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When the terrestrial ancestors of today's whales and dolphins crawled into the seas a long time ago, they acquired many things, including fins, the ability to hold their breath for a long time, and their skin thick and hard. Along the way, they also ignored many features that were no longer relevant or useful.
In fact, as scientists report in a study published Wednesday in Science Advances, the loss of some genes from the common ancestor of whales and dolphins allowed them to shed features that would have been a liability under the waves , which could have contributed to the survival of future generations.
As more and more species genomes are sequenced, researchers can begin to determine which genes are shared by groups of organisms. Presumably, these genes have also been found in the groups last common ancestor.
A team led by Michael Hiller, geneticist at the Institute of Cell Biology and Molecular Genetics Max Planck and author of the new article, has used this technique with modern cetaceans, a group consisting of whales, dolphins and porpoises. They then compared this set of genes to those of the closest relatives of cetaceans, the hippopotamus family, and identified 85 deactivated or inactivated genes in the ancestor of cetaceans during its transition to aquatic life.
These genes have been implicated in a wide variety of processes, such as blood clotting, sleep, and hair growth. Although some genes have already been tagged, others have not been identified. (Dr. Hiller and his colleagues had previously found that genes needed for hair development had been lost in cetaceans, which may have reduced resistance when animals swam in water.)
"Most of the things we found were, to say the least, unexpected to me," said Dr. Hiller.
For example, one of the lost genes produces an enzyme involved in the repair of DNA. As whales and dolphins lack fresh oxygen for long periods, their DNA is often damaged and needs to be repaired. But this repair enzyme does a complicated job, introducing errors even if it corrects. Other less predisposed enzymes are found in cetaceans; This led the researchers to conclude that the messy enzyme gene had perhaps been discarded because, in a life situation with frequent repair of DNA, it was doing more harm than good.
Other lost genes have been implicated in blood clotting. You might think that dropping such genes may seem like a risky evolutionary strategy, but when cetaceans dive, their blood vessels shrink and their blood nitrogen bubbles increase the likelihood of clots forming. Getting rid of clotting genes may have been a healthy long-term gesture.
Four genes related to melatonin, a sleep hormone, have also been lost. In most creatures, melatonin production increases as the body prepares for sleep. But cetaceans do not sleep like most mammals. Instead, half of the brain is sleeping while the other half is monitoring the nearby environment. Living under water, "you can not really sleep for long periods without a body," said Dr. Hiller. In these circumstances, he added, melatonin could have been counterproductive.
As a general rule, Dr. Hiller said, the rule of thumb is that genes that are not actively used tend to disappear or be inactivated. But the new study suggests that the same process can purge the genome of genes that would make a new way of life too dangerous, an additional bag of which emerging species – which have become whales, porpoises, dolphins – would be better off.
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