Sex in the Brain

C. elegans is pointed out on monitor

As Jorgensen, Jamie White and their colleagues wrote in the journal, Current Biology, the study showed that in worms, sexuality is hard-wired in the brain and is not affected by the body or hormones.

"So that means that we can now study the brain and the brain alone and understand how this behavior arises," says Jorgensen. He says that's not true in people, where hormones as well as thinking and consciousness play a role.

Worm Tracks

In a series of elegant experiments, the researchers first had to identify which nerve cells were responsible for the behavior. "We took a jellyfish protein and expressed it in these cells, so now we could see that cell in the brain, and using a laser we can then perform microsurgery and kill that cell and basically eliminate that part of the circuit and then test those animals," he explains. "And we can ask whether those males are still attracted to females. If they weren't, we knew we had identified the correct cells."

The test for attraction, or lack thereof? Simple -- wash some female worms. The females' odors, or pheromones, end up in the wash solution, which they put onto a spot on a petri dish coated with agar. The worms' tracks in the agar record whether they head towards the odor spot or away from it.

Surprisingly, even though male nematodes have some nerve cells that differ from females, White could kill those cells and still get males that were attracted to females. "That tells us that the cells in the brain that are shared by males and females [are] responsible for sexual behaviors," Jorgensen says.

Closer image of C. elegans worms

"Whether differences in this gene activity could control diversity in worms is not something that we see in our experiments," Jorgensen says. "We're not taking a population of worms you would see in nature. We're taking animals that have been bred to be the same, and this behavior is robust and reliable. So it's possible that there is diversity in nature. We wouldn't see it in the lab."

Genes, Brains and Behaviors

Jorgensen says their findings are consistent with findings in flies and other animals that there are groups of brain cells that are dedicated to sexual behaviors.

But he says that finding out that sexual attraction in worms is hard-wired into the brain doesn't tell anything about human sexual orientation -- or any other human behaviors.

"I think that these, what we consider complicated behaviors, sexual attraction, are wired into that brain, so that worms really are hard-wired," he says. "Are humans hard-wired? Are we simply the output of our genes? That's not true-- our brains are much larger, there are phenomena that arise from having such a complicated brain, such as will and consciousness, and we don't understand those things. And it's going to be hundreds of years, I think, before we finally understand these emergent properties of the human brain."

But to ever understand human brains and behaviors, the researchers say the place to start is the wiring of a brain circuit in a tiny worm.

Even though they are primitive compared to humans, nematode worms have most of the same genes as we do. But unlike with humans, scientists possess the complete wiring diagram of these worms' brains.

"Just open up the book and it has this cell is connected to this cell, and so we know exactly how all of the components are connected, and we can begin to figure out how that wiring diagram works, how does a circuit of the brain work," Jorgensen says.

This research was published in Current Biology online ahead of print, October 25, 2007 and funded by the National Science Foundation.