Biology Reference
In-Depth Information
other vertebrates and many central mechanisms underlying reproduction
(Le Page et al., 2010). The root causes for the observed plasticity in sex
determination and differentiation processes of fi shes have been traced to two
unique characteristics: 1. The non-sexualization of fi sh brain permanently,
unlike in mammals and 2. The capacity of fi sh brains to constantly grow
during adulthood and to regenerate after injury, and high aromatase activity
(Table 1). Consequently, the critical window of brain sexualization, which is
limited to the prenatal period in mammals, is extended through the entire
life span of fi shes (Fig. 1). However, this generalization of Le Page may be
true of secondary gonochoric and hermaphroditic fi shes; the critical window
of brain sexualization seems to be closed in most primary gonochoric fi shes
at the juvenile stage or latest by puberty.
In a classical contribution, Le Page et al. (2010) have summarized the
major differences between mammals and fi shes in gonadal differentiation
and brain sexualization (Table 1). The notables are: (i) sex is decisively
determined by a single
Sry
gene located on a morphologically distinguishable
sex chromosome in mammals but it may be by many genes (e.g., Hale et
al., 2010) located on different and not readily distinguishable chromosomes
in fi shes (e.g., Bradley et al., 2011), barring
Dmrt1bY
in a couple of medaka
fi shes (Matsuda et al., 2002; Nanda et al., 2002), (ii) restricted to a small
area, the brain is sexualized during the prenatal period in mammals but
the sexualization continues in the entire brain throughout the development
and adult life in fi shes, (iii) testosterone is the male hormone in mammals,
but it is 11-Ketotestosterone (11-KT) in fi shes and (iv) there is only one
aromatase gene with high expression in the ovary and associated functions
in mammals; however, there are two aromatase genes
cyp19a1a
in the gonad
and
cyp19a1b
in the brain of fi shes (Le Page et al., 2010). Whole genome
duplication inclusive of sex steroid receptor genes, and subsequent loss of
(androgen receptor)
ar
A and substitutions of
ar
B are viewed as a permissive
factor allowing plasticity and evolvability of divergent sex determination
mechanisms in fi shes (Douard et al., 2008).
Of 30,000 and odd fi sh species, cytogenetic and genetic analyses have so
far been made only for 1,700 species. The presence of sex chromosome and
heterogamety has been recognized only in 11% of them. Following many
efforts to search for a series of sex linked genes, it has become possible to
locate the sex determining loci, that is, the SEX in
Salmo salar
(Artieri et al.,
2006). Clearly, it may not bear much fruit to endeavor to identify a single
sex determining gene located on a sex chromosome in fi shes. Under the
infl uence of the mammalian scenario, it appears that many fi sh geneticists
and molecular biologists are searching for a single sex determining gene
residing in a specifi c sex chromosome. As indicated, the piscine genetic
mechanism of sex determination seems to reside in more than one gene
