Thoughts in a Haystack

Pregnancy can be the most wonderful experience life has to offer. But it can also be dangerous. Around the world, an estimated 529,000 women a year die during pregnancy or childbirth. Ten million suffer injuries, infection or disability.

To David Haig, an evolutionary biologist at Harvard, these grim statistics raise a profound puzzle about pregnancy.

"Pregnancy is absolutely central to reproduction, and yet pregnancy doesn't seem to work very well," he said. "If you think about the heart or the kidney, they're wonderful bits of engineering that work day in and day out for years and years. But pregnancy is associated with all sorts of medical problems. What's the difference?"

The difference is that the heart and the kidney belong to a single individual, while pregnancy is a two-person operation. And this operation does not run in perfect harmony. Instead, Dr. Haig argues, a mother and her unborn child engage in an unconscious struggle over the nutrients she will provide it.

In the 1970's, Dr. Trivers argued that families create an evolutionary conflict. Natural selection should favor parents who can successfully raise the most offspring. For that strategy to work, they can't put too many resources into any one child. But the child's chances for reproductive success will increase as its care and feeding increase. Theoretically, Dr. Trivers argued, natural selection could favor genes that help children get more resources from their parents than the parents want to give.

Dr. Haig proposed that pre-eclampsia was just an extreme form of a strategy used by all fetuses. The fetuses somehow raised the blood pressure of their mothers so as to drive more blood into the relatively low-pressure placenta. Dr. Haig suggested that pre-eclampsia would be associated with some substance that fetuses injected into their mothers' bloodstreams. Pre-eclampsia happened when fetuses injected too much of the stuff, perhaps if they were having trouble getting enough nourishment.

In the past few years, Ananth Karumanchi of Harvard Medical School and his colleagues have gathered evidence that suggests Dr. Haig was right. They have found that women with pre-eclampsia had unusually high levels of a protein called soluble fms-like tyrosine kinase 1, or sFlt1 for short.

Other labs have replicated their results. Dr. Karumanchi's group has done additional work that indicates that this protein interferes with the mother's ability to repair minor damage to her blood vessels. As that damage builds up, so does her blood pressure. And as Dr. Haig predicted, the protein is produced by the fetus, not the mother.

Scientists have found that some genes are imprinted in the brain after birth, and in some cases even in adulthood. "Imprinted genes and behavior are the new frontier," said Dr. Lawrence Wilkinson of the University of Cambridge. In a paper to be published in The Philosophical Transactions of the Royal Society of London, Dr. Wilkinson and his colleagues argue that the evidence on imprinted brain genes — preliminary as it is — fits with Dr. Haig's theory. They call it "the most robust evolutionary hypothesis for genomic imprinting."

One major source of conflict after birth is how much a mother will feed any individual offspring. A baby mammal is more likely to thrive if it can get more milk from its mother. But nursing demands a lot of energy from mothers that could be used for other things, like bearing and nursing more offspring. ...

Dr. Wilkinson suspects that conflict between imprinted brain genes may add to the risk for mental disorders, from autism to depression. Because one copy of each of these genes is silenced, they may be more vulnerable. "If you ask me, do I think that imprinted genes are likely in the next 10 years to crop up as mechanisms in mental disorders, I'd say yes," he said.

Link - Posted by John Pieret @ 9:58 PM