Differentiation in Gonochores - Genetic Sex Differentiation in Fish - page 35

is expressed in the liver. In contrast, shbgb is a salmonid-specifi c gene,
and is expressed mainly in the ovary (Bobe et al., 2010).

2.4 Sex steroid receptor genes

From the point of endocrine distruption, sex steroid receptors are recognized
but publications on them are few. Piferrer and Guiguen (2008) are the fi rst
to provide a brief account. The sex steroids exert their biological functions,
when bound to specifi c receptors. The sex steroid receptors are the ligand-
activated nuclear transactivation factors and control gene expression in all
vertebrates. From the point of binding and molecular characterization, they
are grouped into androgen ( ar ), estrogen ( es ) and progestogen ( ps ) receptors.
Within ar , there are two types namely arα and arβ . Table 12 summarizes
available information on identifi cation, characterization and cloning of ar
in a half dozon gonochoric fi sh. The ar mRNA level is shown to coincide
with male differentiation in Dicentrarchus labrax . In zebrafi sh, the levels
seem to determine the initiation of male differentiation period between 16
and 22 dph (Fig. 8).
Four piscine estrogene receptors ( er ) namely erα , erβ , erβ2 and erγ have
been identifi ed (Hawkins et al., 2000). Their identifi cation, characterization
and expression in some fi shes are briefl y summarized in Table 13. Er s
have high affinity for E 2 and specifically bind estrogen agonists like
ethynylestradiol (EE 2 ) and diethylstilbosterol (DES) (Barrero-Gil et al., 2004)

Table 12. Summary of androgen receptors in gonochoric fi sh

Species Observations Reference

Oncorhynchus mykiss ar α fails to distinguish between any

of the natural androgens, as tested by
transactivation assay

Takeo and Yamashita
(2000)

Anguilla anguilla

Identifi cation of ar α, ar β and pr Ikeuchi et al. (2001)

Gambusia affi nis

Tw o ar cDNAs are cloned and
identifi ed as ar α and ar β. Both of them
predominantly express in the distal anal
fi n rays

Ogino et al. (2004)

Dicentrarchus labrax

First assessment on the role of androgen
receptor. Higher ar mRNA levels at
200 to 300 dph coincide with male
differentiation

Blazguez and Piferrer
(2004), Muriach et al.
(2008)

Danio rerio

Molecular cloning and characterization
of a nuclear ar mediates the action of
11-KT

Olsson et al. (2005)

D. rerio

The estimated ar levels determine
the period of initiation of male
differentiation among juvenile
hermaphrodites

Jorgenssen et al. (2008)