Biology Reference
In-Depth Information
the females. Accordingly, the percentage increases from 8% in a 15 dph fry
to 17% in a 29 dph presumptive male fry, in comparison to the increase
from about 20% to 44% in a female fry during the corresponding period.
Hence Uchida et al. have claimed that sexually dimorphic differentiation
has indeed commenced at 15 dph, when the difference in the percentage
of perinuclear oocytes between the presumptive males and females has
become apparent. Hence the occurrence of juvenile hermaphroditism is
evidenced by the presence and persistence of oocytes and the ovarian cavity
in presumptive males; it is however transient for a short period of a few days;
yet their formation precedes the induction of male sex differentiation.
Using a
vasa::
egfp
transgenic line to identify sex transforming juveniles,
Wang and Orban (2007) have shown that the expression of
cyp19a1a
commences in the non-transforming juvenile; it is up-regulated around
oocytes in the ovary until the 3 wpf. During this period of 1-3 wpf,
cyp11b
and
amh
are not detectable. But in the gonad of the transforming juvenile,
the expression of
cyp19a1a
is down-regulated from 3wpf onwards and the
amh
expression is increased by 300-600 times by up-regulation in regions,
where
cyp19a1a
has been expressed earlier. Expression of
cyp11b
is also
up-regulated but later than
amh
and its localization is not related to the
position of degenerating oocytes. Briefl y, at 3 wpf, i.e., at 21 dph dimorphic
sex differentiation becomes more apparent and established, as evidenced
by
egfp-
positive females and
egfp-
negative presumptive males. Apparently,
amh
is one of the prime candidate genes for down-regulation of
cyp191a
leading to the transformation of juvenile ovary into testis. Hence the peptide
hormone
amh
inhibits the expression of c
yp19a1a
(cf Rodriquez-Mari et al.,
2005) and is responsible for the degeneration of oocytes along with the
ovarian cavity, a male specifi c event (Uchida et al., 2002). Notably, Guo et
al. (2005) have shown the strong expression of
Dmrt1
in the perinuclear
oocytes but a fainter one in matured oocytes. Remarkable differences in the
number of developing oocytes in presumptive male zebrafi sh have also been
recently reported (Hsiao and Tsai, 2003). These fi ndings on the molecular
genetic basis of male differentiation in zebrafi sh seem to confi rm the earlier
observations of Uchida et al. that the difference in percentage of perinuclear
oocytes may serve as a morphological index of male differentiation.
Incidentally, Wang et al. (2007) have reported that presumptive males,
in which the differentiation process towards the male commences at the
21 dph, approach completion of testicular differentiation around 32nd,
35th and 43rd dph (Fig. 15). Krovel and Olsen (2002, 2004) have found that
the
vas
::
egfp
transgene is strongly expressed in gonads of the female but
only faintly in that of the male zebrafi sh. Wang et al. have subjected 500
or so individual zebrafi sh at 28 dph for an intensive investigation. They
have found wide variations among the tested individuals not only at the
commencing point and duration of
egfp
expression but also in its intensity
