ABSTRACT

The description and analysis of the sex differentiation process of fi shes in this book series has led to proposing a new conclusions. Sex is more or less decisively determined by Dmy prior to hatching in two species of medaka and Dmrt1 after hatching in Nile tilapia. In all other fi shes, it may indecisively be determined by a host of genes during the post-hatching stages; for example, sox1lb, sox2la and sox3l are overexpressed in the ovary but amh in the testis of zebrafi sh, a secondary gonochore (Santos et al., 2007). Only 11% of fi sh species have cytologically and/or cytogenetically distinguishable male (XX/XY) or female (ZW/ZZ) specifi c heteromorphic chromosomes (Pandian, 2011, p. 23). In majority of fi shes, the processes of sex determination and differentiation are continuous and accomplished by a cascade of genes (e.g., Zhang et al., 2011), each with small additive effects, that are randomly distributed in many or all chromosomes. The expression of most of these genes is under environmental control.