Biology Reference
In-Depth Information
0.030
0.025
0.020
0.015
0.010
0.005
0.000
0
10
20
30
40
50
Fig. 8.
Expression of androgen receptor in juvenile zebrafi sh during sex differentiation. Note
the elevated expression at 16 and 22 dph among high expresser group, suggesting that the male
differentiation is apparently initiated between 16 and 22 dph (from Jorgensen et al., 2008)
as well as some synthetic antagonists like ICI 182,780 (Piferrer and Guiguen,
2008). They produce several different transcripts in identifi ed tissues in
tested fi shes. However, the evidence is limited, and not consistent, as some
of them differ from species to species and sometimes even from tissue to
tissue within a species. Future research may allow some generalizations.
Besides these nuclear steroid receptors, G-protein coupled membrane
receptors have also been identifi ed. Interestingly, they require just a few
minutes and hours to elicit responses, and may play a role in the fi nal
maturation of gametes (Thomas et al., 2006). In the greenback fl ounder
Rhombeosolea tapirina
and rainbow trout
O. mykiss
, Hobby et al. (2000) have
detected a sex Steroid Binding Protein (SBP) that binds E
2
with high affi nity.
Binding of E
2
to SBP is very rapid and occurs within 5 minutes. In trout, SBP
binds E
2
with the highest affi nity followed by Testosterone (T). In contrast,
the relative affi nity of T for fl ounder SBP is more than twice that of E
2
.
In zebrafi sh, expression of mRNAs of gonadotropin receptors
fshr
in
Leydig cells and
lhcgr
in Sertoli cells has been reported. The expression
of
lhcgr
in Sertoli cells, the fi rst report for a vertebrate, may be associated
with the undifferentiated gonochoric mode of gonadal differentiation in
zebrafi sh. In the recombinant zebrafi sh (
rzf
),
rzfLH
and
rzfFSH
stimulate
the release of gonadotropins (Garcia-Lopez et al., 2010).
Incidentally, the evolutionary history of receptor genes seems to provide
an explanation for the indecisive or plasticity of sex determination in
fi shes. Two Whole Genome Duplication (WGD) events are considered to
have occurred before the tetrapod split from the fi sh 360 MYa (Zhou et al.,
2002). In an interesting contribution, Douard et al. (2008) have described
