Whales and dolphins: the lost genes

Tens of millions of years ago cetaceans - whales and dolphins - evolved back into the oceans. To survive, they needed complex new features like fins and blowholes, as well as the genes to make them. But it seems they also lost certain genes as well. 85, in fact, according to a new study by German and American scientists. Producer Mariana Marasoiu heard more from Micheal Hiller...

Mariana - When you hear of whales and dolphins, what comes to mind? Perhaps smooth shiny animals in the depths of the sea, surrounded by dark blue-green water with faint shimmering from the light far above. Sometimes jumping out of the water for a breath of air, or just for a little play.

Wait. But whales and dolphins are mammals! Millions of years ago, they used to live on land. Their ancestors probably looked like wolves on hooves. How did they end up living underwater? Michael Hiller and his colleagues from the Max Planck Institute of Molecular Cell Biology and Genetics investigated a certain type of genetic change that may have helped whales and dolphins in this transition from land  to water.

Michael - We pretty much used the genomes of living whales and dolphins, and we search for genes that exhibit the same mutation in these species. If you have exactly the same mutation in the genomes of different mammals that are related, this tells you that this mutation occurred already in the ancestors of these species. And with this we could single out 85 genes that were lost during this transition from land to water.

Mariana - Why would we want to lose a gene?

Michael - I think there are 2 main principles that could explain when genes actually get lost. One is - and this probably explains the majority of the gene losses that we found in this study - that the function of the gene is no longer required. And then there is no selection pressure to maintain the gene, and it will eventually accumulate mutations that destroy its function and then we would call the gene ‘lost’. In the second, potentially more interesting, although probably also much rarer, mechanism is: that the loss of the genes is actually a benefit. We think we found in this study a few cases where the loss of genes could have been beneficial for the ancestors of whales and dolphins when they actually transitioned from land to water.

Mariana - One example that they found that seems to have been very useful in this transition is the elimination of a protein that repairs DNA damage. Because DNA is basically just a chemical entity, it’s subject to assault from the environment and interaction with other chemical molecules which can damage it. DNA damage happens all the time, but the body has repair proteins that walk over the DNA molecule and try to fix any damage that occurs. Isn’t it a bad thing that whales lost one of these proteins? Well, it turns out that these proteins are sometimes a bit faulty, and some of them are more accurate at repair than others.

Michael - We found that whales and dolphins have lost the protein that’s the most sloppy. What is interesting is that the ability to repair DNA damage is actually increased, so the fidelity is higher. And the mechanism behind why losing this protein then enhances the fidelity of DNA repair is most likely that the other proteins that are more accurate actually compensate for the loss.

Mariana - Having an increased ability to repair DNA damage may explain how whales and dolphins are able to survive for so long under water, as DNA suffers an increased risk of damage during the deep diving and surfacing cycle from oxidative stress,when there is an imbalance between reactive oxygen species and antioxidants. And this is not the only gene loss-based adaptation that seems to help cetaceans thrive. Some of the genes that help with blood clotting were also inactivated, which suggests a reduced risk of blood clots forming inside blood vessels as they get more and more compressed during those deep diving periods. The team may have also found a gene loss that explains how whales and dolphins are able to sleep underwater without drowning.

Michael - The loss of the melatonin synthesising gene, so melatonin being the sleep regulating hormone, is potentially a link or association with a particular form of sleep that cetaceans - whales and dolphins - have: and this is they only sleep with one brain hemisphere at a time while the other brain hemisphere is awake, and likely coordinates movements and coming back to the surface for breathing.

Mariana - While the researchers looked in this study at only one type of change in the genome, the loss of genes, there are many other types of genetic mutations and changes that happened.

Michael - There is definitely much more to learn and our understanding of the changes in the DNA that are required to turn a land animal into an aquatic animal is at the moment quite rudimentary, so hopefully we can make progress on that in the future.