Differentiation in Hermaphrodites - Genetic Sex Differentiation in Fish - page 119

p45011β clades of medaka and rainbow trout. The expression level of
p450 arom is 17 times higher in the ovary than in the testis but that of
p45011β is faint in the ovary, slight in the ovotestis and abundant in
the testis. With concomitant changes in the expression of p450 arom
and p45011β , sex of the eel is changed from female to male.
Recently, a clear sexually dimorphic expression of p450 aromatase
(cyp19a1a) has been described in the yellow tail clownfi sh A. clarkii by
Kobayashi Y, et al. (2010a). The full length cDNA of the p450 aromatase
gene spans to 1,928 bp long and encodes 520 amino acids. No signal
of cyp19a1a is detectable in the bisexual gonad. But a high level of
cyp19a1a transcripts (about 2,300 copies/ng total RNA) is detected in
the ovary of the female phase. cyp19a1a immunoreactivity is confi ned
exclusively to the granulosa cells. A phylogenetic tree depicting the
relationship of various cyp19 proteins shows that the clownfi sh falls
into the cluster of grouper, wrasse, sea bass and fl ounder.
(vi) foxl2 : Kobayashi et al. (2010c) have cloned the foxl2 cDNA of the
protogynous wrasse H. trimaculatus. The isolated foxl2 cDNA is 1989
bp long, encodes 306 amino acid proteins and shows 90% homology to
that of fi shes belonging to the order Perciformes. Figure 45 reveals that
the Forkhead (FH) domain, also known as 'winged helix', is highly
conserved among vertebrates. Using zebrafi sh foxl1 as outgroup, the
phylogenetic tree constructed shows that vertebrate foxl2 sequence,
except for rainbow trout, shares high bootstrap values. foxl2 is known
to specifi cally express in the ovaries. As expected, the wrasse foxl2
mRNA is expressed in the ovary during the female phase and signals
of foxl2 protein are localized in granulosa cells but not in thecal cells;
foxl2 is shown to regulate p450 arom expression in the ovary. However,
the foxl2 mRNA transcript levels in the testes of IP and TP males are
almost equal to that of the ovary suggesting the role of foxl2 may be
quite different from other gonochoric vertebrates; in the sex changing
fi shes, foxl2 controls the steroidogenesis.
(vii) sf-1/nr5a1 and nrOb1/Dax-1 are important genes involved in
steroidogonesis and sexual differentiation. In A. schlegeli, nr5a1
but not nrOb1 is required for differentiation of the early testis.
High expression levels (3-11 times of the initial) of nr5a1 favor the
maintenance testicular differentiation during the male phase. In
contrast, low nr5a1 and high nrOb1 expressions sustain sex change
to the female phase. Obviously, nr5a1 and nrOb1 have a cooperative
function for testicular differentiation and subsequent antagonistic
interaction for oocyte development (Wu et al., 2008).
To understand the underlying mechanism of hypothalmic-pituitary-
gonadal axis involved in gonadal sex change in the honey comb grouper