[Paleopsych] NYT: A Gene for Romance? So It Seems (Ask the Vole)

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


    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

    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

    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

    "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

    "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."

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