 "Photo: Liz Rohde")
A new study on sexual orientation in worms suggests human sexual orientation might be controlled by mechanisms in the brain.
By manipulating circuits in a worm's brain that control behavior, U biologists were able to alter the sexual preferences of male and female worms to mirror the opposite gender. The females became attracted to their own sex. However, male worms that took on female characteristics were not attracted to other males, because the female worms in the species are not drawn to males either.
If a worm's sexual preference can be altered by adjusting circuits in the brain, is human sexual preference controlled solely by the brain as well?
U biology professor Erik Jorgensen led the study investigating worm sexual orientation. "When we started this, we thought there were specific cells in males only that were responsible for male behavior -- it's simply not true," he said.
The study found that cells existing in both male and female worms can be rewired to activate certain modes of behavior.
"You can take a female and activate the male genetic program -- if you do it in the entire female, it will even begin to look like a male," said Jamie White, a postdoctoral researcher in Jorgensen's lab.
The C. elegans nematodes used in Jorgensen's lab are transparent millimeter-long soil worms. Although male worms require females to reproduce, the "female" worms are actually hermaphrodites that produce both sperm and eggs. Jorgensen and White refer to the hermaphrodites as "females" because they reproduce independently.
Mating with males produces more offspring than when the hermaphrodites reproduce by themselves. The researchers took the hermaphrodites and altered neurons in their brains to give them male characteristics. The females became attracted to female pheromones, a scent female worms give off to attract male worms. C. elegans worms are blind and use their sense of smell as a way to find their mate.
When the study began, Jorgensen and White believed that there were specific male parts of the brain that female worms do not have and female brain circuits that males don't have. However, the study proved that both male and female worms have the same neurons controlling behavior. "These behaviors, like sexual preference, don't require female or male external parts -- a male worm will still act like a male even if you take out solely male parts of the brain," White said.
Experiments altering the brain circuits in animals have been successfully conducted in mice and fruit flies in the past. Whether this means that human sexual preference is also tied into the brain is questionable.
"A lot of people would like to say it pertains to human sexual behavior, but a human brain is much more complex," Jorgensen said.
Instead of the four neurons like in the nematode worm brain, a human brain is composed of hundreds of thousands of neurons.
U biology professor Franz Goller believes that humans would be very difficult to study in the same way as worms.
"I think it's possible that humans have sex-specific biases in the brain, but there is no way to be sure yet," he said.
Although scientists can alter the sexual behavior of male and female worms, they have no idea why the worms have the same circuits in the brain, but two different behavior modes.
"We know how to change the behavior, but not what changes it," White said.
The study disproves the idea that a male worm's behavior is caused by certain neurons that only males have in the brain.
"This proves that they don't have to have a male body to have male behavior," White said.
The study also reports another aspect of the research: brain compensation.
If a nematode worm's sexual orientation was destroyed by researchers later in life, the worm would never desire to procreate or be attracted to female worms. But if a worm is born with the disability or mutation, the worm will retain sexual attraction behavior through another part of the brain.
"It's like when a stroke victim loses their language ability because of damage to the brain. Remaining parts of the brain can take over the function," White said.
In experiments with mice, if the mechanism controlling hunger is removed, a mouse will starve. If scientists were to destroy that hunger mechanism at a very early stage, another part of the brain would retain that function, White said.
"It's really surprising that worms can adjust to a disability," White said. "Mice have around 300 neurons and humans have billions, but these nematode worms have much fewer (neurons), yet their simple nervous systems can still readjust," he said.
Goller says he finds the study exciting because the worms' neurons can be pinpointed exactly.
"It allows scientists to dissect the neuron-controlled behavior -- each of the neurons are known," he said.
The study will be published in the journal Current Biology and was released online yesterday.
