ABSTRACT
Le Page et al. (2010) have proposed that non-sexualization of the brain is the root cause for the plasticity in sex differentiation in fi shes. However, this may hold true only for all sex changing hermaphrodites and secondary gonochores alone. The brains of primary gonochoric fi shes are indeed sexualized on or before the juvenile/pubertal stage. This conclusion is based on: (i) all the phenotypic morphotypes undergo intrasexual ontogenetic change suggesting brain sexualization in them, (ii) PGCs from embryos and OSCs/SSCs from differentiated gonad drawn from mature adults that have already spawned/milted, when transplanted into undifferentiated gonads of recipients, express bisexual potency. However, the transplantation of SSCs into sterilized adult testis containing germ cells supporting cells differentiate into testis alone; this clearly indicates that the germ cells supporting somatic
cells have lost the bisexual potency, i.e., brain sexualization has decisively acted on germ supporting cells but not on SSCs/OSCs, (iii) From their experiments on hormonal sex reversal, Nakamura et al. (2003) have reported that both germ cells and germ cells supporting cells in the differentiated ovaries have retained bisexual potency. Hence there is a need for further research to confi rm whether the germ cells supporting cells of primary gonochores retain bisexual potency. Notably, the experiments undertaken by Nakamura et al. (2003) have been limited to the juvenile stage (i.e., labile period) during which the brain is not yet sexualized. Incidentally, the dietary administration of androgens to gravid females induced sex reversal only in the progenies but not in the adult female guppy and molly with differentiated ovary (Kavumpurath and Pandian, 1992, 1993; George and Pandian, 1995, 1998), (iv) Gonadectomized fi shes regenerated testis only, when castrated and ovary only, when ovariectomized, indicating that not only brains of these primary gonochores but also the entire organ system in these fi shes have been sexualized. Interestingly, a clear sexually dimorphic expression in muscles of adult male and female guppy Poecilia reticulata has been reported (Shen et al., 2007). At least one skeletal trait, the size of the operclar bone has also been mapped to the sex chromosome of three spined stickleback Gasterosteus aculeatus (Kimmel et al., 2005). Hence more and more evidences from different spheres are forthcoming to confi rm that the brain and organ systems of adult primary gonochores are sexualized. An exception seems to be Betta splendens, in which contradictory results have been reported indicating that more gonadectomy studies are required, especially in anabandits like B. splendens, Trichogaster and Macropodus.
