Not a new article but something I hadn't seen before....
http://findarticles.com/p/articles/mi_m1511/is_5_20/ai_54432958
The WAR BETWEEN MEN AND WOMEN - tales told by genomic imprinting
Robert Sapolsky
What do you do if a lot of the struggle takes place at a molecular level and
neither person knows it?
As most newlyweds quickly learn, intimate relationships, even the most
blissful, can buzz with tension. Couples typically find themselves
struggling over money, in-laws, ex-lovers, and how much the woman's placenta
should grow when she gets pregnant. That last one is a killer. The guy wants
his woman to have a fast-growing placenta, while the woman does all she can
to keep it down to a reasonable size.
Of course, the fight over the placenta doesn't exactly take place out in the
open. The average man, if asked what he thinks of his wife's placenta, would
probably say he hasn't given it a thought. Instead the placenta conflict
gets played out, unbeknownst to either person, inside the woman's body. A
strange genetic process called genomic imprinting is responsible. And its
existence is only one small example of how males and females have
conflicting evolutionary goals. Understanding that the struggle takes place
can explain a lot of strange behavior and physiology. And it can even
explain how we came to be susceptible to certain horrific diseases.
At first it might seem unlikely that men and women do have conflicting
evolutionary goals. The point of life, according to evolutionary theorists,
is for organisms to pass on copies of their genes to succeeding generations.
Some organisms, such as cockroaches, do this by producing as many offspring
as possible, hoping some will survive. Others, such as elephants and humans,
have many fewer young but shower them with care.
So yes, both mom and dad want their kid to survive. And in monogamous
species, when the father can be pretty sure it really is his kid, he may
well cooperate with the mother. But that's not quite the way it is in more
polygamous animal species such as orangutans, nor was it that way, say many
evolutionary biologists, among our own ancestors. Those males want their
offspring to survive at any cost--even at the cost of the female's health
and future fertility with other males. For example, if the male can figure
out a way to make the female spend most of her food energy making the
offspring grow big and strong, that's okay, even if she starves. That's
where the struggle over the placenta comes in: men and other male mammals
pass on genes to their unborn children that encourage the mother's placenta
to grow big, nourishing the fetus at the mother's expense and at the expense
of her future kids with other guys.
Females want their offspring to survive too, but not at the risk of their
future fertility. For example, in mammals, nursing inhibits ovulation. So a
mammal mom wouldn't nurse her young for the rest of her life, even if doing
so greatly increased its chances of survival. Otherwise, she might never
again ovulate, become pregnant, and bear more young.
This conflict is played out viciously among fruit flies. Rather than growing
old in each other's arms, drosophila mate with multiple partners, none of
whom stick around for a second date. And look at what they've come up with:
male semen contains toxins that kill the sperm of other males. When a male
fly mates with a female who has recently had sex with someone else, his
spermicide goes to work, killing competitors' sperm. That's a great
adaptation, but unfortunately the stuff is toxic to the female and gradually
harms her health. This doesn't bother the male at all. It increases his
evolutionary fitness, and he's never going to see her again.
William Rice, an evolutionary geneticist then at the University of
California at Santa Cruz, did a wonderfully slick experiment in which he
kept female fruit flies from evolving while letting the males compete
against each other. After 40 generations, the evolutionary winners were
males with the strongest toxic punch in their semen. Females who mated with
them had a shorter life expectancy.
The same sort of coevolutionary arms race happens in humans and other
mammals. As a result, we have developed a bizarre set of genes called
imprinted genes, that seem to violate the basic tenets of genetics.
Think back to Gregor Mendel, high school biology, and dominant and recessive
traits. That most monkish of monks taught that genetic traits are coded for
by pairs of genes, one from each parent. He figured out how the pairs of
genes interact to influence the organism, depending on whether the pair have
identical or differing messages. According to Mendel, it doesn't matter
which parent contributed which genetic message. Whether the offspring gets
the vanilla version of a gene from the mother and the chocolate version from
the father or the other way around, the trait coded for by the pair of genes
will look the same.
