Whales lost genes evolving to aquatic life

Whales and dolphins may be aquatic mammals today, but, 50 million years ago, their ancestors were actually land-dwelling animals. Now researchers have tracked down the genes they lost during the transition and adaptation to aquatic life...

Working at the Max Planck Institute of Molecular Cell Biology and Genetics, Michael Hiller and his colleagues investigated the genetic changes associated with the transition from land to water of marine mammals including whales, dolphins and porpoises.

"We could single out 85 genes that were lost during this transition from land to water," says Hiller. The roles of several of these genes were already known, so the researchers could explain the effect of losing them, and how their absence supports the underwater life of whales and dolphins.

Among the lost genes are those concerned with a reduced risk of blood clotting when diving, more effective repair of DNA damage, reduced lung inflammation that could otherwise be the consequence of repeated lung collapse and reinflation during deep dives. Others included genes linked to reduced saliva secretion, and, notably, absence of the sleep hormone melatonin. This latter genetic deactivation may explain how cetaceans are able to sleep with one eye open and half of their brain awake.

To find these genes in the first place, the team combed the DNA of living whales, dolphins and porpoises for sequences carrying fingerprint changes, or mutations, across the species.

Since they’re related to each other, this suggests that the mutations found had already occurred in the ancestors of these species. The search for mutations was then refined by selecting only those that were not present in the closest land relatives of cetaceans, the hippos.

When we think about the evolution and adaptation of organisms to their environments, we usually think either of new genes being added, or existing genes changing, or mutating to give the organism an advantage. But this paper found that losing genes can also play an important role.

"I think there are 2 main principles that could explain when genes actually get lost," explains Hiller. "One is 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 the second - potentially more interesting, although probably also much rarer mechanism - is that the loss of the genes is actually a benefit. And here we think that we found in this study a few cases where the loss of genes could have been beneficial for the ancestors of whales and dolphins in the transition from land to water."

It is an exciting time in the study of evolution. Genomics, the study of genes and DNA, is able to offer such explanations for how species evolved over millions of years and how they have adapted to living in completely different environments from their ancestors.