[Vision2020] A Gene for Romance? So It Seems (Ask the Vole)

[Garrett Clevenger]

A Gene for Romance? So It Seems (Ask the Vole)

By NICHOLAS WADE

Published: July 19, 2005
http://www.nytimes.com/2005/07/19/science/19gene.html?

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 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."


Garrett Clevenger

http://www.icehouse.net/garrett

"What are we doing to our Home?!:("