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Hofsten et al., 2001). Ff1b and Ff1d are functional homologues of
Sf-1 identifi ed in zebrafi sh; they show higher levels of expression in
male gonads, especially in Leydig and Sertoli cells (von Hofsten et
al., 2005a).
(vi) cyp19 ( aromatase ): Despite the enormity of diversity in differentiation,
the most conserved factor common to nearly all fi shes is the control
of ovarian differentiation by the gonadal aromatase gene cyp19a1a.
The gene cyp19 codes for the enzyme cytochrome p450 aromatase
(p450 arom), a key enzyme in conversion of androgens to estrogens
(Simpson et al., 1994). Aromatase is a member of the p450 cytochrome
superfamily of enzymes. The role of aromatase and estrogens in sex
differentiation in fi sh has recently been reviewed by Guiguen et al.
(2010). The proportion of androgens and estrogens is critical for sex
differentiation in many teleost species and is limited by the quantum
of aromatase availability. p450 arom regulates the amount of estrogen,
especially 17β-estrogen and maintains its level within adequate range
for normal ovarian differentiation (Trant et al., 2001; Fenske and
Segner, 2004). Aromatase is a duplicated gene in teleosts (Chiang et
al., 2001), except in eels belonging to an ancient group (Cheshenko
et al., 2008). The isoforms of the aromatase gene cyp19a and cyp19b
encode two structurally different proteins p450 aromA and p450
aromB, respectively, with similar catalytic activities (Kwon et al.,
2001a; Liu et al., 2007). p450 aromA and p450 aromB are preferably
but not exclusively expressed in the gonads and brain, respectively
(Piferrer and Blazquez, 2005). Restricted to gonad, cyp19a1a is directly
involved in ovarian differentiation. Expressed in the brain, cyp19a1b
acts indirectly probably through the hypothalamus-hypophysis-
gonad axis (Chiang et al., 2001; Chang et al., 2005; Sawyer et al., 2006).
However, Kallivretaki et al. (2007) have found that expression level
of cyp19a1b during sex differentiation in zebrafi sh is not assigned
to a particular sex. In the model proposed for sex differentiation in
zebrafi sh by von Hofsten and Olsson (2005), p450 arom plays a key
role.
p450 aromA is reported from the ovary and testis of many fi shes but
its enzyme activity is limited to the ovary only, although it occurs in
O. latipes and D. labrax (Piferrer and Blazquez, 2005). In O. niloticus,
a species with both genetic and environmental sex determination,
high p450 aromA mRNA expression levels are required for ovarian
differentiation (Kwon et al., 2001a,b). Two key steroidogenic aromatase
enzymes known to be highly involved in gonadal differentiation are:
(i) cytochrome p450 aromatase (p450 arom) and (ii) cytochrome p450
11β hydroxylase (p450 arom 11β, Kusakabe et al., 2002). In fi shes,
p450 11β is involved in the production of 11-oxygenated androgens,
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