Biology Reference
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expression in female gonads is at least a few hundred times higher than in
male gonad during the period between 55-127 dpf at 8-10ºC. Hence down-
regulation of
aromatase
gene expression in the male gonad is a critical step
in sex differentiation in the normal male and as well in the artifi cially sex
reversed phenotypic female. Guiguen et al. (1999) have also shown that
aromatase activity in the gonads of the normal female is much higher at
64 dpf fry than that of a masculinized XX neomale fl ounder
P.olivaceus
fry
fed with aromatase inhibitor or MT and concomitant down-regulation of
ovarian
aromatase
gene expression (Kitano et al., 2000).
2.6c Danio rerio
O. latipes
and
O. niloticus
are male heterogametic and primary gonochores,
in which gonadal differentiation proceeds from undifferentiated gonad
directly to the ovary and testis. Based on gynogenic (but not cytogenic)
evidence adduced by Uchida et al. (2002), the zebrafi sh
D. rerio
is regarded
as female heterogametic,
albeit
doubts still remain on its gamety. Braat et
al. (1999) have shown that the migration of PGCs is completed and cluster
with precursor PGC supporting somatic cells are recognizable at 5 dpf.
The PGCs proliferate between 5 and 10 dpf to produce about 30 PGCs;
however, Braat et al. (1999) remain silent on any difference in the number
with regard to the possible presumptive sex. In
G. aculeatus
, a stickleback
characterized by differentiated gonad, proliferation of PGCs precedes
directing the ovarian differentiation (Lewis et al., 2008). In zebrafi sh embryo,
meiosis occurs as early as 10 dpf and oocytes appear between 14 and 29
dpf (Takahashi, 1977; Maack and Segner, 2003). Gonadal differentiation in
the male involves a transformation from juvenile ovary to testis. Briefl y,
the ovarian development is initiated by 10 dph and progresses until 20
dph. Between 15 and 25 dph testis development is initiated in presumptive
males, simultaneously with apoptosis of ovarian cells including the ovarian
cavity (Jorgensen et al., 2008). For details on the mechanism of apoptosis,
Krumschnabel and Podrabsky (2009) may be consulted. It may also be
indicated that many authors have endeavored to search for the gene or genes
(Bradley et al., 2011) responsible for sex differentiation in zebrafi sh with a
differentiated gonad. This topic has made an attempt to trace the sequence
of mile stone events in the search for sex determining/differentiation in
zebrafi sh.
In the absence of any established biochemical marker for genetic sex
of zebrafi sh, Uchida et al. (2002) have used a morphological feature, the
percentage of perinuclear oocytes (see Fig. 30, Pandian, 2011) as an index
to distinguish a presumptive male from a female. They have found that the
percentage of perinuclear oocytes with regard to the total number of germ
cells in an 'ovary' is signifi cantly lower in the presumptive males than in
