[Vision2020] A Gene for Romance? So It Seems (Ask the Vole)
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Mon Jul 18 21:49:19 PDT 2005
A Gene for Romance? So It Seems (Ask the Vole)
By NICHOLAS WADE
Published: July 19, 2005
Biologists have been making considerable progress in identifying members of
a special class of genes - those that shape an animal's behavior toward
others of its species. These social behavior genes promise to yield deep
insights into how brains are constructed for certain complex tasks.
Some 30 such genes have come to light so far, mostly in laboratory animals
like roundworms, flies, mice and voles. Researchers often expect results
from these creatures to apply fairly directly to people when the genes
cause diseases like cancer. They are much more hesitant to extrapolate in
the case of behavioral genes. Still, understanding the genetic basis of
social behavior in animals is expected to cast some light on human behavior.
Last month researchers reported on the role of such genes in the sexual
behavior of both voles and fruit flies. One gene was long known to promote
faithful pair bonding and good parental behavior in the male prairie vole.
Researchers discovered how the gene is naturally modulated in a population
of voles so as to produce a spectrum of behaviors from monogamy to polygamy,
each of which may be advantageous in different ecological circumstances.
The second gene, much studied by fruit fly biologists, is one known to be
involved in the male's quite elaborate suite of courtship behaviors. New
research has established that a special feature of the gene, one that
operates differently in males and females, is all that is needed to induce
the male's quite complex behavior. Social behavior genes present a
particular puzzle since they involve neural circuits in the brain, often set
off by some environmental cue to which the animal responds. Catherine Dulac
of Harvard has found that the male mouse depends on pheromones, or air-borne
hormones, to decide how to behave toward other mice. It detects the
pheromones with the vomeronasal organ, an extra scent-detecting tissue in
The male mouse's rule for dealing with strangers is simple - if it's male,
attack it; if female, mate with it. But male mice that are genetically
engineered to block the scent-detecting vomeronasal cells try to mate rather
than attack invading males.
The mice have other means - sound and sight - of recognizing male and
female. But curiously, nature has placed the sex discrimination required for
mating behavior under a separate neural circuit aroused through the
"It was very surprising for us," Dr. Dulac said.
The gene that was eliminated from the mice is a low-level member of a
presumably complex network that governs the inputs and outputs necessary for
mating behavior. The most striking behavioral gene discovered so far is a
very high level gene in the Drosophila fruit fly.
The gene is called fruitless because when it is disrupted in males they lose
interest in females and instead form mating chains with other males. The
male's usual courtship behavior is pretty fancy for a little fly. He
approaches the female, taps her with his forelegs, sings a song by vibrating
his wing, licks her and curls his abdomen for mating. If she is impressed
she slows down and accepts his proposal. If not, she buzzes her wings at
him, a gesture that needs no translation.
All these behaviors, researchers discovered several years ago, are
controlled by the fruitless gene - fru for short - which is switched on in a
specific set of neurons in the fly's brain. The gene is arranged in a series
of blocks. Different combinations of blocks are chosen to make different
protein products. The selection of blocks is controlled by a promoter, a
region of DNA that lies near but outside the fru gene itself.
So far four of these fru gene promoters have been found. Three work the same
way in both male and female flies. But a fourth selects different blocks to
be transcribed, making different proteins in males and in females. This
difference, it seemed, was somehow the key to the whole suite of male
Last month Barry J. Dickson of the Austrian Academy of Sciences provided an
elegant proof of this idea by genetically engineering male flies to make the
female version of the fruitless protein, and female flies to generate the
male version. The male flies barely courted at all. But the female flies
with the male form of fruitless aggressively pursued other females,
performing all steps of male courtship except the last.
How does the male form of the fruitless protein govern such a complex
behavior? Dr. Dickson and his colleagues have found that the protein is
produced in 21 clusters of neurons in the fly's brain. The neurons, probably
connected in a circuit, presumably direct each step of courtship in a
Surprisingly, female flies possess the same neuronal circuit. The presence
of the male form of fruitless somehow activates the circuit, in ways that
are still unknown.
Fruitless serves as a master switch of behavior, just as other known genes
serve as master switches for building an eye or other organs. Are behaviors
and organs constructed in much the same way, each with a master switch gene
that controls a network of lower level genes?
Dr. Dickson writes that other such behavior switch genes may well exist but
could have evaded detection because disrupting them - the geneticist's
usual way of making genes reveal themselves - is lethal for the fly.
(Complete loss of the fruitless gene is also lethal, and the gene was
discovered through a lucky chance.)
Though researchers like to focus on specific genes, they are learning that
in behavior, an organism's genome is closely linked to its environment, and
that there can be elaborate feedback between the two.
Honeybees spend their first two to three weeks of adult life as nurses and
then switch to jobs outside the hive as foragers for the remaining three
weeks. If all foragers are removed from a hive, the nurse bees will sense
the foragers' absence through a pheromone and assume their own foraging
roles earlier. As the colony ages, however, there are too few nurses, so
some bees stay as nurses far longer than usual.
Gene Robinson, a bee biologist at the University of Illinois, has found that
a characteristic set of genes is switched on in the brains of nursing bees
and another set in foraging bees. This is an effect of the bees' occupation,
not of their age, since both the premature foragers and the elderly nurses
have brain gene expression patterns matched to their jobs.
Evidently the division of labor among bees in a hive is socially regulated
through mechanisms that somehow activate different sets of genes in the
A remarkable instance of genome-environment interaction has been discovered
in the maternal behavior of rats. Pups that receive lots of licking and
grooming from their mothers during the first week of life are less fearful
in adulthood and more phlegmatic in response to stress than are pups that
get less personal care.
Last year, Michael J. Meaney and colleagues at McGill University in Montreal
reported that a gene in the brain of the well-groomed pups is chemically
modified during the grooming period and remains so throughout life. The
modification makes the gene produce more of a product that damps down the
brain's stress response.
The system would allow the laid-back rats to transmit their behavior to
their pups through the same good-grooming procedure, just as the
stressed-out rat mothers transmit their fearfulness to their offspring.
"Among mammals," Dr. Meaney and colleagues wrote in a report of their
findings last year, "natural selection may have shaped offspring to respond
to subtle variations in parental behavior as a forecast of the environmental
conditions they will ultimately face once they become independent of the
A full understanding of these behavior genes would include being able to
trace every cellular change, whether in a hormone or pheromone or signaling
molecule, that led to activation of the gene and then all the effects that
followed. Dr. Robinson has proposed the name "sociogenomics" for the idea of
understanding social life in terms of the genes and signaling molecules that
The genes discovered so far mostly seem to act in different ways and it is
hard to state any general rules about how behavior is governed.
"It's early days and we don't have enough information to develop theories,"
Dr. Robinson said.
A question of some interest is how far the genetic shaping of behavior
exists in people. Larry J. Young of Emory University, who studies the social
behavior of voles, said that in people, activities like the suckling of
babies, maternal behavior and sexual drives are likely to be shaped by
genes, but that sexual drives are also modulated by experience.
"The genes provide us the background of our general drives, and variations
in these genes may explain various personality traits in humans, but
ultimately our behavior is very much influenced by environmental factors,"
Researchers can rigorously explore how behavioral genes operate in lower
animals by performing tests that are impossible or unethical in people.
"The problem with humans is that it is extremely difficult to prove
anything," Dr. Dulac said. "Humans are just not a very good experimental
"What are we doing to our Home?!:("
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