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Sex gene discovery opens door for new studies of genes, brains, behavior

Students and faculty crowded into a Medical Center lecture hall Nov. 6 for a sneak preview of research that biology professor Bruce Baker and his colleagues would publish five weeks later. Afterward, legendary Stanford geneticist Charles Yanofsky strode down the hall's steep steps to shake Baker's hand.

"I'm jealous," Yanofsky said with a broad smile.

Baker's lab, with scientists from three other universities, had discovered a gene with an unexpected power: It oversees virtually every aspect of the elaborate sex ritual of the male fruit fly Drosophila melanogaster. When the group published the research Dec. 13 in the scientific journal Cell, the headlines read like this one from the New York Times:

"Mating Game of Fruit Fly Is Traced to a Single Gene."

Journalists were fascinated by the possibility that genes might control the sexual habits of animals more complicated than a laboratory fruit fly ­ humans, for example. But scientists like Yanofsky were more excited by the scientific surprise of the work: Few geneticists, including Baker and his colleagues, expected to find a single gene acting in the brain to coordinate a complex pattern of behavior.

"This finding of a single gene that can determine behavior in that way is a major discovery," Yanofsky said. "Most biological processes ­ vision, hearing and so forth ­ are proving to be so complex that it's surprising that a single gene can have such a profound effect."

Scientists also were excited about the discovery because it offers a new way to look at the links between genes and behavior. "[This is] one of the first opportunities where scientists can open the door to a behavior pattern and work out the events from the gene to the behaviors," Yanofsky said.

Could such studies shed light on the sex lives of humans? So far, a search of genome data bases has not uncovered a gene like fru in any other animal. However, Baker said he and his colleagues would not be surprised if such a gene showed up.

"It seems to me likely that there is a gene-brain-behavior circuit in higher organisms," Baker said. However, he said, even a fly depends on environment and experience, as well as on a stage-managing gene, to play out the intricacies of courtship. "When it comes to sex in more complex organisms, those non-biological influences will undoubtedly be stronger and more varied, and the variety of outcomes is undoubtedly much greater," he said.

In other words: It's not likely that human romance will ever be reduced to the biological equivalent of a computer chip.

The gene-brain-behavior circuit

Baker and his colleagues did more than find and clone the gene, which is named "fruitless" and nicknamed fru. They showed how it fits in a family of genes that work together to control all other aspects of sex differentiation in fruit flies, including development of male and female organs. They showed how alterations in the gene disrupt the male fly's courtship ritual, step by step. And they showed that fru does its work in a selection of about 500 cells in the fly's brain, neurons that appear to command other brain cells to carry out the fly's mating minuet.

Baker said that next, under continuing grants from the National Institutes of Health and other funders, the scientists in the consortium plan to continue looking for fru or genes like it in other animals. And they plan to work out the details of how the gene works to control fly behavior.

He said their data so far indicate that the fru gene may function somewhat like a computer program, setting up certain brain cells to work as command-and-control centers that process incoming information, then send signals on to other neurons to command several actions in response.

If this model turns out to be correct, the gene and the commanding brain cells operate as a circuit that allows the male fly to respond to cues from the female and to learn from his experience. For example, an incoming signal that indicates "female fly nearby" might lead to a chain of commands that enable the male to extend one wing and vibrate it at the proper tempo for a fruit fly love song.

Knowing how to perform that song is crucial for future fly generations: The scientists showed that when the fru gene is mutated or altered, the male fly does not reproduce because it cannot perform the song or the other steps in courtship and mating.

That same type of "dedicated circuit" of signals from genes to brain cells to specific actions may be laid down for other behaviors like migration, hibernation and nurturing, Baker said. "All these behaviors are complex, but they all need to be done right," he said. "If they're not, then the organism or its progeny is likely to die."

A four-way consortium

Baker said the key function of the "fruitless" gene would not have been uncovered without the work of four different labs, each approaching the gene from a different perspective. "This was scientific cooperation at its best," he said.

The principal investigators on the National Institutes of Health-sponsored study are Baker, professor of biological sciences at Stanford; Steven Wasserman, associate professor of molecular biology and oncology at the University of Texas Southwestern Medical Center in Dallas; Jeffrey Hall, professor of biology at Brandeis University; and Barbara Taylor, assistant professor of biology at Oregon State University.

Lisa Ryner, a research associate in Baker's lab at Stanford who did major work to clone the gene and discover its products, was the lead author on the scientific paper published in the Dec. 13 issue of the journal Cell. Other authors were postdoctoral research fellow Anuranjan Anand of Stanford; postdoctoral research fellow Stephen F. Goodwin and research associate Adriana Villella of Brandeis; and Diego H. Castrillon, formerly an M.D./Ph.D. student at the University of Texas Southwestern Medical Center, now a resident in pathology at Brigham and Women's Hospital and Harvard University Medical School.

