Selfish DNA

How could I not read about a bit of DNA called R2d2. But it turns out this DNA does not share any of the personality of its namesake (a droid from the original Star Wars movies).

R2d2 is a piece of DNA found in mice. It contains multiple copies of a specific gene (Cwc22). When there are seven or more copies of that gene R2d2 turns selfish during egg development.

So, how this is supposed to happen is that there are two copies of each chromosome (the chromosomes are bunches of genes and other genetic information that are grouped together onto a single strand). You get one chromosome from each parent. This means they are not identical (you get different genes from each parent). Normally, the two versions of each chromosome will separate during gamete (egg and sperm formation). In males, you end up with four sperm from each original cell (the DNA replicates before it divides). But in females, you only get one. The DNA separates the same as it does in sperm, but the rest of the cell does not separate equally. This is why eggs are so much bigger than sperm, and why things like mitochondrial DNA can be traced through your mother. The male really only contributes genetic material, the rest of the things you need in the cell come from the female.

Normally, there is a fifty-fifty chance of an egg getting a chromosome from the father or from the mother. It gets a copy of each chromosome but which one is determined randomly. But R2d2 changes this. It is more likely to be incorporated into the viable egg. This wouldn’t be a problem if R2d2 had an evolutionary advantage, in other words if it would help the mice who had it to survive and reproduce. But it doesn’t. It has a negative effect. Female mice that have one copy of R2d2 produce smaller litters.

In normal situations, having smaller litters would mean that R2d2 would be selected against. More mice should be being born that don’t have R2d2. But lab studies have shown that R2d2 can increase in a population incredibly quickly.

How does this affect wild populations? It seems that the results are really variable depending on the population. Some populations have higher percentages of R2d2 and others have very small or even no evidence, yet. More research is definitely required.

So what does all this mean in the wider world of population genetics and studies of evolution?

Generally speaking, we assume that genetics that are selected for offered some sort of advantage. Being a better jumper as a kangaroo resulted in increased survival rates or increased offspring or both. But based on this evidence, it is possible that some traits that we think must have some evolutionary advantage actually didn’t, they just violated Mendel’s laws (that’s the fifty-fifty chance).

Reference:

https://www.sciencenews.org/article/selfish-dna-flouts-rules-inheritance?tgt=nr