[EMAIL PROTECTED]
Fascinating!
--------------------------------------------------------------------------------
July 19, 2005
A Gene for Romance? So It Seems (Ask the Vole)
By NICHOLAS WADE
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 known to be involved
in the male's elaborate suite of courtship behaviors. New research has
established that a special feature of the gene, one that works differently in
males and females, is all that is needed to induce the male's 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 nose.
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 vomeronasal organ.
"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 courtship
behaviors.
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 coordinated sequence.
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 bees'
brains.
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 parent."
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 mediate them.
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," he said.
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 system."
a.. Copyright 2005 The New York Times Company
[Non-text portions of this message have been removed]
Yahoo! Groups Links
<*> To visit your group on the web, go to:
http://groups.yahoo.com/group/scifinoir2/
<*> To unsubscribe from this group, send an email to:
[EMAIL PROTECTED]
<*> Your use of Yahoo! Groups is subject to:
http://docs.yahoo.com/info/terms/
