Within each of our cells we have two kinds of DNA. We are most familiar with ‘nuclear DNA’, which is what most people mean when they talk about DNA. This DNA is inherited from both parents, and contains the code for almost everything we need to function.
The other type of DNA is called ‘mitochondrial DNA’. As opposed to nuclear DNA, mitochondrial DNA is inherited only from the mother. This DNA is found inside special structures in our cells called mitochondria. Mitochondria generate energy from the food we eat, and mitochondrial DNA contains code that programmes them to do this.
Scientists can use both nuclear and mitochondrial DNA to understand the evolutionary history of a species. They do this by looking, for example, at how similar DNA of the same type is between populations. If DNA between populations is very different, this suggests that the populations have not interbred much in the past. If it is very similar, this suggests that populations have interbred, and/or are currently interbreeding.
Usually, both nuclear DNA and mitochondrial DNA tell the same evolutionary story. But sometimes, they tell opposing stories. This is called ‘mitonuclear discordance’.
An example of mitonuclear discordance is where nuclear DNA is very similar between some populations, suggesting that those populations regularly interbreed, while mitochondrial DNA is very different between those populations, suggesting that those populations do not mix. This is a pattern seen in several shark species.
For a long time, scientists thought that mitonuclear discordance in these shark species was due to a difference in mating behaviour between the sexes. Females tend to breed in the place they were born, while males tend to roam around and breed wherever they can. This means that nuclear DNA, inherited from both males and females, becomes well-mixed across populations, but mitochondrial DNA, which is inherited only from females, remains specific to each population.
A recent study on great white sharks has thrown this theory up in the air, though.
In the new study, the researchers first confirmed what previous research has found: that in great white sharks, nuclear DNA suggests some populations interbreed, while mitochondrial DNA suggests these populations do not mix much. Then, using state-of-the-art simulations, they tested the theory that this mitonuclear discordance is because females breed where they are born while males do not. They found no support for this theory.
Female great white sharks do tend to breed where they are born, and males do tend to roam around, but this does not explain the opposing stories their DNA tells.
Now that we know that the mating behaviour of the sexes is not the cause of mitonuclear discordance in great white sharks, the research team suggest we need to rethink this assumption across shark species.
“We would like to dig more into the potential selective processes shaping the mitonuclear discordance,” lead author of the study, Romuald Laso-Jadarta, tells BBC Widlife. This would require more data, which means more wild great whites need to be sampled.
Until that happens, there aren’t any other theories to really sink our teeth into. All we know is that something a bit fishy is going on with shark DNA, and more research needs to done to figure out why.
Top image: great white shark. Credit: Getty
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