ABSTRACT
Sex determination, a delicate process, is one of the earliest and most basic ‘decisions’ made by a developing embryo. The decision whether to become male or female is conveyed very early in embryonic development by a primary gene(s); this signal is transmitted through a cascade of genes, which ultimately completes the production of distinct sexes. It is therefore not unreasonable to expect the genetic mechanism of sex determination to be among the most conserved developmental process. The entire sequence of the sex determination system commencing from the primary sex determining gene(s) down the genetic cascade completing the sexualization process seems to be very stable in some fi shes but not in others. For instance, cyprinids like the male heterogametic crucian carp Carassius auratus, irrespective of being fertilized by the homologous sperm or by the male heterogametic sperm of the common carp Cyprinus carpio, sustains the expected sex ratio of 50% females and 50% males among their (F1-F16) progenies . Nor do the other cyprinids like Puntius gonionotus, on induction of gynogenesis, display any departure from the expected 100% female ratio (see Table 19). Likewise polyploidization of these hybrid progenies to 3n or 4n too does not alter the normal ratio. This observation holds good for the cobitid loach Misgurnus anguillicaudatus also. On the other hand, the departure from the expected sex ratio is ubiquitous within a single pair or when hybridized with male or female heterogametic cichlids . Apparently, the sex determination system is stably fi xed in the cyprinids like the crucian carp but not in the cichlids. In fact the stability of the sex determination system appears to be continuum from the stably fi xed one to the most labile one (see Baroiller et al., 2009a,b). In some fi shes, one or more genes in the
cascade seem to be infl uenced by environmental factors, rendering the sex determining system a labile one.
