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undifferentiated gonad of juvenile recipients, i.e., during the labile period.
On transplantation of the SSCs into the sterilized testis containing germ cells
supporting cells, the SSCs differentiate into testis only in the Nile tilapia
and pejerrey; this indicates that the germ cells supporting somatic cells have
lost the bisexual potency and the hormonal climate prevailing in the adult
sterilized testis permits the gonad to express unisexual potency of SSCs
alone. Administration of one or another androgen even at relatively higher
doses to gravid guppy and molly masculinized the gestated progenies
with undifferentiated gonad but not the gravid females. Lastly, surgical
experiments undertaken by different authors in all the primary gonochores
have shown that following the removal of the testis and ovary, the adults
regenerate only the testis and ovary, respectively; these observations
clearly indicate that as the organ system perhaps including the brain is
sexualized, the adult fi shes regenerate only the testis following castration
and the ovary following ovariectomy. In fact, the grass carp, which is not
amenable to the induction of hormonal sex reversal even during the juvenile
stage, regenerates the testis and ovary following the removal of the entire
testis and ovary including the adjoining mesentery, respectively. Clearly,
the sex is decisively determined and the brain of grass carp is sexualized
prior to or immediately after hatching. However, many gonochoric teleosts
are amenable to hormonal induction of sex reversal, when the hormone
is administered during the labile period mostly restricted to the juvenile
stage. In them sex is stably determined, even if phenotypic sex is different
from genotypic sex, as in the rainbow trout, and their brains are perhaps
sexualized at the end of the labile period. It is very likely that the brains
of these gonochores are sexualized at the juvenile stage and their window
for reversible sex change is certainly closed before the fi shes reach sexual
maturity (Fig. 56).
4.2 Abnormals and steriles
Among gonochoric species, the presence of abnormal hermaphrodites has
been frequently reported. These non-functional or very rarely functional
abnormals possess (i) ovotestis (e.g., Oncorhynchus keta ), or (ii) ovotestis +
ovary (e.g., Salmo trutta, or (iii) ovotestis + testis (e.g., O. kisutch ) or (IV)
ovary and testis (e.g., Takifugu rubripes (Fig. 57) . Of these, the incidence
of ovotestis (≈ 36%) and ovary + testis (> 33%) is more common (see Atz,
1964). In 1964 Atz summarized more than 90 historical examples. Later
Devlin and Nagahama (2002) also summarized the subsequent reports.
Pandian (2010) noted that the abnormals are scattered across 23 taxonomic
families. For obvious reasons, the incidence frequency is the highest among
the commercially important cyprinid species. The reports on incidence of
abnormals are mostly limited to the morphological description. Histological
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