Biology Reference
In-Depth Information
S. meridionalis
belongs to the second group. Incidentally, the oogonial entry
into meiosis and formation of the ovarian lumen occur simultaneously
in
Carassius auratus, Cottus bairdii
and
O. mossambicus
(Nakumara et al.,
1998).
In an attempt to trace the cause for all-female production in the Southern
catfi sh
S. meridionalis
, Liu et al. (2008) have described a histologically sex-
wise indistinguishable gonad until 8 dph at 24ºC. Remarkably, the female
gonad commences two processes, which gradually fuse to form an ovarian
cavity at 12 dph fry, a clear sign of morphological sex differentiation. In
the male gonad, the cavity is not visible at 12 dph. Hence
S. meridionalis
may not belong to secondary gonochore like the zebrafi sh, in which the
ovarian cavity is formed along with oocytes in all fry known as 'juvenile
hermaphrodites'.
Notably, the proliferation of PGCs commences only in the 29 dph fry,
clearly after the ovarian cavity is formed on the 12 dph; the PGCs enter into
the fast proliferation phase in 35 dph fry and end with the terminal number
of 712 in a 85 dph fry (Fig. 13). In the male gonad, they also commence
around the same time and reach the fast phase in a 85 dph fry ending
with the terminal number of 1,159 PGCs (Zhang, 2005). Hence there is a
clear difference between the chronological sequences of dimorphic sexual
differentiation in medaka and tilapia on one hand and catfi sh on the other:
The number of PGCs is signifi cantly higher in the male than in female
catfi sh, indicating perhaps the relatively lower expression of the
Dmrt1b
in the male catfi sh; this is in contrast with the presence of larger numbers
of PGCs in female medaka and tilapia.
Liu Z et al. (2007, 2008) have also estimateed the levels of
cyp 19, foxl 2
and
Dmrt1a
and
b
in the developing gonads, unfortunately at a later period
of differentiation. High levels of expression of
cyp19
,
foxl2
and
Dmrt1a
and
Dmrt1b
in the female gonad at 65 dph, when the sex dependent PGC
proliferation has just become apparent but certainly later, after the formation
of the ovarian cavity at 12 dph. However, the expression levels of
Dmrt1a
and
Dmrt1b
are higher testes. Moreover, the expression of
cyp19b
in the
brain pituitary and gonad is suggestive of its role in B-P-G axis (Lui Z et
al., 2007). Besides
gthα, fshβ
and
lhβ,
known for their expression in zebrafi sh
(So et al., 2005) and protandrous gilthead sea bream
Sparus aurata
(Wong
and Zohar, 2004), are expressed in the catfi sh, especially the
gth α
and
fsh
β
from 25 dph and
lhβ
40 dph again later than the day, when the ovarian
cavity is formed.
2.7 Genetic cascades
Figure 17 is a schematic representation of the genetic cascades through
which the dimorphic sex differentiation is realized in the selected models
