A male marmoset carrying infant twins. Marmoset young have been found to exchange cells in the womb.
Credit: Jeff Fite
CANBERRA: In a bizarre process, developing marmoset twins have been found to exchange genes in the womb.
The unusual method of genetic exchange means that marmosets can have organs - even sex organs - that include their sibling's cells. The process can create young with ambiguous paternity and may hold the key to successful human organ transplant, say U.S. researchers behind the find.
"We've discovered … that when an individual parents their offspring, they may actually be parenting their brother's offspring," said primatologist Corinna Ross from the University of Texas in Tyler. "And that's very odd — the fact that you can give birth to your brother's kids."
Ross is co-author of a paper detailing the find published today in the U.S. journal Proceedings of the National Academy of Sciences.
Marmosets (Callithrix kuhlii) are small South American monkeys and they typically give birth to fraternal twins; that is twins that develop from two separate eggs. According to Ross, as the eggs develop within the womb, they often exchange embryonic stem cells via a shared blood supply.
This genetic exchange within the womb creates 'chimeras', individuals that have cells with two distinct genomes. A developing organ within this chimera could potentially have a mixture of cells that originated both in its self and its twin.
Researchers already knew that marmoset bone marrow and blood sometimes contained cells that originated in a twin. But Ross and her team conducted a new genetic survey to reveal that all types of tissue can be chimeric, including the liver, blood, lung, muscle, and most importantly, sperm.
This gene transfer "is likely to be a highly controversial finding," said co-author Jeff French, who studies animal behaviour at the University of Nebraska in Omaha. "Chimerism has long been viewed as having [negative] outcomes, like sterility. Marmosets, in contrast, appear to have specific adaptations to promote the production of genetic chimeras."
According to French, the ambiguous paternity created by chimerism could underlie the highly cooperative parenting behaviours seen in marmosets - fathers and unrelated adults often care for infants, which is rare in animals.
"In marmosets, we assume that the high cost of reproduction for females has driven the evolution of infant care by fathers," he said. Chimerism may encourage males to look after many young, because they might have indirectly fathered them.
These chimeras may also provide some clue to aid human organ transplant research, said French. Normally, cells are recognised by the body as self or non-self - and non-self cells are destroyed by the immune system. Therefore, marmoset chimeras must have a mechanism that allows cells with different genomes to coexist, he said.
Currently recipients of organ transplants have to take immune suppressant drugs with unpleasant side effects and many find that their organs are rejected within five years regardless.
"The primary reason that organ transplants in humans fail is because the body's immune system recognises the donated organ as 'non-self'," said French. "Exploring the mechanisms that allow marmosets to maintain both self and sibling cells could speak to this question."
This unusual type of genetic exchange may not be restricted to marmosets, added Ross. "It is possible for humans to exchange genetic material in this way, although it is still believed to be a very rare event."
More information:
Germ-line chimerism and paternal care in marmosets (Callithrix kuhlii), PNAS.
