HUMAN NATURE

It Takes One To Tango

By William Saletan
Sunday, June 10, 2007

It's time to talk about the birds and the bees.

No, I don't mean sex. You've already heard that story: Boy meets girl, sperm meets egg, a baby grows in Mommy's tummy. That's the way of all flesh. Or so you were told.

Well, you know how it is with the stories we tell kids. We leave a few things out of that birds-and-bees yarn. The truth is that many bees don't have daddies. Neither do some birds. It's called parthenogenesis -- literally, virgin birth. We're finding more and more animals that can reproduce this way, and we're learning how to engineer it in others. We're even tinkering with mommy-daddy procreation in humans. If you think sex is kinky, wait till you see the alternatives.

In bees, fertilized eggs become females. Unfertilized eggs become males. Even without a queen, some female honeybees can keep a colony going by laying eggs that fertilize themselves.

Birds do it, too. Up to 30 percent of unfertilized turkey eggs can spontaneously begin to develop. In one study, selective breeding boosted the rate above 40 percent. A 1975 U.S. government report documented more than 50 mature turkeys that weren't fathered.

Lately, parthenogenesis has been verified in various snakes and lizards, extending the list to about 70 vertebrate species, including fish, frogs and chickens. But the beasts at the top of the food chain -- lions, sharks, Komodo dragons -- seemed impervious.

Then, in December, Nature delivered the first shock: "Parthenogenesis in Komodo dragons." Tests showed that seven juvenile dragons in European zoos were unfertilized offspring, known as parthenogens, the journal reported.

Still, the process hadn't been proved in sharks or mammals. And there seemed to be a good reason why. An egg that fertilizes itself makes two identical sets of chromosomes, including sex chromosomes. In birds, snakes and most lizards, two identical sex chromosomes make a male. That allows parthenogenesis to function as a DNA survival mechanism, because an isolated female -- close your ears, kids -- can produce a son and mate with him. But in sharks and mammals, this wouldn't work, because two identical sex chromosomes -- XX -- make a female.

Or so we thought. Three weeks ago, Biology Letters delivered the second surprise: "Virgin birth in a hammerhead shark." A perfectly formed baby shark had appeared in a tank in Nebraska that held only females of her kind. Tests proved the baby, too, was a parthenogen.

Why hadn't we found parthenogenesis in these animals before? We hadn't looked. Several sharks have mysteriously reproduced in captivity in recent years. Scientists now think that parthenogenesis is responsible. Of the two sexually mature female Komodo dragons in Europe, both are now known to have reproduced this way.

For explaining everyday life -- babies, puppies, puberty -- the mommy-daddy story of procreation works fine. But at life's edges, conventional biology, like conventional physics, breaks down. As you approach the speed of light, time slows and distances shrink. And as you approach extinction, genes find new ways to pass themselves on. Scientists call it "reproductive plasticity." A shark's got to do what a shark's got to do.

Parthenogenesis is just one of nature's survival tricks. Worms can divide into smaller worms. Female turtles can store sperm for years. Sea bass can change their sex. Male ants can commandeer eggs to clone themselves.

Mammals are different. We have a mechanism called imprinting, which foils parthenogenesis. But we've also developed an organ that can foil imprinting: the human brain. A few years ago, scientists produced 10 mice, two of them apparently normal, by manipulating a couple of genes so that eggs could fertilize each other. The scientists predicted "even greater improvements in the efficiency of parthenogenetic development in mice," and they vowed to try next with pigs.

Will we try parthenogenesis in humans? We already have. Biotech companies are rushing to industrialize it, with one claiming "a dominant patent position in the production of human embryonic stem cells by parthenogenesis." The stem-cell version of parthenogenesis can't make babies, but the mouse version might be able to. Theoretically, it would make it possible for two women to create a child together -- not a clone, but a mixture of genes from each parent, just like you or me.

We're not there yet, but we're on the way. Two years ago, British officials authorized human procreation using sperm, nuclear DNA from one woman, and mitochondrial DNA from a second woman. Three weeks ago, they proposed legislation that would approve the creation, for research, of "a human embryo that has been altered by the introduction of any sequence of nuclear or mitochondrial DNA of an animal." The categories we've always taken for granted -- mommy, daddy, people, animals -- are blurring.

But there's no going back to the days when humans weren't beasts and all babies had two parents. Those days never existed. O brave old world, that has such creatures in it.

human@slate.com

William Saletan covers science and technology for Slate, the online magazine at www.slate.com.


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