Abstract
Animals and plants display a remarkable diversity in the way they determine sex. This implies that changes in the mode of sex determination occur over evolutionary time. Changes that involve a modification of the identity of sex chromosomes are called sex chromosome turnovers, and are caused by the spread of a mutant gene with sex-determining properties or the translocation of the ancestral sex determiner to a new genomic location. Through both empirical and theoretical studies, the mysteries surrounding the proximate and ultimate mechanisms responsible for turnovers, and their consequences on sex chromosome evolution, are gradually being unravelled. Nevertheless, many questions surrounding these evolutionary transitions remain unanswered: Why do turnover rates vary across taxa? What are the driving evolutionary forces? Why do certain types of transitions seem more common than others?