Jeff Fite

Two for the road.
A marmoset dad carries his twin offspring.

I'll Be a Monkey's Uncle

We get half our genes from mom and half from dad, but marmosets get something extra from their siblings. While in the womb, these small South American monkeys swap stem cells with their fraternal twins, giving each a genetic piece of the other. Now scientists have discovered that this chimerism extends to sperm cells, which means that when one of these twins grows up, he can actually sire his brother's offspring. The finding may help explain the unusual parenting behavior seen in these monkeys, researchers say.

Marmoset females almost always give birth to fraternal twins. This type of twinning results when two egg cells are fertilized by separate sperm. Because each sperm and egg cell contains different DNA, one twin has a different genetic makeup than the other. Scientists have known for years that marmoset twins exchange stem cells--which can develop into a variety of cell types--through their shared blood supply during pregnancy, but they had only seen evidence of genetic mixing in blood cells.

In the new study, Corinna Ross, a primatologist at the University of Texas Health Science Center in San Antonio, and her colleagues discovered chimerism in a variety of other tissues, including the liver, heart, and sperm. As a result, the pool of sperm of one twin can be a mix of his own sperm and his brother's sperm. When the twin breeds, he could potentially sire a nephew instead of a son, the researchers report online this week in the Proceedings of the National Academy of Sciences.

This unusual inheritance may explain the selective parenting behavior exhibited by marmoset dads, who tend to pay more attention to some offspring than others. Ross's team found that marmoset fathers carry their chimeric infants more often than nonchimeric infants. That makes sense, says Ross, because when one twin receives stem cells from another, he is essentially inheriting "extra" DNA from his father's side of the family. Somehow, dads are able to pick up on this chimerism to more easily recognize their own offspring in a population where paternity can be in doubt, she notes.

"It's a really exciting result and ... adds a lot more depth" to our understanding of marmoset behavior, says Charles Snowdon, a biological psychologist at the University of Wisconsin, Madison. But, he adds, genetic chimerism isn't the only thing influencing family life in these monkeys. A host of other factors, such as previous parenting experience, group social dynamics, and hormones, can also affect how much attention dads pay to their kids.

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Posted in Biology