Biology Reference
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from secondary gonochores. It must be noted that after all the processes
de-differentiation and re-differentiation in the same original genetic sex
of the gonads do occur in seasonal/annual spawners repeatedly. But
the most crucial step is the genes that are responsible for switching the
redifferentiation process quite to the opposite sex. Hence it may prove to
be exciting in undertaking comparative studies on some of these secondary
gonochores, hermaphrodites and hermaphrodites that have returned to
gonochorism (e.g.,
Leptoscarus vaigiensis
) to identify the genes that are
responsible for switching the re-differentiation process on and off.
Incidentally, diandric hermaphrodites have undelimited type gonad.
The testis is reported to be derived from its wall. Most available reports are
based on histological studies of the ovarian wall of the Japanese diandric
fi shes. From the picture and description of these ovarian walls, the existence
of germ cells supporting cells like the Sertoli cells and Leydig cells is not
discernible. An electron microscopic study like the one made for the sterile
gonad of tilapia, is desirable.
The process of sex differentiation has been described considering
gonochores and hermaphrodites separately. For gonochores, with the
exceptions of a few (e.g.,
Macropodus opercularis, Crenicara punctulata,
see
Pandian, 2011) do not undergo natural sex change as adults. However,
hermaphrodites do undergo natural sex change once in a single direction
or more than once in either direction; the exceptions may be a fractions of
females in
Pagrus pagrus
and males in
Diplodus vulgaris
(see Pandian, 2010).
Nevertheless, our description of sex differentiation genes and sex steroid
receptor genes has clearly shown that they are not only common to both
of them but also homologous, confi rming that the hermaphrodites have
originated from secondary gonochores and have retained the same genetic
mechanism of gonadal differentiation.
Though much is known about the phenotypic morphotypes (e.g.,
Pandian, 2010) and their endocrine profi les (e.g., Oliveira, 2006), this topic
recognizes that all phenotypic morphotypes reported among gonochores
are
intrasexual,
i.e., fall within a single sex. But those among hermaphrodites
are
intersexual
, i.e., they cross the borders from one sex to the opposite; for
example, primary male to female and vice versa. More reports are required
to confi rm this conclusion.
Genotypic morphotypes include the jack and hooknose among
salmonids, parental and cuckolder among centrarchids and red and
yellow morphs among cichlids. The XY hooknose is known to sire both
hooknose and jack offspring. It is not known whether jacks too sire jacks and
hooknoses. Also it is not known whether the same is true of the centrarchid
and cichlid genetic morphotypes, as has been observed for the hooknose
of salmonids.
