Ross, C., J. French, and G. Ortí. 2007. Germ Line Chimerism and Paternal Care in Marmosets (Callithrix kuhlii). Proc. Natl. Acad. Sci. USA, 104 (15): 6278–6282.
The formation of viable genetic chimeras in mammals through the transfer of cells between siblings in utero is rare. Using microsatellite DNA markers, we show here that chimerism in marmoset (Callithrix kuhlii) twins is not limited to blood-derived hematopoietic tissues as was previously described. All somatic tissue types sampled were found to be chimeric. Notably, chimerism was demonstrated to be present in germ-line tissues, an event never before documented as naturally occurring in a primate. In fact, we found that chimeric marmosets often transmit sibling alleles acquired in utero to their own offspring. Thus, an individual that contributes gametes to an offspring is not necessarily the genetic parent of that offspring. The presence of somatic and germ-line chimerism may have influenced the evolution of the extensive paternal and alloparental care system of this taxon. 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. Consistent with this prediction, we found a significant correlation between paternal care effort and the presence of epithelial chimerism, with males carrying chimeric infants more often than nonchimeric infants. Therefore, we propose that the presence of placental chorionic fusion and the exchange of cell lines between embryos may represent a unique adaptation affecting the evolution of cooperative care in this group of primates.
Translation: According to the seminar I went to today, what this all means is that Marmosets and their relatives almost always produce fraternal twins, and the embryos grow in close proximity, with out the usual membranes separating them. The two developing embryos can actually have blood vessels in common, meaning that blood born cells can move from one embryo to the other. And stay there. And develop. So when the little monkeys are born and grow up, they can be riddled with cells that are genetically part of their sibling. This is what we call a chimera, when one individual has cells that are of different genetic lineages.
So then one of the chimeric monkeys mates. But some of his germ line cells (the ones that make sperm) are genetically his brother. So he's doing the mating, but the young could be genetically his nephews. Weird, I know. And one outcome of all this is that marmosets put a lot more energy into taking care of their nieces and nephews than would otherwise be expected. Ain't evolution weird and wonderful.