H-Y antigen and the evolution of heterogamety

In natural populations of platyfish, Xiphophorus maculatus, there are three sex chromosomes--W, X, and Y. Females may have one of three genotypes: WY, WX, or XX, and males may have either XY or YY. The W chromosome can be considered a modified X that blocks the male-determining function of the Y. The platyfish may represent an evolutionary stage at which female heterogamety arises through a single mutation, in the midst of a male heterogametic system. Our serological analysis revealed presence of H-Y antigen in XY and YY males, but not in XX, WX, or WY females, indicating that H-Y antigen may not be associated invariably with the heterogametic-type gonad--especially in transitional systems.