A large number of cells within the brains of marmosets actually come from their siblings, potentially influencing their behaviour 鈥� and the same may be true of human fraternal twins.
We already know that marmosets, small monkeys native to South America, carry their siblings鈥� cells in their blood. This is because, unlike other primates, most marmosets have non-identical twins or triplets, which exchange blood cells while in the uterus via a shared circulatory system. These swapped cells are retained after birth, meaning that up to .
What has been less clear is whether sibling cells can also find their way into marmosets鈥� brains and other organs. To find out, at Harvard University and his colleagues collected a total of over 2 million brain cells聽from 11 deceased marmosets and analysed their genomes. Two of the animals were twins, another two were non-twin siblings, while the rest were unrelated.
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They discovered that two types of immune brain cells 鈥� microglia and macrophages 鈥� often came from siblings, but other brain cells like neurons did not. In total, 20 to 52 per cent of the marmosets鈥� microglia and 18 to 64 per cent of their macrophages contained their siblings鈥� genomes. These cells, unlike neurons, develop from blood stem cells, which are exchanged between siblings in utero, and presumably make their way to the brain.
Microglia and macrophages are best known for responding to infections and damage inside the brain. However, they are also increasingly recognised for their role in regulating the number of neurons in the brain and the wiring of neural circuits.
That means that sibling-derived microglia and macrophages 鈥渃ertainly have the capacity to influence the brain鈥檚 development and therefore the marmosets鈥� behaviour,鈥� says at Georgetown University in Washington DC, although the nature of this influence is still unclear. 鈥淚t makes you wonder whether the sibling cells could serve that sibling at the expense of the resident [marmoset], like sibling rivalry from within.鈥�
The presence of sibling cells inside marmoset brains may help to explain why they are one of the few primate species with a cooperative care system, in which fathers and older siblings assist mothers with infant care, says at the Texas Biomedical Research Institute. 鈥淚t may lead to an alteration in kin recognition or what they recognise as 鈥榮elf鈥�,鈥� she says.
Ross and her colleagues previously found that cell-swapping between marmoset siblings in utero can also bizarrely lead to males carrying their brothers鈥� sperm, adding further complexity to sibling relationships because male marmosets can father children that, from a genetic point of view, are actually their brothers鈥�.
Human twins also share blood in utero and fraternal twins, who have different genomes, have been found to after birth. As a result, it is possible that, like marmoset fraternal twins, their brains are partly made up of each other鈥檚 cells too, says Ross. One way to investigate this would be to conduct a similar analysis on brains donated from deceased twins, says del Rosario.
bioRxiv
