Evolution of dosage compensation does not depend on genomic background

Organisms have evolved various mechanisms to cope with the differences in the gene copy numbers between sexes caused by the degeneration of Y and W sex chromosomes. Here we document that two reptilian lineages (XX/XY iguanas and ZZ/ZW softshell turtles), which independently co-opted the same ancestral genomic region for the function of sex chromosomes, follow the traditional dichotomy between male and female heterogamety. The independent co-option of the same genomic region for the role of sex chromosome, as in the iguanas and the softshell turtles, offers a great opportunity for testing scenarios on sex chromosome evolution under the explicit control for the genomic background and for gene identity.