About 50 million years ago, ancestors of the modern whale transitioned from land to sea, undergoing remarkable transformations in the process. They gained collapsible lungs, thick layers of blubber and blood that stores more oxygen. But they also shed many traits that were critical for terrestrial life, such as genes involved with sleep, blood clotting and DNA repair, a new study published in the journal Science Advances suggests.
Researchers compared the active genes found in modern cetaceans, which include whales, dolphins and porpoises, with those of other mammals such as their closest living relatives, the hippo family. They identified 85 genes that became inactive when cetaceans became fully aquatic, 62 of which had not been reported before, reports Veronique Greenwood at the New York Times.
Previous studies found that the genes that enabled hair growth, sweat and hind limbs had been lost in cetaceans. But the new findings go even further to describe the genetic reasons behind such major physiological, behavioral and anatomical changes.
“There have been a lot of studies like this, but this has probably been the most comprehensive in terms of the number of genes,” Michael McGowen, research scientist and curator of marine mammals at Smithsonian Institution’s National Museum of Natural History, who was not involved in the study, tells Smithsonian magazine.
Some of the inactive genes that Hiller and his team identified simply became obsolete in a marine environment. These “neutral losses” include a gene that produces saliva. Other losses seem to be driven by the necessity of adapting to a new aquatic lifestyle.
Blood clotting, for example, may seem like an advantageous mechanism in mammals. Yet, when cetaceans dive, their blood vessels constrict and nitrogen bubbles make the blood clot more easily, restricting the flow of much-needed oxygen in the bloodstream. Ridding the body of clotting genes makes diving less dangerous.
Though they are air-breathing mammals, whales and dolphins often go for long periods of time without taking in fresh oxygen. This behavior can cause DNA damage that may result in the formation of tumors and other maladies. The enzyme that repairs this type of DNA can be faulty enough to cause serious harm. Because cetaceans undergo frequent DNA damage, researchers suspect that this enzyme was eventually ditched in favor of less harmful restorative enzymes.
“We think that by losing the sloppiest protein involved, you probably increase the fidelity of DNA repair,” Hiller tells Tina Hesman Saey at Science News.
Additionally, modern cetaceans are missing four genes related to the production of melatonin, a sleep hormone. Unlike most aquatic creatures, at least half of a cetacean’s brain is alert at all times to signal when to surface for a breath of air. Melatonin can put the body into a deeper restive state, which is dangerous for whales and dolphins who can sink or drown during long stretches of inactivity.
While evolutionary scientists commonly accept that underutilized genes tend to disappear or become inactive during the evolutionary process, this study suggests that genes potentially dangerous to a new lifestyle can also be abandoned or become non-functional.
"We found new evidence that loss of genes during evolution can sometimes be beneficial, which supports previous results from our lab suggesting that gene loss is an important evolutionary mechanism,” says Hiller in a statement.