During pregnancy, fetal cells enter the mother (and vice versa). They linger in the mother for decades at minimum – probably for life. The first thought has been that this might be the cause of higher levels of autoimmune diseases in women, and that might be so. But there are more interesting possibilities.
There is evidence that fetal cells may be beneficial. Experimental work in mice shows that a sub-population of fetal cells (pregnancy-associated progenitor cells, or PAPCs) have stem-cell-like properties. They show up at injury sites, and there are some indications that they contribute to tissue repair. For example, after heart damage in mother mice, fetal cells engraft in the heart and differentiate into cardiomyocytes. They cross the blood-brain barrier and apparently differentiate into neurons.
We now know that fetal cells also enter the brain in humans, but we do not yet know if they act as neurons there. But that’s the way to bet.
The question is, what acts by these fetal cells would be favored by natural selection? Aiding Mom’s heath and survival would obviously be favored: that would allow her to care for the child that threw off those fetal cells, which would increase the kid’s fitness. This would be true even after menopause, in humans. Which raises the question: what can those fetal cells do that Mom can’t do for herself? Well, for one thing, they’re younger, have longer telomeres, have fewer somatic mutations. Maybe they’re good at repair. If they’re contributing to immune defense, well, they bring in different HLA alleles.
Of courser, they have their own genetic interests, which sometimes differ from those of the mother. The kid would benefit if Mom somewhat changed her ways in the direction of investing more in that child (acting as if it was twice as important as other full sibs) and if she had her later children with the same father: such children are twice as closely related to the shedder than stepsibs. That may not be the optimal course for the mother’s genetic interests, especially if the father is something of a loser – but the increased relatedness of full sibs would usually trump that, from the fetal cell line’s point of view.
Of courser the fetal cells of those later children have their own interests and will push in different directions. If these fetal cells can recognize the degree of kinship in future pregnancies, life gets complicated.
Since some women have only had male children, their fetal cells are easy to recognize (they have a Y chromosome) and I suppose we could figure out a way to zap them, possibly returning their mother’s brain to its original unmanipulated state. Except for Toxo and advertisers and whatnot. Feminists take note: if you have ever been pregnant, you’re not the same person anymore. With any luck, we will soon see a new version of Morgellons syndrome, with genetic formication.
One fun possibility is that some of these cells are passed on, generation after generation – from mother to a female fetus , and so on indefinitely, rather like mitochondria. It is easy to show that such cells would have to have a positive effect on female fitness. We know plenty of similar examples of symbiotic bacteria in insects – here, we’re talking about a symbiotic (technically, mutualistic) cell line that started out human. Such a symbiotic cell line would co-inherit with a particular type of mitochondrial DNA: if it conferred extra fitness, that mtDNA haplotype would expand rapidly.