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an individual's life suggests that these fish have in place an inherited mechanism
of switching to the opposite sex, which may be very relevant in understanding the
mechanism behind simultaneous hermaphroditism, which still remains mysterious.
Ample empirical evidence shows that sequential hermaphroditism occurs in many
species that are known to have genetically determined sex, including birds, amphib-
ians, and fish (
Kuntz et al., 2004
).
Sex chromosomes are identified only in 176, or about 10%, of 1700 fish species
investigated cytogenetically (
Devlin and Nagahama, 2002
), but even these species
can switch to the opposite of their genetic sex. Our knowledge of the genetic deter-
mination of sex in this group seems far from reliable when we read: “Male hetero-
gamety (males are XY and females XX, as is generally the rule in mammals) and
female heterogamety (females are WZ and males ZZ, the system at work in birds)
are sometimes observed within the same fish genus, and even the same fish species.
More complicated systems can involve multiple sex chromosomes and multiple gene
loci (influence from autosomal loci on sex determination and polyfactorial sex deter-
mination)” (
Volff, 2002
).
The empirical evidence of sequential hermaphroditism in fish indicates that this
is a function of an epigenetic mechanism (the switching to the opposite sex occurs
within the life of an individual). As early as the mid-1970s,
Tang et al. (1974
)
observed that injection of the mammalian pituitary LH in the females of the rice-
field eel,
Monopterus albus
(Zuiew), induces the transformation of ovaries into male
gonads displaying normal spermiogenesis.
Removal of a female fish from a protandrous (i.e., develop first as males) group
or of a male from a protogynous group (i.e., develop first as females) induces the sex
inversion of the largest individual into the sex of the removed fish. Such sex inver-
sions are determined in the brain by the secretion of gonadotropins from hypotha-
lamic neurons (
Baroiller et al., 1999; Grober and Bass, 1991
).
Unlike other vertebrate classes, the brain in fish is not fully sexualized, and hence,
it is plastic enough to enable sex inversion in adult fish (
McCarthy, 2009
). The estab-
lished role of social factors in sex inversion also supports the role of the brain in
sex determination in fish. Effects of cortisol in sequential hermaphroditism in fish
provide support to the idea that stress, a neurobilogical process, may be involved in
environmental sex determination (
Hattori et al., 2009
).
Essential changes during sex inversion also occur in the number of neurons of
the fish hypothalamus. Masculinization of fish is associated with an increase in the
number of hypothalamic GnRH neurons in cases of socially determined sex inver-
sion from female to male (
Elofsson et al., 1997
).
Sex Evolution and Sex Determination in Eumetazoans
There are many hypotheses on the evolution of sex. The oldest, and still prevalent
among them, is that of the German biologist August Weisman (1834-1914), put for-
ward by the end of the nineteenth century. According to Weisman, sexual reproduc-
tion generates variation upon which the natural selection acts. However, the opposite
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