"Well, I'll be a monkey's uncle" has now taken on a new meaning. Scientists studying marmosets have discovered that over half the males carry their brother's sperm. Marmosets, small monkeys that live in South America, have long been a genetic enigma. Marmoset mothers almost always give birth to fraternal twins, which develop from two eggs and are thus genetically distinct. In 1962, scientists at Dartmouth Medical School discovered that almost all marmosets carry some blood-generating stem cells that began in their twin sibling.
Animals that carry cells from another individual are known as chimeras. Aside from marmosets, chimeras have been discovered in humans, cats and cows. But scientists have long thought that chimerism was a rare fluke.
Marmosets were different. Almost all of them had chimeric blood, and they were all healthy. It appears that they swap cells so often because of their peculiar development. In the womb, their placentas grow quickly and fuse, creating a network of blood vessels through which cells can travel from one twin to the other.
In 1991, Jeffrey French, a primatologist at the University of Nebraska, Omaha, set up a colony of marmosets. He and his colleagues have tracked their family history ever since. Recently one of his graduate students, Corinna Ross, set out to find a paternity test for marmosets. She developed a method much like the test used to identify human fathers. Small regions of DNA known as microsatellites are very prone to mutating, creating a kind of genetic fingerprint that's different from one family to another.
"We were hoping that we could use hair for paternity analysis, but that's not quite how it worked out," Ross said. "The initial results were very odd."
Different hairs from the same marmoset had different genetic fingerprints. Some of it matched the DNA of the marmoset's twin.
Working with French and Guillermo Orti, a University of Nebraska geneticist, Ross then studied other tissues from marmosets. "We found chimerism throughout everything," she said.
The pattern was different from one marmoset to the next. "A single individual might be chimeric for hair and liver, for example, and not for anything else," Ross said.
One of the most surprising results of the study is that over half of male marmosets have chimeric sperm. Ross and her colleagues discovered cases in which the DNA of male marmosets turned up in babies supposedly fathered by their fraternal twins. In other words, the sperm came from one male, but it had the DNA of the male's brother. A paternity test would show that the baby's genetic father was actually its uncle. The scientists were not able to isolate DNA from marmoset eggs, but they did find that two out of 21 marmoset ovaries were chimeric. It's possible that a female marmoset can give birth to nephews and nieces.
The discovery of rampant chimerism in marmosets led the Nebraska scientists to wonder if it affected how parents treated their children. Primates can recognize their offspring by distinctive odors. But a marmoset with chimeric skin would give off two odors — its own, and that of its twin sibling.
The scientists found that mothers carried babies with chimeric skin less than they carried babies with only one set of skin cells. Fathers, on the other hand, carried chimeras over twice as much as non-chimeras.
Scientists have long noted that marmo-sets are unusual among primates for their doting fathers. Chimeras may be the source of their attention. It's possible, Ross suggests, that chimeras give off a wider range of odors that signify that they are related to a father, increasing his attention.
The results of the marmoset study appeared last week in The Proceedings of the National Academy of Sciences.
"It's potentially a really interesting finding," said David Haig, an evolutionary biologist at Harvard. Haig is particularly intrigued by the genetic relatedness of chimeric twins. On average, two normal fraternal twins would share about half their genes. Marmoset twins are more closely related because they have more genes in common.
That common bond might drive the evolution of unusual kinds of behavior. Haig notes that in a band of marmosets, only the oldest female can reproduce, while the younger individuals help raise her offspring.
Ross, who now works at the South Texas Centers for Biology in Medicine, is continuing to study the marmosets to understand the effects of chimerism. The questions she hopes to answer are not just scientific, but philosophical.
"This changes how we think of marmosets as individuals, but it also changes how we think of the term at all," she said. A male mates with a female, who gives birth to his brother's offspring. "But most of his body also has his brother's genes. So what is he as an individual?"
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