Biology Reference
In-Depth Information
seminiferous tubules, indicating that
tDmrt1
is specifi cally expressed in
Sertoli cells lineage only.
The expression of
sox9a
has been detected in the PGC supporting
somatic cells but no difference between sexes has been noted until the age of
25 dph in the fry. In contrast to
tDmrt1, sox9a
is most expressed in epithelial
cells of efferent testicular duct in testes. RT-PCR and
in situ
hybridization
studies have conclusively shown that unlike
tDmrt1,
the
sox9a
is expressed
in XY gonads only after the appearance of intratesticular efferent duct or
formation of ovarian cavity (Fig. 14). After the induction of XY embryo
and followed by sex reversal with estrogen, the expression of
tDmrt1
is
suppressed (by down-regulation) and then disappeared completely. In
contrast,
tDmrt1
is expressed by up-regulation in the surrounding cells of
PGCs in XX reversal with androgen (Kobayashi et al., 2008).
Kwon et al. (2001a) have cloned the brain
aromatase
gene and studied
the expression of brain aromatase and ovarian aromatases. Their semi-
quantitative RT-PCR analysis reveals that the expression of brain and ovarian
aromatase
genes is initiated between 3 and 4 dpf (day post-fertilization).
The level of brain aromatase mRNA gradually increases throughout the
period upto 27 dpf with little difference between sexes. A marked sexual
dimorphism of ovarian aromatase mRNA becomes apparent between 15
and 27 dpf, i.e., the level is gradually up-regulated in females, while that in
males is dramatically down-regulated. Incidentally the treatment with oral
feeding of aromatase inhibitor (ATD, 1, 4, 6 androstatriene 3-17 dione) from
11 to 17 dpf or immersion of 13 dpf-fry in a solution containing aromatase
inhibitor signifi cantly increases the percentage of male tilapias (Kwon et al.,
2000). These fi ndings clearly indicate that the ovarian
aromatase
gene plays
a decisive role in initiation of sexual dimorphism in
O. niloticus.
A study on 17 sex differentiation genes in
O. niloticus
fry (5-15 dph
and 25-70 dph) by Ijiri et al. (2008) has not only confi rmed the described
observations but also indicated the role of other sex differentiation genes.
The expression of aromatase (
cyp19a1a
), an enzyme responsible for
producing 17β-estrodiol, has commenced at 5 dah with marked elevation
thereafter in XX gonads. The close relationship of
foxl2
in XX gonads
suggests an important role in the transcriptional regulation of
cyp19a1a
.
In contrast,
tDmrt1
shows male specifi c expression from 6 dah onward,
suggesting its decisively important role in testicular differentiation.
sox9
and
amh
showed testis specifi c expression subsequently. From 35 dph only
the mRNA expression of 11α-hydroxylase (
cyp11b2
), an enzyme responsible
for the synthesis of a potent 11-Ketotestosterone (11-KT), has been found
in XY gonads.
In
Oncorhynchus
too, the
aromatase
gene expression is observed at 55
(day post fertilization) dpf (= 20 dah) before the fi rst occurrence of meiosis
in the early oocytes (70 dpf = 35 dah) (Guiguen et al., 1999). The level of its
