Nebraska research on marmoset paternity leads to genetic discovery

Research at the University of Nebraska sheds light on how a pre-birth genetic exchange between marmoset twins plays a part in parenting behavior among these tiny New World monkeys.

Writing in the March 26-30 online edition of the Proceedings of the National Academy of Science, Corinna Ross, a former graduate student and postdoctoral researcher at the University of Nebraska, showed how in utero genetic exchange between marmoset twins may help explain the unique patterns of parental care in this primate species. Marmosets (Callithrix kuhlii) typically give birth to twins. Fraternal twins, which develop from two separate fertilized eggs, often exchange embryonic stem cells through shared blood vessels early in development. This exchange produces genetic chimeras, or animals that carry the genes of their siblings in some tissues. Chimerism in marmosets is well recognized in their blood cells and bone marrow but had not been previously described in other bodily tissues.

In this study, Ross, Jeffrey French of the University of Nebraska at Omaha and Guillermo Orti of the University of Nebraska-Lincoln show that chimerism is common in many of the tissues of twin marmosets, including reproductive cells such as sperm. These chimeric animals can then pass on their sibling's genes to their offspring, producing offspring that are not genetically their own.

"What we've discovered in this animal is that when an individual parents their offspring, they may actually be parenting their brother's offspring," said Corinna Ross, a postdoctoral fellow at the Barshop Institute for Longevity and Aging Studies at the University of Texas Health Center in San Antonio. "And that's very odd -- the fact that male marmosets sometimes fertilize their offspring with their brother's sperm."

This ambiguous paternity could underlie the highly cooperative parenting behaviors seen in marmosets -- explaining why fathers and unrelated adults often care for infants. This genetic exchange could have driven the evolution of cooperative infant care in this group of primates, the authors suggest.

French said that this discovery, like so many in science, came unexpectedly. Ross initially was looking at different tissues in marmosets to find the best type for DNA paternity testing.

"Initially when I started my dissertation, I was interested in paternity, and whether fraternal twins can have a different father," Ross said.

"Every single tissue she looked at had evidence of chimerism," said French, a recognized expert on marmosets who runs the Callitrichid Research Center at UNO. The marmoset research center studies social behavior, reproduction and hormones of the species. Ross, originally from New York, later worked on her post-graduate studies with Orti at UNL after working in the Callitrichid lab.

"Cory was looking at molecular tools and markers to trace the genes, to see if they're exchanging genes or not. Because she needed individuals of known pedigree to do the genotyping on the tissues, she had access to all that at the marmoset center at UNO through the archiving and record-keeping of the specimens," said Orti, a biologist at UNL who advised Ross on the genetics aspects of the research.

UNO's marmoset colony, established in 1991, provided Ross with a wealth of genealogic information. The history of individuals and family groups is completely detailed and now goes back three and four generations. The discovery not only opens up new clues into what's underlying the marmoset behavior, but also highlights important areas of research into chimerism, which could affect what scientists know about DNA testing.

"We used to think chimerism was very rare and always led to negative effects, like sterility," said Ross, who earned her master's degree at UNO and her doctorate at UNL. "Now, in the marmosets, we know it doesn't necessarily mean sterility, and more is being discovered and written about it all the time. Medical journals releasing papers about human chimerism, and there have been popular programs like the Discovery Channel's 'I am my Own Twin.' These studies are going to shed new light on paternity testing, what it means to be an individual, and other things that we take for granted."

Ross said her research at the Barhop Institute also uses marmoset populations and is on aging and genetics. She said she is privileged to have had great mentors in Nebraska. "It was a great environment that encouraged graduate students to excel," she said. "It led to this publication and being co-PI on an National Science Foundation grant. I've been really lucky in my career."

"This is a real tour de force for Cory," French said.

CONTACTS: Jeffrey A. French, University of Nebraska at Omaha, (402) 554-2558;
Corinna Ross, Univ. of Texas Health Science Center, San Antonio, (210) 562-6012 or (210) 846-3196;
Guillermo Orti, University of Nebraska-Lincoln, (402) 472-3433