ABSTRACT
In vertebrates two general mechanisms of sex determination are recognized. They are (i) Genetic Sex Determination (GSD) and (ii) Environmental Sex Determination (ESD) (Crews and Bull, 2008). In GSD species, as in homeiothermic birds and mammals, sex is determined at fertilization and differentiation is driven by expression of a cascade of genes, whose expression is neither sensitive nor responsive to changes in the environment. In poikilithermic fi shes and reptiles, sex is also determined by gene (e.g., Dmy, Dmrt 1) or combination of genes during embryonic and/or postembryonic stage(s); in them the process of sex differentiation is, however, regulated by a host of genes, whose expression is sensitive and/or responsive to changes in environmental factors like temperature, pH and oxygen levels. As a consequence, environmental factors may more decisively regulate the sex differentiation process. The effects of changes in pH and oxygen levels are more important to aquatic fi shes but may be less relevant to reptiles, whose eggs are incubated in land. The role of pH and hypoxia in disruption of genetic sex differentiation has already been discussed in the earlier chapters. Within Environmental Sex Differentiation (ESD), TSD is designated to convey Temperature-dependent and Thermo-labile Sex Differentiation. Unlike the usage by herpetalogists (e.g., Wibbels et al., 1994), the TSD is used here to convey temperature-dependent sex differentiation in fi shes. To denote Genetic, Environmental and Temperature-dependent Sex Differentiation, the abbreviations, GSD, ESD and TSD are used to distinguish Genetic (GSD), Environmental (ESD) and Temperature-dependent (TSD) Sex Differentiation.
