Foreign Drivers

Generally, a given nuclear allele has a 50% chance of showing up in an offspring.  But sometimes this is not the case –  some cut in line, and having a >50% of  transmission. Such an allele tends to increase, even if it gives no advantage to the organism carrying it.  In some case such an allele can become common, or even go to fixation (100% frequency) while reducing fitness.

The best-known example is the t-allele in mice.  It is transmitted well over 50% of the time,  but never goes to fixation, because male mice with two copies are sterile.

Sometimes  driving genes involve the centromere, a region at the pinched waist of each chromosome that plays an important role in directing chromosomal movement in meiosis.  The cheating opportunity results from the fact that there are four cells produced in female meiosis – the egg and three polar bodies that go nowhere.  A change in the centromere that increased its chance of ending up in the egg, rather than the polar body, would give it a big transmission advantage. The rapid rate of adaptive evolution seen in centromeres makes many suspect that centromeric drive occurs fairly often.

There are probably associated costs in some cases.  Meiosis might be messed up to an extent in males, or a deleterious gene variant might exist near the driving centromere and be  carried along with it.  Over time,  you would expect other genes to change in ways that minimized those costs.

A driving centromere that hadn’t been around long would show the usual signs of recent selection and might still be causing noticeable trouble.  One that been fixed for a long time would be hard to detect, and probably wouldn’t cause much trouble.

This has all been an introduction – the question is, what would happen if we admixed with a long-isolated group of archaic humans that had their own driving genes?  We were similar enough that such genes would probably have succeeded on an AMH genetic background.  If current thinking is correct, and there were several such admixture events in the last 50k years or so ( Neanderthals, Denisovans, and the unknown, highly divergent group in Africa (Mangani !) ),  such sweeps might be easy to see, and negative effects might also still exist and be detectable – the Revenge of the Neanderthals.

Partly I wonder about this because of the anomalously low success rate of human fertilization.  It’s very unusual and nobody yet has an any explanation.  Partly, also, because there is a version of 6.5 megabyte stretch across the centromere of the X-chromosome that is common in Africans and way more divergent than than any other  large chunk of the human genome. There have also been claims of sweeps in  the centromeric regions on several chromosomes in Eurasians.

If this happened to be true, it would also imply that gene flow between  the two groups question would have had to have been extremely low [zero, most likely] for a long long time, since even a wee bit would be enough to transmit something like this, whose success probably would not depend much on local factors like climate or ecology.