BioEd Online

As a general rule, a man who learns that his children are genetically his brother's offspring would have good cause for distress. But for one group of primates, that wouldn't necessarily mean that mum has been unfaithful, a new study finds.

The reason, says Corinna Ross of the University of Texas Health Science Center in San Antonio, is that these primates are often genetic mosaics containing some cells that belonged to their siblings. And when those cells happen to be sperm, a male can sire offspring that are genetically nephews and nieces rather than sons and daughters.

This strange genetic mixing could be one of the reasons why these animals tend to raise their families in large collectives, with everyone lending a hand; animals are thought to generally give more parental attention to children with a strong genetic similarity to themselves.

The discovery was made accidentally when Ross was studying small, tree-dwelling primates called black tufted-eared marmosets (Callithrix kuhlii) at the University of Nebraska in Lincoln. The marmosets that she studied were being kept in captivity, with two senior males for every reproducing female. Ross wanted to test the paternity of the colony's offspring, to find out which male was the father of each child; to do this, she looked at hair samples of various animals to determine their genetic make-up.

But her results were confusing: some hair samples showed the genetic fingerprint of a sibling rather than that of the individual being tested.

Cellular mix

Scientists have known for years that marmosets — which are typically born in pairs — share a blood supply with their twin while developing in the womb. As a result, most marmosets contain blood cells from their siblings, and therefore carry their sibling's genes as well as their own. But the phenomenon — known as chimaerism — had only been observed in marmoset tissues that produce blood cells.

Ross went on to test 15 different tissue types — from kidney to skin to sperm — in 36 sets of marmoset twins from 15 families. A percentage of samples from every tissue tested contained the genetic information from a sibling rather than the animal that provided the tissue sample, she reports in Proceedings of the National Academy of Sciences¹. Of the hair samples, 17% were chimaeric.

Finding chimaerism in reproductive tissue is particularly surprising, says David Haig, an evolutionary biologist at Harvard University in Cambridge, Massachusetts. "That means that when a marmoset reproduces, some of the cells from that marmoset are producing sons and daughters, whereas others are producing nephews and nieces," says Haig.

Ross found chimaerism in four of the seven sperm samples that she analysed; in 5 of the 15 families, members passed a sibling's genetic information on to their offspring. In one particularly odd case, a female passed her brother's genetic information on to her children - suggesing the intriguing possibility that the mother may have passed a Y chromosome to her offspring.

Family ties

All this could have an impact on the family dynamics, says Haig.

Marmoset families are highly cooperative, and the entire family helps to raise the young. Marmoset mums hand much of the childcare over to dad, who carries his offspring on his back as he scampers from tree to tree. Older siblings also help out, and often opt to help raise their younger brothers and sisters rather than have children of their own, freeing up mum to get pregnant again more quickly.

Chimaerism could facilitate that family harmony, agrees Ross, because a sibling carrying a mosaic of the family's genetic information is more likely to share some of that information with each family member, strengthening kinship bonds among siblings. "For example, their parents could be having infants that are more closely related to them than their own infants might be," says Ross.

It does seem to have an impact on families, Ross found. She looked at the amount of chimaerism in the skin of baby marmosets, to see if that external (and relatively easy to measure) signal of a genetic mix had an effect on parenting. It did.

Fathers spend more time carrying infants with chimaeric skin than infants who were not skin chimaeras, she found. Mothers, on the other hand, spend less time with these chimaeric offspring, perhaps knowing that others would pick up the slack. Exactly why dads seem to invest more in chimaeric offspring isn't yet clear.