Organizational-activational concept revisited: sexual differentiation in an atherinomorph teleost.

Because of its numerous sexually dimorphic characters, the Western mosquitofish Gambusia affinis affinis is an excellent vertebrate system for addressing questions concerning sexual differentiation. In this review, the actions and limits of gonadal sex steroids, specifically testosterone, on the development of the sexually dimorphic anal fin and its axial and appendicular support are described. Data from our laboratory show that the critical period in this species for the anterior transposition of the male anal fin and its appendicular support occurs during the late embryonic period and that this differentiation of the male phenotypic genital areaduring the critical period is regulated by androgen.