Biology Reference
In-Depth Information
gonadal primordium. The fate of these cell lineages, however, has not yet
been fully elucidated.
foxl2
is involved in ovarian development and in medaka, it commences
its expression in PGC-supporting somatic cells in XX gonad, just after the
initiation of ovarian differentiation. It is maintained in granulosa cells
throughout ovarian development and is involved in differentiation of
granulosa cells. In the adult ovary,
foxl2
is expressed among pre-vitellogenic
and vitellogenic follicles but not in post-vitellogenic follicles (however see
Nakamoto et al., 2006).
Recording differences in the frequency of spontaneous occurrence of
male progenies among XX♀ x XX♂ breeding pairs of
O. latipes,
Scholz et
al. (2003) have found that the
Dmrt1bY
expression is strictly correlated
with the genotype but not with phenotype, i.e., its expression is restricted
to XY and YY gonads, regardless of the phenotypic sex. In contrast, the
expression of
fi g1a
is correlated with the phenotypic sex.
Dmrt1bY
seems
to play a key regulatory role at an early event of sex determination and
remains hormone-insensitive, as against the hormone-sensitive
tDmrt1
in
O. niloticus
(Kobayashi et al., 2008).
More recently, a new factor named gonadal-soma-derived factor (
Gsdf
)
has been found to be associated with testicular differentiation (Sawatari
et al., 2007). There are two forms
Gsdf
in rainbow trout
Gsdf1
and
Gsdf2,
while only one in medaka. A rise in expression levels of
Gsdf
in XY gonads
around the 6th dpf (see Paul-Prasanth et al., 2011) leading to testicular
differentiation. Whereas the expression of
Dmy
is linked to the gonotype
of the gonads, that of
Gsdf
is closely linked to the gonadal phenotype.
2.6b Oreochromis niloticus
In the Nile tilapia, males are heterogametic (XY), as in medaka. The sequence
of sexual dimorphic differentiation is similar to that of medaka but the
position of male inducing gene
DMY/Dmrt1bY
in medaka is assumed
by
Dmrt1
called
tDmrt1
in tilapia
.
Kobayashi et al. (2008) have shown
that
tDmrt1
expression occurs specifi cally in Sertoli cell lineage and the
expression precedes testicular differentiation. Examining the sexually
dimorphic proliferation of PGCs and the expression profi les of
tDmrt1
, they
have detected strong expression in supporting somatic cells surrounding
the PGCs at 5 dph fry, prior to the sexually dimorphic proliferation numbers
of PGCs and histogenesis such as the differentiation of intratesticular
efferent duct or ovarian cavity. The number of PGCs, which are around
30 at 5 dph female fry, begins to progressively increase to about 350 at 20
dph fry. During the period from 5 dph to 20 dph, the expression of
tDmrt1
becomes progressively stronger. But the PGC number in the XY fry begins
to increase to about 45 in an 8 dph fry and after a period of arrest, it peaks
