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Sexual Orientation Is Genetic in Worms, Study Says

Anne Casselman
for National Geographic News
October 25, 2007
 
The sexual preferences of nematode worms can be changed by flipping a genetic master switch in their brains, a new study says.

A worm's sex is determined by chromosomes found in its DNA. If a worm is male, for instance, all the cells in the body—including those in the brain—will be male.

The scientists weren't able to change the gender of worms. But they were able to fool their nerve cells into acting like those of the opposite gender by manipulating the worms' genes.

"We'd like to ... understand how a group of [brain] cells actually produces ... a behavior," said study co-author Erik Jorgensen, a molecular biologist at the University of Utah.

Nematodes are tiny round worms, about a millimeter in length, that live in the soil and eat bacteria.

The species has two sexes, males and hermaphrodites. The hermaphrodites are basically females that can fertilize their own eggs.

The worms have behaviors that govern their sexual attraction and reproduction. For example, male worms are attracted to hermaphrodites and hermaphrodites avoid one another.

Sexual Brains

In the experiment Jorgensen and colleagues genetically manipulated a hermaphrodite worm's nervous system.

They took a gene that is responsible for sexual characteristics of male cells and turned it on in the hermaphrodites' brains.

This tricked their brains into acting like male brains. As a result, the hermaphrodite worms adopted male sexual behavior and became attracted to other hermaphrodites.

"They look like girls but they act like boys," study lead author Jamie White, a postdoctoral fellow at the University of Utah, said in a statement.

Many scientists believe that such sexual behaviors stem from parts of the brain that are larger in each respective gender.

Instead this experiment suggests that these behaviors can also stem from brain regions in which there are no noticeable size differences.

"This tells us is that the brain is sexualized," study co-author Jorgensen said.

The study also found that the same nerve cells, or neurons, that produce male sexual behavior are present in hermaphrodites, only they serve a different purpose.

This means that males and females have identical brain cells that nevertheless produce different behaviors.

The study will be published online today in the journal Current Biology.

From Worms to People

"You might think this is a study about sexuality, but it's really our foothold on understanding the brain," Jorgensen said.

Worms and humans also share genes that are involved in brain function, according to Jorgensen.

"So what we learn about how the brain works in a worm will apply to our understanding of humans." (Related news: "Lesbians Respond Differently to 'Human Pheromones,' Study Says" [May 8, 2006].)

But there are huge gaps between species: Male nematodes have 383 neurons, while humans have a hundred billion neurons.

Those billions of human neurons enable more complex traits such as consciousness and create vast differences between individuals.

Odor Switches

In a similar experiment—also published in an upcoming issue of Current Biology—Douglas Portman and graduate student KyungHwa Lee, both molecular geneticists at the University of Rochester in New York, changed the sexual identity of nerve cells in nematodes' brains and sensory systems.

The researchers switched a female worm's odor preference to that of a male.

Male and hermaphrodite nematodes are drawn to different smells. For example, when given a choice between the two, hermaphrodites prefer a buttery odor, while males lean toward a vegetable smell. (Related news: "Sexual Prime Peaks When Males 'Smell' Mates, Spider Study Shows" [June 16, 2006].)

Portman believes this might reflect nematodes' differing metabolic needs.

These results also support White and Jorgensen's research findings: that the hardware to create sexual behaviors is not unique to either sex.

"The sexual identity of neurons that are shared between both sexes is critical for generating sex-specific behaviors, and that is a big surprise," Portman said.

"The more splashy part of this paper gets at the question of how the sex of the nervous system determines behavior," he said.

"These studies show that the sexual identity of the cell does somehow tweak its function. But how that happens is not known."

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