Imprinted genes violate Mendel's rules. With these genes, only the gene from
one parent has input--the matching gene from the other parent is silenced,
losing its influence over the trait expressed. Most experts in this new
field believe that there are only a couple hundred of these genes in humans,
but they can be quite influential.
About half the genes in question have something to do with growth--of the
placenta, the fetus, or the newborn. And the genes derived from the father
favor greater, faster, more expensive growth, while the maternally derived
genes counter that exuberance. As the evolutionist David Haig of Harvard
first suggested in 1989, imprinted genes--including these genes in
humans--are a case of intersexual competition, fruit fly sperm wars redux.
The first battleground is the placenta, a tissue that can seem more than a
little creepy. It's only partially related to the female, but it invades (a
term used in obstetrics) her body, sending tentacles toward her blood
vessels to divert nutrients for the benefit of a growing fetus. The placenta
is also the scene of a pitched battle, with paternally derived genes pushing
it to invade more aggressively while maternally derived genes try to hold it
back. How do we know? In rare diseases, maternal or paternal genes related
to placental growth are knocked out of action. Lose the paternal input and
the antigrowth maternal component is left unopposed--the placenta never
invades the mother's endometrium, so the fetus has no chance to grow. In
contrast, remove the maternal input, leaving those paternal genes unopposed,
and the placenta grows into a stupendously aggressive cancer called
choriocarcinoma.
The imprinting struggle continues during fetal development. One gene, which
codes for a powerful growth-stimulating hormone in mice and humans, is
expressed only by paternally derived genes. This is a classic case of dad
pushing for maximal fetal development. In mice (though not in people), the
mother counteracts the pro-growth tumult by expressing a gene for a cellular
receptor that regulates the growth hormone's effectiveness. Thrust and parry
Once a baby is born, imprinted genes take a particularly impressive turn.
Certain paternally expressed genes help make kids active nursers. On the
surface, this looks like another example of the usual picture: faster
development at the cost of mom's lactational calories. But now we're talking
about imprinted genes that influence behavior. Other genes influence brain
development in even stranger ways (see Brain Genes, below).
The discovery of imprinted genes may pave the way for curing a number of
unpleasant diseases involving tumors, infertility, and fetal overgrowth or
underdevelopment. But philosophically, the findings are disturbing. They
appear to have some deflating implications about human nature. Among fruit
flies, sperm-war genes show that males care little about the females'
future. What about us? "In sickness and in health," we promise, "until death
do us part." We're the species that came up with Paul Newman and Joanne
Woodward. For monogamous animals, the future health and fertility of the
female is as much in the male's interest as hers. So what are these
imprinted genes doing in a human couple pondering which appetizers to serve
at their golden wedding anniversary?
The answer is that reports of our monogamy are greatly exaggerated. Features
of human anatomy and physiology argue against it. Most human cultures allow
polygamy. And most studies, ranging from genetic paternity tests to Cosmo
questionnaires, suggest that there's a lot of action going on outside the
pair-bond, even in monogamous societies. We have more in common with fruit
flies than commonly believed. (Mind you, we're not one of the more
polygamous species around. Even the busiest patriarchs weigh in with only a
few hundred kids.)
Does nature have to be so bloody in tooth and claw and gene? Must everything
be based on competition? Why can't we all just get along? Here's where the
evolutionary biologists, with Bogart esque weariness, pull out the great
cliches of their field. Biology isn't about what should be, they explain,
but what is. It's a tough evolutionary world out there. It's dog
outreproduce dog. But a recent experiment by Rice and evolutionary
behaviorist Brett Holland hints that intersexual competition needn't be
inevitable. With careful manipulation, it can be derailed. The researchers
isolated pairs of mating flies, forcing them to be monogamous. They then
bred the offspring with the offspring of other such enforced monogamous
pairs, continuing to maintain the monogamy. And after only 40 generations,
the monogamous descendants produced less harmful seminal fluid.
They also treated their mates with unusual courtesy--normal fruit-fly
courtship looks like sexual harassment. Once competition between males was
no longer a selective force, producing toxic chemicals apparently became a
maladaptive waste of energy. Freed of the cost of intersexual warfare, these
monogamous flies actually outbred the usual competitive flies.
