Like stripping down to swim, the ancestors of whales and dolphins may have shed some genes during their transition from being landlubbers to aquatic dwellers.
Ancestors of orcas, bottlenosed dolphins and other cetaceans lost function of at least 85 genes as the animals adapted to live full time in water, researchers report September 25 in Science Advances.
Scientists compared DNA of whales and dolphins with that of other mammals to find 236 genes missing from cetaceans. Of those missing genes, 85 are still present in hippopotamuses, cetaceans’ closest relatives, suggesting that the genes were lost during the land-to-water transition about 50 million years ago.
Cetaceans may have adapted to diving by jettisoning genes involved in regulating blood pressure and blood clotting, and in repairing DNA. DNA undergoes damage from cycles of low and high oxygen as animals dive to deep water and resurface again. One of the lost genes, POLM, encodes a DNA repair enzyme that is error-prone even under the best of circumstances, so getting rid of it may have given cetacean ancestors an advantage. “We think that by losing the sloppiest protein involved, you probably increase the fidelity of DNA repair,” says evolutionary genomicist Michael Hiller of the Max Planck Institute of Molecular Cell Biology and Genetics in Dresden, Germany.
Giving the heave-ho to other genes involved in lung function may ultimately have helped whales and dolphins avoid damage when their lungs temporarily collapse during deep dives.
And losing another gene, SLC4A9, which regulates saliva production, may have been either a use-it-or-lose-it scenario (SN: 4/4/19) or a less-is-more bonus (SN: 3/9/11) or possibly a bit of both, Hiller says. Because they live in water, dolphins and whales don’t need saliva to lubricate or help break down food, so that gene could safely go. Producing less saliva may also help the animals retain fresh water inside their bodies, which would be a perk when living in moisture-sapping saltwater, he says.
Cetaceans also lack the enzymes needed to produce the sleep-inducing hormone melatonin, Hiller’s team found. That hormone may no longer be necessary for these animals, which sleep using only one side of their brains at a time (SN: 10/9/09).