TRANSITION OF SETS. “The emergence of a new master sex-determining gene on a different chromosome can also lead to a shift of the heterogametic sex, [e.g., transforming a XY system into ZW (nonhomologous transition)]. Such a situation is again represented in the ricefishes. Two species have ZW sex determination on two linkage groups, which are different from those that became the X and the Y in the other five species.”
http://www.pnas.org/content/112/34/10575.full