While the scientists can't say what this means for sexual orientation in humans, it does raise the possibility that sexual preference is wired in an organism's brain.
Biologist Erik Jorgensen and his research team at the University of Utah took female nematodes with normal reproductive organs and activated genes that determine "maleness," thereby converting female brains to male brains.
"So we did that and now the females are attracted to other females," Jorgensen said from Salt Lake City. "That told us that the brain was sufficient for all of the sex-specific behaviours, for sexual attraction."
In other words, co-author Jamie White, a postdoctoral researcher in Jorgensen's lab, said in a statement: "They look like girls, but act and think like boys. The (same-sex behaviour) is part of the nervous system."
Nematodes, or C. elegans, are millimetre-long worms that live in soil and eat bacteria. Many of the same genes found in nematodes are also found in other animals - including people - making them a good model for human research.
Because the worms have no eyes, sexual attraction is based only on their sense of smell. Male nematodes are drawn to sex-attractant odours, called pheromones, produced by the females.
The researchers, whose work is published in the most recent issue of Current Biology, first tested pheromones on male nematodes.
"We put a spot of that beautiful odour on a plate and we put the males down on the plate," said Jorgensen. "They went straight . . . for the source of that pheromone."
"And when we make these transgendered females, they do the same thing. As soon as she detects that there are other females on the plate from the odours, she zips right up there," he said.
"Normally a female would avoid that spot. They actually have an active repulsion."
But the scientists found there is more to the mating ritual than just sexual attraction.
"The females that are now attracted to other females, they get there and they don't know what to do," said Jorgensen, pointing out that there are other brain cells involved in the actual mechanics of sex.
Under certain adverse conditions - what Jorgensen calls "life on the prairie . . . when there are no boys around" - female nematodes can become hermaphrodites, producing both sperm and eggs to ensure propagation. Those used in this experiment were "absolutely female," he said.
While not published in the current paper, preliminary results from subsequent experiments show sexual preference of male nematodes can also be altered using genetic manipulation. Rewired male wrigglers avoided female pheromones.
Still, Jorgensen said he's reluctant to make strong inferences about human behaviour from studies of worms.
"Humans are animals and humans are subjected to the same evolutionary pressures as any organism, so there will be a strong genetic component to sexuality in humans as well as worms, flies, mice," he said.
"But in addition, our brains are very complex. And because our brains are so large, an emergent property has arisen, which is consciousness and will. We can make choices. So to say that personal choice or free will doesn't play a role in human sexuality is a simplification."
Furthermore, the issue of human sexual orientation is politically loaded, he said, with the gay community for the most part favouring the idea that genes underlie sexual preference, while religious conservatives deem homosexuality a choice that goes against God's will.
In his personal view, deciding the truth of the matter "is really an inappropriate pressure on science."
What the researchers are more interested in is what their results say about how the brain functions overall.
"We understand electricity: you put these components together and you can get a light to turn on," Jorgensen said. "But how does the brain work? This is biological tissue. How do we sense the things we sense? How do we remember the things we remember? And how do we think the things we think?"
The worm attraction studies lay some of the groundwork for figuring out how the brain's wiring translates into such behaviours, he said.
"What this gives us for future studies is a circuit and behaviour that we can pick apart and say: 'Look, these are the connections.' That is our goal really, to understand how the brain works."