Just imagine carrying out the same experiment in people. Isolate some humans
and force them and their descendants into monogamy for a millennium, and we
would probably begin to disarm our mammalian weapons of intersexual warfare,
namely imprinted genes. They are an evolutionary burden, making possible
some truly horrendous . cancers. Remove their advantages by eliminating
polygamy, and natural selection should edit them out.
Having arrived at what sounds like a surreal moral--an exhortation to
remember the Sixth Commandment as part of the "Let's Whip Choriocarcinoma by
the Year 3000" campaign--it's time to take a step back Understanding how
intersexual competition started with flies is relatively easy. Thanks to
random genetic variability, some male flies stumbled into ever so slightly
toxic sperm, which the females had to detoxify or die. And from there, the
competition spiraled upward. The story of imprinted genes is a bit more
complicated, but once paternally derived genes began pushing for
the-hell-with-the-mom growth, the battle inevitably escalated. If the tribe
next door shows up at the Paleolithic watering hole with clubs that seem
just a wee bit on the big side for the purpose of bonking prey animals over
the head, the home team will naturally respond by getting even bigger clubs,
just in case. And soon we have a world with choriocarcinoma, toxic fruit-fly
semen, and umpteen times the education budget buying $600 toilet seats for
the military. As in so many other arenas of conflict, it's easier to ratchet
up than down.
Stay-at-Home Dads
Conflict between males and females takes a complicated turn in the rare
species with high "male parental investment." In such species, a male
instinctually takes care of youngsters he thinks are his own. He mates with
someone, and babies appear an appropriate length of time later, so he
protects them from predators and takes them to ice-skating lessons. A lot of
bird species fit this profile. Sometimes the male does more parenting than
the female. At the moment when the egg hatches, she has invested far more
energy in the gestation than he has, but afterward the tide gradually turns.
In those cases, what should a heartlessly practical female do if she wants
to maximize her reproductive success? She should figure out the exact moment
when he's spent more cumulative energy than she has on passing on copies of
his genes via these offspring. When that moment arrives, she should abandon
the youngsters. He will likely continue taking care of them, guaranteeing
her reproductive success and leaving her free to start a new round of
reproduction with someone else. --R. S.
Brain Genes
Mom wants you to be clever, but dad cares more about your metabolism. At
least, that's the way the evidence of genomic imprinting points. Experiments
with mice show that some maternally expressed genes favor a bigger cortex,
the intellectual part of the brain. Mutations that knock out those genes
cause retardation. In contrast, some paternally expressed genes favor growth
of the hypothalamus, which controls many unconscious body functions.
How do these imprinted genes related to brain function fit into the scheme
of intersexual warfare? Instead of the usual scenario, in which dad wants
more growth while mom wants less, each parent's genes favor a different kind
of growth. Does a female enhance her future health and fertility by
supplying her young with a thick cortex and super SAT scores? Does a bigger
hypothalamus produce a kid who is able to sap more of mom's resources? No
one knows, but Eric B. Keverne of the University of Cambridge, England, who
has done much of the work on brain-related imprinted genes, continues to
wrestle with these intriguing findings, trying to fit them into the
framework of intersexual warfare. Meanwhile, some evolutionary biologists
speculate that the traits might have evolved for entirely different reasons.
--R. S.
Self-Conscious Scrub Jays
When biologists talk about male orangutans "realizing" that they have less
to lose in mating than females, or female scrub jays "calculating" exactly
when to abandon their young to a caring father, they're speaking
metaphorically. Except for the most cognitively sophisticated primates,
animals don't sit there with an evolution textbook and a calculator,
strategizing consciously. Instead phrases like "the scrub jay wants to do,
this, decides that this is the right time," and-so on are shorthand for the
more correct but cumbersome, "Over the course of evolution, scrub jays who,
at least in part through genetically influenced mechanisms, are better able
to optimize the timing of their behavior leave more copies of their genes,
thus making this attribute more prevalent in the population." Personifying
the animals is just an expository device agreed upon to keep everyone from
falling asleep during conferences. --R. S.
COPYRIGHT 1999 Discover
COPYRIGHT 2000 Gale Group