A monkey's uncle may actually be its father, according to a new study showing that germ-cell chimerism is rampant in marmosets. These animals often carry another individual's sperm or egg cells—in this case from their fraternal twin—not only complicating family trees but potentially affecting the parenting practices of these tree-dwelling primates.

Since the 1960s, scientists have known that marmosets are usually hematopoietic chimeras. Marmoset pregnancies generally produce fraternal twins. During gestation, the two placentas fuse, mingling the blood-producing cells of the developing fetuses. Until now, it was unclear whether this chimerism extended beyond tissues of hematopoietic origin.

While studying parenting and social behavior in the colony of black tufted-eared marmosets (Callithrix kuhlii) at the University of Nebraska (Omaha), Corinna Ross discovered that other tissue types of these animals are often genetic mosaics, including germline tissue. As Ross tells Lab Animal, “Marmosets are well known for having large cooperative breeding groups in which both males and siblings—and sometimes animals that are thought to be unrelated to the breeding pair—help care for infants. In both captivity and in the wild, researchers have noted the formations of not only monogamous pairings, but also polyandrous pairings, in which two males appear to be mating with a breeding female.” Ross conducted paternity tests using hair samples from various animals and found that the genetic profile of many of the samples actually matched the sibling of the animal that provided the sample (Proc. Natl. Acad. Sci. USA, doi: 10.1073/pnas.0607426104, published online 26 March).

Ross, who is now at the University of Texas Health Science Center (San Antonio, TX), went on to genotype samples from 15 tissue types, including liver, lung, heart, and gonads, taken from 36 sets of twins from 15 families. She found some degree of chimerism in all tissue types sampled, including the gonads and sperm, suggesting that the in utero exchange of nonhematopoietic stem cells is common during marmoset development. If some of the sibling's cells take up residence in the gonads, in adulthood the marmoset produces sperm or egg cells containing the genetic material belonging to its twin brother or sister; indeed more than half of the male marmosets sampled produced chimeric sperm.

Wondering how widespread chimerism might affect parenting, Ross tracked the amount of time marmoset parents spent caring for infants. She tells Lab Animal, “We found that males care for chimeric infants more than they care for nonchimeric infants. While we do not know exactly how kin recognition functions in these primates, we are proposing that chimerism may alter a male's attraction to infants because of an altered perception of individuality in these infants.”