Here is how the four research teams worked together:

Baker's own work has already earned him honors, including membership in the National Academy of Sciences. He is a leader in discoveries about a hierarchy of genes that influence each other to control sex differentiation in the fruit fly. However, research by Taylor at Oregon State showed that there must be an undiscovered branch in that genetic hierarchy.

"At the time, we had no idea that this missing gene would be as interesting as it turned out to be," Baker said. Ryner began a search for it, working on a hunch about how it might be controlled by other genes in the hierarchy. She designed a tool called a molecular probe and found the mystery gene. She also discovered, from its location on the fly's chromosomes, that the gene was likely to be fru.

Her discovery led to a phone call to Brandeis, where Jeffrey Hall is a leader in methods to carefully document how flies act during courtship and mating. Hall had done several studies on the fru gene, which has been known since the early '60s to be involved somehow in the male fly's choice of a sex partner. Flies with certain mutations of the gene court both males and females. Sometimes the male flies form long chains, with each male courting the fly in front and being courted by the fly behind it.

Most mutations of the fru gene studied so far leave the flies unable to complete the act of mating, making them behaviorally sterile. In the tradition of Drosophila genetics, the normal gene was named after its mutation and dubbed "fruitless." But until the four research teams began to study it, there was little evidence that it controlled a broad spectrum of sexual behavior.

Hall told Baker and Ryner that Wasserman's research team at the University of Texas-Southwestern Medical Center was also working on fru. Wasserman is an expert on the genetics of fertility; he was working with other labs to find mutations that make fruit flies sterile when he found new mutations that caused male flies to form long courtship chains. After trading information about earlier-known mutations with Hall, Wasserman and his student Castrillon began using information about those mutations to do a painstaking procedure called a "chromosomal walk" ­ a step-by-step assembly of the ladder of the gene's DNA.

That turned out to be a daunting task. A typical Drosophila gene contains about 2,000 base pairs, or "rungs" of the DNA ladder. The fru gene has at least 70 times that many rungs ­ 140,000 base pairs.

By the time the Baker, Taylor, Hall and Wasserman teams met at a Drosophila genetics meeting in 1993, all four realized that they were studying the same gene. They formed a consortium to test their hunches about fru.

First, Ryner and Castrillon teamed up to clone the entire fru gene. (Scientists in Japan independently cloned some segments of fru, in research reported earlier this year.) Now that fru has been cloned, it should have practical uses as a scientific tool. It may also prove valuable in agriculture, where some other species of fruit flies destroy crops. It could give pest-control experts a reliable way to raise large numbers of sterile insects, as "birth control" to prevent the spread of an infestation of flies. Stanford and UT Southwestern have applied for a patent on the gene.

In collaboration with Goodwin in Hall's lab, Ryner went on to show that fru is governed by genes higher in the sex-determination hierarchy, so that it makes different products in males than it does in females.

She also showed that it probably acts as a transcription factor, meaning that it may govern the actions of other genes needed for specific aspects of sexual behavior. And she isolated copies of many of the transcripts of the gene ­ pieces of RNA that convey the gene's instructions to the cell.

Armed with Ryner's and Castrillon's data about the gene and its transcripts, other members of the consortium went on to find out more about fru. Goodwin and Villella of Hall's lab worked with Anand in Baker's lab to make new mutations in the fru gene. Villella used video methods developed by Hall to record and analyze the courtship and mating rituals of these altered flies.

The key finding of these behavioral studies was that the normal fru gene controls far more than the fly's choice of a sexual partner. The stronger the damage to the gene, the more aspects of courtship and mating were disrupted ­ including the male's ability to recognize a female, to sing his courtship song, or to bend his abdomen to mate. With the most severe mutations, the flies were barely interested in courting at all. However, they appeared normal in other ways, able to fly and walk as usual.

At Oregon State, Taylor's expertise in fly anatomy proved key to showing where the gene does its work. She used RNA transcripts of the gene's sex-specific messages to find out where in the body each of those messages was being made ­ that is, in which cells the gene was expressed, or turned on.

She found that the particular transcripts of the gene that control male sexual behavior were expressed only in cells located in the central nervous system and nowhere else in the fly.

Those 500 cells ­ one-half of one percent of all the cells in the fly's central nervous system ­ will be a powerful tool to find out how the fly's brain is wired up for behavior, Taylor predicted. "Here we have a gene that allows us to extract some behavioral element and then go into the nervous system and see how it is organized," she said.

She also found that fru is expressed in some of the same brain cells in female flies. So far, the geneticists have detected no effect of the gene on female behavior ­ but that may be only because female sex behavior in Drosophila is subtle and little understood by fly researchers.

For related stories on the Web, see Scientists Identify Gene and Behavioral Gene Background.

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By Janet Basu