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The study reinforces the notion that sexual orientation is hard-wired in the brain, said Erik Jorgensen, scientific director of the Brain Institute at the University of Utah.
"They look like girls, but act and think like boys," Utah researcher Jamie White, who worked on the study published in the journal Current Biology, said in a statement.
Researchers in Jorgensen's lab switched on a gene in female worms that makes the body develop male structures, but they only activated the gene in the brain.
As a result, the female worms still had female bodies, but they behaved like males.
‘Sexual behaviour encoded’
"It suggests sexual behaviour is encoded in our genes," and not caused by extra nerve cells specific to males or females, Jorgensen said in a telephone interview.
Animals such as nematodes, fruit flies and mice share many of the same genes as humans and are often used as models to understand human genetics.
But Jorgensen said the study is not likely to resolve the burning question about the genesis of sexual orientation in humans.
"A human's brain is much more complex than a worm's brain," he said.
Many scientists think a host of factors such as genetics, hormones and environment may play a role in determining sexual orientation in humans, but this has not been proven.
About the study
Jorgensen said the study is interesting because it suggests rather than being caused by extra, sex-specific nerve cells, attraction behaviours are part of the same brain circuit.
The finding was part of a study looking at areas in the worms' brains involved in sexual attraction.
Nematodes, or C. elegans, are tiny worms about one millimetre long that live in the dirt, chomping bacteria. They have no eyes and rely on smell for navigation and propagation.
There are few males, only one in 500, so most of these female nematodes are hermaphrodites, meaning they have both male and female sexual organs.
This gives the female worms the ability to fertilize their own eggs and produce offspring in the absence of a male.
Female worms are hermaphrodites
"For the most part they are females," Jorgensen said. "It's really hard to tell that they are hermaphrodites, but they do make these few sperm."
When they do mate with males, female worms produce 1 200 progeny, compared with just 200 when they produce their own sperm.
The researchers were trying to understand the underpinnings of sexual attraction in the male nematodes.
They reasoned it could arise from four extra smell-related nerve cells found only in male worm brains, from four core nerves found in both males and females or from a mix of both.
When they systematically neutralised the male-only neurons, mature male worms still responded to the females.
What the study revealed
The findings imply nerve cells common to both male and female worms are central to sexual attraction and sexual orientation.
"They have genes for both male behaviour and female behaviour in them," Jorgensen said. "It suggests the brain determines behaviour."
The study expands on prior studies suggesting a genetic component to sexual orientation.
This is one more observation. We've seen this in flies and in mice," he said. "The difference is we know what cells are involved."
The study was funded by the National Science Foundation. – (Julie Steenhuysen, ReutersHealth)
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