Sexual orientation lies in the brain

November 14th, 2007 - 2:51 am ICT by admin  
Nematode worms lack eyes, so attraction is based only on the sense of smell. There are no true females and only one in 500 nematodes is male. Most are hermaphrodites, with both male and female organs. Jorgensen and White loosely refer to hermaphrodites as females because they produce offspring.

“They look like girls, but act and think like boys. The [same-sex attraction] behaviour is part of the nervous system,” said Jamie White, a postdoctoral researcher and first author of the new study.

“The conclusion is that sexual attraction is wired into brain circuits common to both sexes of worms, and is not caused solely by extra nerve cells added to the male or female brain,” says laboratory leader and biology Professor Erik Jorgensen, scientific director of the Brain Institute at the University of Utah and an investigator with the Howard Hughes Medical Institute.

“The reason males and females behave differently is that the same nerve cells have been rewired to alter sexual preferenc. Our conclusions are narrow in that they are about worms and how attraction behaviors are derived from the same brain circuit. But an evolutionary biologist will consider this to be a potentially common mechanism for sexual attraction.

“We cannot say what this means for human sexual orientation, but it raises the possibility that sexual preference is wired in the brain. Humans are subject to evolutionary forces just like worms. It seems possible that if sexual orientation is genetically wired in worms, it would be in people too. Humans have free will, so the picture is more complicated in people.

“A hermaphrodite makes both eggs and sperm, She doesn’t need to mate [with a male] to have progeny,” but can fertilize her own eggs. “Most of the time, the hermaphrodites do not mate. But if they mate, instead of having 200 progeny, they can have 1,200 progeny,” Jorgensen said.

Male attraction involves the combination of both accessory and core nerve cells. The involvement of the core neurons was a surprise.

“‘We thought the extra CEM neurons provided sexual preference’ because fourth-stage males are not attracted to hermaphrodites but adult males are,” Jorgensen said.

“We found instead that the brain - which is the same in young males and hermaphrodites - is rewired during the worm equivalent of puberty - the fourth larval stage - to make the males attracted to hermaphrodites.”

“What we show is that the shared nervous system [common between male and hermaphrodite] is broadly sexualized,” and sexual attraction can be changed by essentially flipping a genetic switch in that common brain, he adds.

In the study, the researchers used laser microsurgery to kill the male-only CEM neurons in young larval males. The resulting adult males still were attracted to hermaphrodites.

That suggests the core neurons - brain cells common to both the male and hermaphrodite brain - are sexualized, and that the extra CEM nerve cells found only in males are not necessary for sexual attraction even though they normally play a role in it.

“The nervous system can compensate for lost neurons as it goes through puberty,” Jorgensen said.

“Normally there are eight sensory neurons in nematodes,” says White. “You can take away seven of the eight prior to sexual maturation, and as long as there is one left, he can still be sexually attracted. … Why would an organism that has only 383 nerve cells use eight of them for sexual attraction” It must be that the behaviour is very important. There is redundancy. The system is flexible.”

“We took the hermaphrodite brain and we activated the genes that determine maleness, but only in the brain and not in the rest of the worm. Hermaphrodites with masculine brains “were attracted to other hermaphrodites,” Jorgensen said.

The results show sexual orientation is wired into the brain in both sexes of worm.

To masculinize the brains of hermaphroditic worms, the researchers switched on a gene named fem-3, but only in the nervous system. With the gene active only in the brain, the hermaphrodites still had the same bodies and genitalia, but their brains were male, so they were attracted to other hermaphrodites.

When the scientists genetically altered hermaphrodites’ brains to change their sexual orientation, they crawled toward the pheromones of other hermaphrodites.

“People debate whether the brain is influenced by sexual hormones from the gonads or whether the behaviour is derived from the brain alone, In this case, it’s clear the brain is sexualized. … The surprise was that sensory neurons found in the hermaphrodite brain are involved in sexual attraction in males,” Jorgensen said.

The study is available in the current online edition of Current Biology, and will appear in the journal’s Nov. 6 print edition. (ANI)

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