ABSTRACT
Our understanding of the molecular and cellular mechanisms of sex differentiation in fishes has increased exponentially over the last several decades. A number of critical steps involved in this process and some of the specific regulatory genes have been identified (see reviews by Yamamoto, 1969; Hunter and Donaldson, 1983; Nakamura et al., 1998; Baroiller et al., 1999; Striissmann and Nakumara, 2002). The majority of the literature seems to support the concept promoted by Yamamoto (1969) that natural steroid hormones are key regulatory factors in the cascade of events associated with gonadal sex differentiation (Adkins-Regan, 1987; Redding and Patino, 1993; Patino, 1997; Nakamura et al., 1998; but also see conflicting views in Kawahara and Yamashita, 2000; Striissmann and Nakumara, 2002). Furthermore, steroidogenic enzymes and steroid receptors are elements that can be influenced by environmental temperature (Crews et al., 1994; Baroiller et al., 1999), and this could explain the growing number of reports of temperaturedependent sex determination among fishes (Striissmann and Patino, 1995, 1999; Pandian and Koteeswaran, 1999). Notwithstanding these recent advances in knowledge, our understanding of the mechanisms of sex differentiation in fishes is far from complete. Past experience has demonstrated the risk of making generalizations based on observations made with only a handful of species. Such generalizations may be possible only when information is available for species representing the full spectrum of the forms of sexuality found in fishes. Fishes include species that reproduce gonochoristically, hermaproditically (sequential or simultaneous), and parthenogenetically (unisexual) (Yamamoto, 1969). Moreover, sex differentiation among gonochorists can proceed directly from the undifferentiated gonads into a male or female pathway ("differentiated gonochorists") or through an initial female phase followed by sex reversal into males in half of the individuals ("undifferentiated gonochorists") (Yamamoto, 1969; Nakamura et al., 1998).
