It's not easy being a marmoset mother. But the tiny New World monkeys often get babysitting help—and the key may be a bizarre interchange of DNA that can take place in the creatures' wombs, a new study says.
Marmoset moms typically give birth to fraternal twins, which develop from two separately fertilized eggs. Unlike identical twins, fraternal twins have differing DNA.
Now scientists from the University of Nebraska in Lincoln say that embryos of fraternal twins can exchange cells of practically every type of tissue—including reproductive, or germ line, cells.
It's the first evidence for genetic chimerism—the mixing of genetic lineages between siblings—in primate reproductive tissue, claims the research team.
Chimerism is rare in mammals, but it had previously been found in marmosets' blood-derived tissues, the study authors point out.
The new research, however, shows that the genetic mixing is much more extensive than was previously believed—and that the chimerism can be passed to offspring, meaning the marmoset family tree gets oddly tangled.
"The use of genotype data and genealogical data allowed us to determine that a male may be producing the sperm that fertilizes the egg, but the genetic material of that sperm is actually his [fraternal] twin brother's," said study author Corinna Ross.
Ross and her colleagues report their findings this week in the journal Proceedings of the National Academy of Sciences.
The head-scratching parental ambiguity caused by the chimerism could be an evolutionary driver of the marmosets' expansive system of child care.
Oftentimes, adult marmoset males will care for unrelated infants, a blessing for a tired mother whose twins may weigh as much as 20 percent of her weight, the study says. (Related story: "All-Star Animal Dads" [June 18, 2004].)
The researchers suggest that the helpful males are in fact responding not from kindness but subtle "cues of relatedness" created by the genetic chimerism.
The researchers found a "significant correlation" between males who babysat and the presence of germline chimerism: These males were found to rear chimeric infants more often than nonchimeric ones.
"Although the exact mechanisms of sociobiological change associated with chimerism have not been fully explored, we show here that chimerism alters relatedness between twins and may alter the perceived relatedness between family members, thus influencing the allocation of parental care," the study says.
Study author Ross said the findings could also cause researchers "to redefine what it means to be an individual in this species."
"This study is exciting," added David Haig, an evolutionary biologist at Harvard University, "because, in a genetic sense, twin marmosets could be cousins instead of siblings."