Epigenetic Control of Animal Development - Building the Most Complex Structure on Earth - page 166

the sperms derived from grasshopper spermatogonia had the mysterious chromo-
some that he called the “accessory chromosome” (it turned out to be identical to the
X chromosome). He related that the “accessory chromosome” was present in only
half of sperm cells with sex determination. But it was the American geneticist Nettie
Stevens (1861-1912) who, in 1905, identified the Y chromosome and posited that
the gender of insects and worms depends on the presence of the Y chromosome.
The chromosomal determination of sex soon became almost a truism in biol-
ogy. And the discovery of the temperature-dependent sex determination took biolo-
gists by surprise. Some amphibians have temperature-dependent sex determination,
although they have “sexual chromosomes.” Genotypically, female larvae of the sal-
amander Hynobius retardatus reared at a high temperature (28°C) during the ther-
mosensitive period (15-30 days after hatching) develop into female adults. Thus, a
nongenetic factor, such as the temperature during the thermosensitive period, over-
rides the chromosomal sex determination.
A paracrine effect of a neuroendocrine factor would be to act as morphogen in the
undifferentiated gonad to induce differentiation into the ovaries or testicles. Salame-
Mendez et al. (1998).
The role of gonads and the brain in primary sex development and the develop-
ment of sexual dimorphism is the source of some controversy. According to the pre-
vailing view, the development and functioning of male and female sexual organs
causes the masculinization/femininization of the brain via the secretion of sex hor-
mones. Recent evidence, however, challenges the above view and may indicate that
the reverse is true.
The embryonic mouse brain starts displaying sex differences in its expression of
at least seven genes “before any gonadal hormone influence” ( Dewing et al., 2003 ).
In chicks, sex-specific expression of genes in the brain begins on the fourth embry-
onic day; that is, 1 day before it occurs in any other embryonic structure, including
gonads ( Scholz et al., 2006 ).
In marine turtles, and in many other reptiles, sex is also determined by the incuba-
tion temperature rather than “sex chromosomes.” Generally, low temperatures induce
the production of males and higher temperatures females, regardless of the genetic sex.
Sexual differentiation in the sea turtle ( Lepidochelis olivacea ) starts in the brain
(diencephalon) rather than gonads. Here, the temperature is assessed and estradiol
expression first rises. Salame-Mendez et al. (1998) observed that nerve fibers from
the spinal cord pervade the gonads of the L. olivacea embryos and suggested that “[t]
he spinal cord and the innervation derivating from it could play a role in driving or
modulating the process of the temperature-dependent gonadal sex determination and/
or differentiation.”
The study was challenged by others ( Pieau and Dorizzi, 2004 ; Davies and
Wilkinson, 2006 ; Kuntz et al., 2004 ), but the most recent evidence seems to sug-
gest central neural control of the gonadal differentiation. It is reported that sensory
nerves innervating the genital ridge before gonadal differentiation are located in the
dorsal horn of the lumbar spinal cord. Shifts in the incubation temperature are asso-
ciated with the increased synthesis of c-Fos-like protein (a reliable marker of neu-
ronal activity) in the sensory neurons innervating genetically female L. olivacea