One child, two fathers -- marmoset-style

Marmosets are close-knit primates, but siblings are incredibly tight -- as embryos, fraternal twins trade stem cells that become part of a wide range of tissue types, including sex organs and sperm cells, according to an article appearing online this week in the Proceedings of the National Academy of Sciences. As a result, some chimeric marmosets can pass a sibling's DNA onto their offspring. Humans and other animal species often carry a mother's or a twin's blood cells along with their own, but "this is the first strong claim that chimerism is much more extensive than just in the blood cells," said David Haig, a professor of biology at Harvard University who was not involved in the study.Chimerism may even have an effect on marmoset behavior, the authors note. In this study, marmoset fathers were more likely to carry their chimeric offspring than their non-chimeric offspring, Corinna Ross, lead author of the study and a primatologist at the University of Texas Health Sciences Center in San Antonio, told The Scientist. "What we were able to show is that male care differs based on whether an infant was chimeric or not."Ross and her colleagues uncovered marmoset chimeras while trying to develop a paternity test. "This project actually started to find out whether hair tissue represented an individual -- it turns out it doesn't," Ross said. Instead, a single hair sample often pointed to more than one father. To investigate further, the researchers genotyped five microsatellite DNA markers in tissue samples from 92 individuals, including 36 twin pairs and their parents. The team identified marmosets carrying three or more variants of one marker as potential chimeras, since primates typically carry only two variants of each marker (one from each parent). The researchers considered 38 of these individuals as true chimeras, because their markers matched those of their own twins, as well as their parents. Of the 17 tissues tested, including liver, skin, spleen, and blood, all were chimeric in at least one marmoset. "We genotyped directly from isolated sperm samples, and we found that those were chimeric," noted Ross.However, Kurt Benirschke at the University of California San Diego, who was not part of the study, suggested that the chimeric cells found in different tissue types might all be chimeric lymphocytes that contaminated the samples. "With the exception of the [chimeric] sperm, there is the possibility that these are lymphocytes that are being [identified] here," he said. (Sperm samples are less susceptible to blood contamination.)Individuals in five mating pairs appeared to pass a sibling's DNA markers on to their own offspring. One mother passed on markers from her twin brother's DNA, suggesting that his primordial germ cell took up residence in her ovary and matured to produce a viable egg. Still, "I don't think that you could conclude this was germ-line chimerism as yet, because you really have to show that the sperm passed on a twin's genotype" beyond a few markers, Benirschke said. To test whether parents favored chimeras, the team compared the amount of time marmosets spent carrying 10 chimeras and 20 non-chimeric infants. Although fathers appeared to favor offspring with chimeric hair and saliva, mothers did the opposite. "I would like to see if [this behavior pattern] stands up looking at larger samples. I think it's an unexpected and interesting observation," said Haig. Chris Womack mail@the-scientist.comLinks within this articleC. N. Ross et al., "Germ line chimerism and paternal care in marmosets (Callithrix kuhlii)," Proceedings of the National Academy of Sciences Early Online Edition, March 26, 2006. http://www.pnas.orgC. Kittredge, "A question of chimeras," The Scientist, April 11, 2005. http://www.the-scientist.com/article/display/15405/J. L. Nelson, "Microchimerism and autoimmune disease," N Engl J Med, April 23, 1998. http://wwwl.the-scientist.com/pubmed/9554866G. D. Niblack et al., "T-and B-lymphocyte chimerism in the marmoset," Immunology, February, 1977. http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1445224David Haig http://www.oeb.harvard.edu/faculty/haig/HaigHome.htmD. Haig, "What is a marmoset?" Am. J. Primatol., December 1999. [pdf] http://www.oeb.harvard.edu/faculty/haig/pdfs/99Marmoset.pdfCorinna Ross http://golab.unl.edu/people/cross/index.htmlR. Lewis, "Of sheep and grapes: DNA fingerprinting tracks ancestry," The Scientist, September 27, 1999. http://medicine.ucsd.edu/cpa/home.html

One child, two fathers -- marmoset-style

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