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.

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