IF YOUR sperm or eggs contained genes from another individual, how would you feel about your offspring? This is the extraordinary situation facing marmosets, in which such germ line chimerism has been identified for the first time.
Wied鈥檚 black tufted-ear marmosets (Callithrix kuhlii) are unusual in that a high proportion of pregnancies result in fraternal twins, and most of those are chimeras. Genetic analysis of the twins shows that all tissues 鈥� including skin, hair, brain, lung and muscle 鈥� may contain cells from their sibling. It happens early in development when the placentas of the twins fuse and blood stem cells pass between the two fetuses, 鈥渟ettle鈥� and grow in various locations in the body.
What Corinna Ross and colleagues from the University of Nebraska in Lincoln have now discovered is that embryonic stem cells also pass from twin to twin, and can settle in the reproductive organs. When the marmoset has offspring of its own, some of those animals may not actually be genetically theirs. Ross鈥檚 team found that in five out of 15 family lines they analysed using genetic markers, individuals had passed on their sibling鈥檚 genome to their offspring (Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.0607426104). To make matters more complex still, gestating twins may themselves become chimeric in the germ line.
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鈥淲hen the marmoset has offspring of its own, some of those animals may not actually be genetically theirs鈥�
Animals seem to 鈥渒now鈥� if they are dealing with chimeric offspring, in the sense that the chimerism influences their behaviour. Marmosets live in family groups, and engage in cooperative care where both fathers and older siblings look after the infants as well as mothers. Ross found that fathers carry chimeric offspring more than non-chimeric infants, while mothers carry chimeric infants less. 鈥淐himerism may promote paternal care by providing fathers with multiple cues of relatedness,鈥� says Ross.
He speculates that chimeric offspring may carry more genes that match the father, say in the skin and hair, than the mother. Such cues would give fathers greater confidence in their paternity.
In an evolutionary sense, Ross says, chimerism may promote the system of social care by increasing relatedness between twins, just as the increased relatedness in social insects such as ants and bees is thought to underpin their social system. Normally, full brothers and sisters share 50 per cent of their genes. Chimeric siblings are even more closely related. 鈥淚n many ways the marmoset social system appears similar to that of social insects, which have very complex relatedness patterns,鈥� Ross says.
Gavin Dawe, who works on stem cells and brain repair at the National University of Singapore, says it is remarkable that chimerism seems to affect the way mothers and fathers look after their offspring. Some kinds of chimerism do occur humans, he says, but germ line chimerism usually leads to sterility. 鈥淗owever, twins that 鈥榲anish鈥� in utero may not be that uncommon,鈥� says Dawes. 鈥淓ven if we do not have a living twin we may be carrying chimeric cells from our vanished twin.鈥�
David Haig, who works on intragenomic conflict at Harvard University, says marmosets provide an interesting setting for studying conflict in an individual. 鈥淚f the germ line is chimeric you could get sperm competition within the ejaculate of a single male.鈥�
