Great white sharks DNA difference mystery defies scientific explanation

Scientists have discovered a strange mystery in the DNA of great white sharks that they still cannot fully explain. To understand this, it helps to know there are two kinds of DNA in animal cells: nuclear DNA, which comes from both parents and is found in the center of the cell, and mitochondrial DNA, which is passed only from the mother and is found in the cell’s energy parts called mitochondria.

This research, led by Gavin Naylor and an international team of scientists, was published in the journal Proceedings of the National Academy of Sciences.

Here’s what the researchers found:


About 10,000 years ago, during the end of the last ice age, great white sharks were limited to a single group living in the southern Indo-Pacific Ocean because ice and low sea levels reduced their available habitat. As the Earth warmed and ice melted with time, sharks spread out into different parts of the world.

Today, there are three main groups of great white sharks: one near Australia and South Africa, another in the northern Atlantic Ocean, and a third in the northern Pacific Ocean.

Despite being spread across the globe, great white sharks are still rare—only about 20,000 exist worldwide, which is very low compared to many animals. This small population size means their DNA can show unusual patterns.

Scientists noticed that the sharks’ nuclear DNA is very similar across these groups, but their mitochondrial DNA shows big differences. They thought this was because female great whites tend to return to their birthplace to mate (called philopatry), while males travel widely and mix the nuclear DNA among populations. Since mitochondrial DNA only comes from females, it would stay unique in each group while nuclear DNA would be more mixed.

Q. Wha is philopatry?
A. Philopatry means that female sharks tend to return to the same specific area where they were born to mate and give birth. While female sharks travel long distances to feed, they come back to these familiar breeding grounds regularly. This behavior keeps the mitochondrial DNA, which is inherited only from the mother, more distinct within different shark populations because females don’t mix widely between groups.

But when scientists analyzed a large amount of DNA from sharks worldwide, they found that this idea doesn’t completely explain the differences. The mitochondrial DNA had changed more than could be expected from female breeding behavior alone.

They also considered other explanations:

• Maybe only a few females in each generation had babies (called reproductive skew), but tests showed this wasn’t the case for great whites.
  
• Maybe random genetic changes in small populations (genetic drift) caused the mitochondrial differences, but if this were true, nuclear DNA would be affected similarly—and it isn’t.
  

That leaves natural selection as a possible cause. Natural selection means certain traits become common because they help animals survive or reproduce better. In this case, it would mean that sharks with certain mitochondrial DNA survive while others do not, and so only that DNA type is passed on. But natural selection usually works better in large populations, not small ones like great whites, making this explanation seem unlikely.

So, the big question remains: why is there such a big difference between nuclear and mitochondrial DNA in great white sharks? Scientists admit they do not yet have a clear answer.

To sum up, scientists are still puzzled by why the two types of DNA in great white sharks show such different patterns. The study shows the difficulties of studying such rare animals and that more research is needed to solve this mystery.