Biological Look At Marmosets And Chimerism

Funky monkeys (Subscription)
March 29, 2007 -- The Economist

Marmosets give birth to their genetic nieces and nephews.

It does not take a biologist to tell that there are two sorts of twins. Twins can be genetically identical or they can be as different as siblings that were born at different times. Most marmosets, though, fall somewhere in between. These small New World monkeys may be cute, but they are chimeras nonetheless. Like the monster from Greek mythology, many marmosets are a mixture of more than one individual. They are, genetically, both themselves and their sibling at the same time.

There are two other odd (and indeed cute) things about marmosets. One is that they are always conceived as twins. The other is that they are unusually caring towards one another. Fathers are particularly and peculiarly doting. Now Corinna Ross and her colleagues at the University of Nebraska have found evidence to suggest the former oddity explains the latter. They have done so by showing that chimerism extends to all sorts of tissues, including marmoset sex cells.

That insight arose when Dr Ross DNA-fingerprinted 12 types of tissue from 39 dead marmosets, in order to work out how frequently cells containing a twin's genome occurred in different parts of the body. Marmosets are not equal mixtures of two genetic individuals. They become chimeras not because their embryos merge but because more often than not their placentas do. They thus share their embryonic blood supplies. That allows them to exchange stem cells, which then develop into more specialised sorts of cell in their new bodies.

Chimeric cells were scattered everywhere, including the sex cells. This means that, in principle, a marmoset can either father (if male) or give birth to (if female) a baby that is its niece or nephew rather than its son or daughter.

To find out if this actually happens, Dr Ross turned her attention to living marmosets. She used hair, blood and saliva samples to identify chimeric animals among the 36 sets of twins in her colony. First, as she reports in this week's Proceedings of the National Academy of Sciences, she proved the marmosets made babies from their chimeric sex cells. This happened even when the twins were brother and sister rather than being of the same sex.

That is bizarre, because female mammals have two copies of the X chromosome, and males have one X and one Y. Any chimeric female that passed on her twin brother's genes must have developed eggs from an XY stem cell instead of the normal XX. Conversely, a chimeric male that passed on his twin sister's genes grew sperm from an XX stem cell, rather than XY. Dr Ross found examples of both.

This done, she tried to work out whether chimerism influenced parental care in marmosets. Zoologists think of caring for others as selfish behaviour, which animals only bother with to improve the chances of their genes being passed on to future generations. Creatures therefore help to bring up babies only to the extent that they believe they are related to them. In marmosets, caring means carrying, so Dr Ross recorded the time each baby marmoset was carried by its mother and by its father during the first fortnight of life. Mothers spent less time carrying those of their offspring who were chimeras than they did carrying those that were non-chimeric. Fathers, however, behaved in the opposite way.

Why that happened is not clear. But one possibility is that, in a species in which females routinely mate with several males, chimerism evolved as a way of duping males into looking after offspring that are not their own. Chimerism, in other words, might confuse a male who fathered one twin into thinking he is dad to both.

This could happen because a chimeric twin that grew from an egg which his sperm did not fertilise would nonetheless have some skin cells containing his genes. The scent-producing cells of that skin would give off pheromones signalling his paternity. Thus by mixing two fathers' genes between two infants, a female creates a situation in which both males consider it in their interest to care for both offspring—a double bonus.