An evidence-based model of preexisting heterogeneities in dosage sensitivity shaping mammalian and avian sex chromosome evolution. In this model, preexisting heterogeneities in dosage sensitivity determined the trajectory of Y/W gene loss in both mammals and birds, and of subsequent X-inactivation in mammals and dosage compensation in birds. Colored arrow widths are scaled approximately to the number of ancestral genes in each class. (A) The dashed orange line represents the possibility that a subset of X-linked genes may have not undergone compensatory X up-regulation following Y gene decay. (B) Ancestral Z genes with no W homolog follow a gradient of preexisting dosage sensitivity (top; gray to white), which determined the degree of dosage compensation following W gene loss (bottom; gray to white).
