Biology Reference
In-Depth Information
E. merra,
Kobayashi et al. (2010b) have studied the expression pattern of
mRNAs for gonadotropin (GTH) subunits
Cga, fshb and fhb
in the pituitary
at different sexual phases. The
fshb
mRNA level, which remains low during
the female phase, dramatically increases with concomitant development
of the testis. After 3 weeks, FSH treatments (50-500 mg/g fi sh) induce the
female to the male phase and up-regulate endogenous androgen levels
and
fshb
transcripts, while LH treatment shows no effect. Hence FSH may
trigger the female to male sex change in the groupers (see also Alam et al.,
2010). It remains to be seen whether LH triggers the male to female sex
change in protandrics.
Sex steroid receptor genes: In the protogynous
Halichoeres trimaculatus,
ar mRNA
levels are higher in the testis but decrease in the following order:
functional ovary < bisexual-gonad and its ovarian tissues < testis (Kim et
al., 2002). But its level in the protandrous black porgy
A. schlegeli
is higher
prior to sex change (He et al., 2003). Taken together, it is clear that the
androgen receptor is important for the male phase in both protogynous
and protandrous hermaphrodites.
Estrogen receptor cDNAα type has been cloned from the ovary and
liver of
A. schlegeli
(Lee et al., 2001). The
er
α
is signifi cantly more strongly
expressed in the vitellogenic ovary than in that of immature ovarian
tissue. There is no difference in the level of
er
α in the functional male and
bisexual gonad. The
er
is also expressed testicular gonad of the porgy and
is consistent with that reported for
O. mykiss
(Baroiller et al., 1999) and a
combination of
er
α expression and at low doses (0.25 mg) of E
2
signifi cantly
increases the number of spermiating males, milt volume and GSI of the
porgy (Chang et al., 1995).
In view of the role of Hypothalamic-Pituitary-Gonad (HPG) axis in
sexual maturation of fi shes, it is considered that the HPG axis may also
be involved in sex change among the hermaphroditic fi shes. For example
pre-optic GnRH and Arginine Vasotocin (AVT) axis have been implicated as
responsible for sexual plasticity in fi shes (Foran and Bass, 1999), monoamine
transmitters as a stimulating factor in sex reversal of
T
.
duperrey
(Larson et
al., 2003), neuropeptide Y (NPY) as an inducer of sex reversal in
T. bifasciatum
(Kramer and Imbriano, 1997) and hGC as an effector of sex reversal in
Coris
julis
(Reinboth and Brusle-Sigard, 1997). The gobiid fi sh
Trimma okinawae
is
known to rapidly change its sex back and forth from male to female and then
to male serially, depending on the social status in the harem. Kobayashi et
al. (2009) investigated the role of gonadotropin receptor (
GtHR
) gene during
the onset of sex change from female to male and male to female. They also
examined the role of
FSHR
(Follicular Stimulating Hormone Receptor)
and
LHR
(Luteinizing Hormone Receptor) genes, as the action of GtH is
mediated through its receptors
FSHR
and
LHR
located in the gonad. The
gonad of the goby has both ovaries and testes simultaneously. During the
