June 3, 2005
For Fruit Flies, Gene Shift Tilts Sex Orientation
By ELISABETH ROSENTHAL,
International Herald Tribune
When the genetically altered fruit fly was released into the observation chamber, it did what these breeders par excellence tend to do. It pursued a waiting virgin female. It gently tapped the girl with its leg, played her a song (using wings as instruments) and, only then, dared to lick her - all part of standard fruit fly seduction.
The observing scientist looked with disbelief at the show, for the suitor in this case was not a male, but a female that researchers had artificially endowed with a single male-type gene.
That one gene, the researchers are announcing today in the journal Cell, is apparently by itself enough to create patterns of sexual behavior - a kind of master sexual gene that normally exists in two distinct male and female variants.
In a series of experiments, the researchers found that females given the male variant of the gene acted exactly like males in courtship, madly pursuing other females. Males that were artificially given the female version of the gene became more passive and turned their sexual attention to other males.
"We have shown that a single gene in the fruit fly is sufficient to determine all aspects of the flies' sexual orientation and behavior," said the paper's lead author, Dr. Barry Dickson, senior scientist at the Institute of Molecular Biotechnology at the Austrian Academy of Sciences in Vienna. "It's very surprising.
"What it tells us is that instinctive behaviors can be specified by genetic programs, just like the morphologic development of an organ or a nose."
The results are certain to prove influential in debates about whether genes or environment determine who we are, how we act and, especially, our sexual orientation, although it is not clear now if there is a similar master sexual gene for humans.
Still, experts said they were both awed and shocked by the findings. "The results are so clean and compelling, the whole field of the genetic roots of behavior is moved forward tremendously by this work," said Dr. Michael Weiss, chairman of the department of biochemistry at Case Western Reserve University. "Hopefully this will take the discussion about sexual preferences out of the realm of morality and put it in the realm of science."
He added: "I never chose to be heterosexual; it just happened. But humans are complicated. With the flies we can see in a simple and elegant way how a gene can influence and determine behavior."
The finding supports scientific evidence accumulating over the past decade that sexual orientation may be innately programmed into the brains of men and women. Equally intriguing, the researchers say, is the possibility that a number of behaviors - hitting back when feeling threatened, fleeing when scared or laughing when amused - may also be programmed into human brains, a product of genetic heritage.
"This is a first - a superb demonstration that a single gene can serve as a switch for complex behaviors," said Dr. Gero Miesenboeck, a professor of cell biology at Yale.
Dr. Dickson, the lead author, said he ran into the laboratory when an assistant called him on a Sunday night with the results. "This really makes you think about how much of our behavior, perhaps especially sexual behaviors, has a strong genetic component," he said.
All the researchers cautioned that any of these wired behaviors set by master genes will probably be modified by experience. Though male fruit flies are programmed to pursue females, Dr. Dickson said, those that are frequently rejected over time become less aggressive in their mating behavior.
When a normal male fruit fly is introduced to a virgin female, they almost immediately begin foreplay and then copulate for 20 minutes. In fact, Dr. Dickson and his co-author, Dr. Ebru Demir of the Institute of Molecular Biotechnology, specifically chose to look for the genetic basis of fly sexual behavior precisely because it seemed so strong and instinctive and, therefore, predictable.
Scientists have known for several years that the master sexual gene, known as fru, was central to mating, coordinating a network of neurons that were involved in the male fly's courtship ritual. Last year, Dr. Bruce Baker of Stanford University discovered that the mating circuit controlled by the gene involved 60 nerve cells and that if any of these were damaged or destroyed by the scientists, the animal could not mate properly. Both male and female flies have the same genetic material as well as the neural circuitry required for the mating ritual, but different parts of the genes are turned on in the two sexes. But no one dreamed that simply activating the normally dormant male portion of the gene in a female fly could cause a genetic female to display the whole elaborate panoply of male fruit fly foreplay.