initial degradation z₀ of expression in the heterogametic sex due to the loss of one gene copy, which here is set at z₀=1. Other parameters were also held equal across the two sex chromosome systems (N_eX=N_eZ=1125, μ=0.0003).