Epigenetics of sex determination and its evolutionary implications

It is very difficult or almost impossible to find direct
proofs of evolutionary events that occurred million
years ago or to trace processes of speciation during
the short time of human lifetime. However, it is a
possibility that the same processes that led to those
evolutionary events may be in action in our time
(Darwin used the Lyellian uniformitarian principle in
support of his evolutionary history too).

In relation to evolution of sex the knowledge of
processes of sex determination during the
individual develoment of extant dioecious
organisms may be elucidating to the evolution
of sexuality in the meaning that it may show us
the possibility and likelihood of evolution of sex
from unisexual, parthenogenetic organisms, to
bisexual organisms. Ultimately any evolutionary
process taker place and is tested in the process
of individual development.

It was believed that sexual differentiation in humans
depended on activation of the sry gene located in
the Y chromosome, which came to be known as
"the primary sex determining gene". It was believed
that the product of this gene induced expression of
the gene for a transcription factor, Sf1 (steroid factor1),
which induced a signal network leading to the gonadal
determination of male development.

However, recent evidence has shown that sexual
differentiation of embryos does not start in the genital
Anlage but in the brain: there are about 50 genes
that are differentially expressed in the brain of prospect
male and female embryos before the expression of
the sry gene and related genes in the genital Anlage.
The idea that the hormonal influences from gonads
determine the initial characteristic differences in embryonic
brains of females and males proved to be incorrect.
The truth seems to be the other way around. It is the
differential expression of genes in the brain that
determines the sexual development of the male
and female sexual organs. This does not mean
that there is not an essential genetic component in
sex determination in sexually reproducing organisms.
This only arises the question of the relationship between
the genetic and epigenetic factors in sex determination.
Experimentaly it is demonstrated that in Japanese quails
transplantation of the female forebrain primordium into
genetically male embryos, before gonadal differentiation,
suffices to prevent differentiation of male sex organs in
genetically male embryos (Gahr, 2003).

In many reptiles neural signals in response to
different environment temperatures override the
genetic determination of sexuality of embryos.
The brain responds to the input on the body
temperature with a chemical output that induces
production of aromatase which converts brain
androgens into estrogen, thus determining female
development of genetically male embryos.

In the sea turtle Lepidochelys olivacea it is
the brain (diencephalon) that senses
the temperature and starts the sexual
differentiation of embryos. The undifferentiated
genital primordium is pervaded by nerve fibers
from the spinal cord during the thermosensitive
period of sex determination (stages 20-27)
and it is believed that

"The spinal cord and the innervation derivating
from it could play a role in driving or modulating
the process of temperature-dependent gonadal
sex determination and/or differentiation "
(Gutierrez-Ospina et al., 1999).

These examples show the high lability of the
process of sexual differentiation in vertebrates,
which is important in view of the fact that the
developmental lability is related to (and is a source
of) evolutionary lability of animal morphology
and physiology.

In regard to the understanding of mechanisms
of evolutionary transition from unisexuality to
sexuality elucidating is the example of a
unisexual, female-only crustacean, Daphnia magna,
which in response to unfavorable environmental
conditions, produces sexual offspring via a well-known
epigenetic mechanism, a brain signal cascade.

Numerous cases of sex conversion in fish,
in response to social and other stimuli, also
prove the lability of the sexual development of
vertebrate embryos. These sex-convertible
species develop morphological (including
gonads), morphometrical, and behavioral characters
of the opposite sex. It is adequately demonstrated
that these sex conversions are determined by
specific changes in neurons, especially in the
hypothalamic GnRH neurons:

"Such changes in the POA GnRH cell poipulation
phenotype may reflect a proximate central
mechanism in the induction of the dramatic
gonadal and behavioral transformations that are
associated with sex change in hermafroditic fish."
(Elofsson et al., 1997).

Generalizing the extensive evidence on sex
conversion in fish it is concluded that

"The initiation of the sex reversal is often
controlled by social, behavioural factors,
and since the only way behaviour can affect
the gonads is through the brain, there must
be a central neuronal mechanism underlying
the gonadal change" (Elofsson et al. 1997)

This neural mechanism is based on the processing
and integration of the social stimuli in the brain, a
non-genetic computational process.

N.C.

.

Relevant Pages

  • Re: Scientists learn why some men may be more interested in football than sex
    ... You seem to post almost exclusively about sex (usually sexual ... sexuality itself in terms of romantic and sexual feelings. ... brain were activated. ... that of goals being scored," the researchers conclude. ...
    (soc.men)
  • Teen sex can cause addictive behaviours, says new book
    ... Teen sex can cause addictive behaviours, ... "When we are involved in sexual relations, the brain is actually ... But teens are looking for that sort of information. ... Repress their sexuality until ...
    (uk.legal)
  • Re: Are men discriminated against as child-minders?
    ... the house my aunt and uncle had rented out to a "nice ... there were all kinds of outrageous group sex ... Plus, boys mature slower than girls of that age, ... that you don't view male sexuality as dirty, ...
    (soc.men)
  • Re: American Buddhism - Sexuality - Quotations by Zen Master Rama, Dr. Frederick Lenz
    ... I think that the writer try to make a distinction in the article to "sex" as ... >>How could sexuality be anything but God? ... >>opens up his heart, to be very selective about the people they have sex ... >>You and I might define a 'soul mate' differently. ...
    (soc.culture.thai)
  • Sex Myths
    ... What is it about sex ... the vagina dentata? ... But it's not just women who are the targets of absurd sexual myths. ... Illinois University and author of “The Physician and Sexuality in ...
    (rec.sport.pro-wrestling)