[Vision2020] Could Scientists Have Found A Gay Switch?

[Art Deco]

 Could Scientists Have Found A Gay
Switch?<http://www.popsci.com/science/article/2012-12/being-born-gay-isnt-your-genes-its-them>
[image: The Sex Files]<http://www.popsci.com/category/category-badges/sex-files>
  Epigenetic molecules that regulate genes may influence homosexuality.
 By Jennifer Abbasi<http://www.popsci.com/category/popsci-authors/jennifer-abbasi>
Posted
12.13.2012 at 9:00 am  *Popular Science*

 [image: Pop Art Couple]
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Gayness may not be in our genes, but in the molecules that regulate them.
New research suggests that epigenetic factors -- chemical "switches"
attached to genes that turn them on or off -- are a more plausible
heritable mechanism behind homosexuality than DNA itself.

Non-genetic changes to gene expression are called epi-marks, for
epigenetics, the field of research dealing with the molecular on/off
switches. Epi-marks are normally erased between generations, but there's
recent evidence that they're sometimes passed from parent to child.

Researchers at the National Institute for Mathematical and Biological
Synthesis (NIMBioS) looked at how epi-marks that influence testosterone
sensitivity in the womb might contribute to homosexuality. Late in
pregnancy, natural variations in testosterone levels can alter a fetus'
sexual development. Sex-specific epi-marks protect female fetuses from
masculinization in the presence of too much testosterone; boys are
protected from feminization if too little testosterone is present.

According to computer
modeling<http://www.jstor.org/page/journal/quarrevibiol/forthcoming.html>by
the group, testosterone-buffering epi-marks passed from a parent to an
opposite-sex offspring may result in the reverse effect: Girls who inherit
sex-specific instructions from their fathers will be partially
masculinized, while boys who get epi-marks from their mothers will be
partially feminized. In this model, homosexuality occurs when
stronger-than-average epi-marks influencing sexual preference from an
opposite-sex parent escape erasure and are then paired with
weaker-than-average sex-specific epi-marks produced in opposite-sex
offspring.

The model suggests that sex-specific epi-marks have survived because
they're highly beneficial for parents' fitness and are only infrequently
passed on to offspring, where they may reduce reproductive fitness.

"The study provides a very interesting, but as yet untested, genetic
mechanism for the evolutionary maintenance of human homosexuality," says
Nathan Bailey, an evolutionary biologist at the University of St Andrews in
Scotland who was not involved in the research. "We are going to have to
wait until more evidence is in, but I do think it would be exciting to know
whether epi-marks contribute to the expression of sexual orientation in
humans."

Study co-author Sergey Gavrilets, a professor of ecology and evolutionary
biology and mathematics at the University of Tennessee, Knoxville, and
associate director for scientific activities at NIMBioS, says there could
still be a "gay gene" or genes, but that there are problems with the idea:
"Nobody has been able to present solid experimental evidence for this in
spite of significant effort."

The search for genes that control sexual orientation is based on increasing
evidence of a strong genetic component. Studies clearly show that
homosexuality runs in families, with an increased rate among siblings and
the maternal uncles of gay men, according to a 2011
review<http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3030621/>.


A study<http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0051088>published
online just last week by Italian researchers Andrea Camperio
Ciani and Elena Pellizzari found that the maternal aunts and grandmothers
of gay men have more children than those of straight men. A few years ago,
Ciani used genetic modeling to explain the 2004 finding that sisters and
maternal aunts of homosexual men have more children than the females in the
maternal line of straight men. According to that
model<http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0002282>,
at least one unknown gene on the X chromosome predisposes female carriers
to higher fertility and male carriers to homosexuality. "The genes evolved
for the fecundity benefit in females, at the reproductive cost of an
increase in homosexuality in males," Ciani explains.

Gavrilets says the theory is compatible with his. Both "provide an example
of sexually antagonistic selection, when some traits are maintained in the
population -- in spite of being deleterious in one sex -- because they are
advantageous when expressed in the other sex," he says.

Meanwhile, a study<http://rsbl.royalsocietypublishing.org/content/9/1/20121038>published
online yesterday provides what may be the first evidence of a
direct reproductive benefit for same-sex sexual behaviors in males.
Researchers at the University of Frankfurt found that female Atlantic
mollies, a type of fish, may switch their usual preference for colorful
males to drab males after observing the less-desirable fish engaging in
sexual behaviors with their more vivid counterparts.

The researchers theorize that bisexuality directly benefits male Atlantic
mollies thanks to the phenomenon of female mate choice copying, in which
females prefer males that they've seen engaged in sex. ("Displaying mating
behavior conveys information not only about a male's readiness to mate, but
also mate quality, as performance is associated with costs," the authors
write.) Usually female mate choice copying occurs after observing
heterosexual couplings, but this study demonstrated the same result with
same-sex partners.

Female mate choice copying has been observed in animals ranging from fruit
flies to birds to humans. Evolutionary biologist David Bierbach, the
study's lead author, says it can't be ruled out as a potential driver of
male bisexuality in our species.

Evolutionary adaptations, of course, aren't the whole picture. Heritable
factors, whether genetic or epigenetic, only explain perhaps 20 to 50
percent of variation in sexual preferences, Gavrilets says. "Recent studies
suggest that sexual orientation (as nearly all behaviors) is based on the
genetic constitution of an individual, as well as on environmental factors
the individual experienced in certain life stages," Bierbach says. "Nature
and nurture."

Bailey adds, "The idea of a 'gay gene' is overly-simplistic. Biologists
know that complex traits such as sexual orientation develop from
interacting genetic and environmental effects, and [the NIMBioS] study
doesn't change that view."

*Jennifer Abbasi is a science and health writer and editor living in
Portland, OR. Follow Jen on Twitter
(@jenabbasi<http://www.twitter.com/jenabbasi>)
and email her at popsi.thesexfiles at gmail.com.*


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