Sleight of evolution: Unlocking genes that turned whales into giants
By Darren Incorvaia
NEW YORK: With names like titanosaurus and giganotosaurus, you’d be forgiven for thinking that dinosaurs were the most massive animals to ever live. But if you’re after gargantuan creatures, chartering a boat is better than building a time machine — the largest animals to ever exist are whales, topped off by the truly titanic blue whale.
Maxing out at almost 100 feet long and 190 metric tons, the blue whale dwarfs the competition.
Despite being well-known for their enormous size, big bodies actually came into fashion relatively recently among whales — only five million to 10 million years ago, practically yesterday in geologic time.
The first whales were the size of wolves and sea lions, a far cry from the submarine-size behemoths that swim the seas today.
A prehistoric boom of ocean nutrients and the freedom from gravity that water provides played a role in making whales so large, but the actual genetic changes underpinning their ballooning have remained a mystery.
To examine how changes to whale DNA over time may have caused their gigantism, Mariana Nery, a biologist at the State University of Campinas in Brazil and colleagues used a targeted approach. The results, published Thursday in the journal Scientific Reports, implicate genes that have functions in growth hormone and insulin pathways in causing the enormousness of whales.
Dr. Nery and her team took advantage of the fact that humans have long studied the genes that affect body size in some of whales’ closest relatives, like horses, sheep and cows.
When comparing nine of these body size-related genes across 19 whale species of different sizes, the researchers found evidence of positive natural selection in four genes — that is, there were changes in the DNA that correlated with a bigger body.
The study’s finding about positive selection “isn’t the definitive answer,” but rather a first step to saying whether these genes are the cause of whales’ colossal size, says Michael McGowen, a biologist at the Smithsonian National Museum of Natural History in Washington, D.C., who was not involved with the study.
Dr. Nery agreed: “This is a little piece of the big puzzle,” she said. Future work will involve a look at the regulatory DNA regions that control these genes and also a thorough sweep of entire whale genomes for more potential culprits, she said.
One result that the team wasn’t expecting came from a gene called E.G.F., or epidermal growth factor; or rather, from the lack of it.
The E.G.F. gene of the biggest whales, the filter-feeding baleen whales, has become non-functional over evolutionary time, turning into what scientists call a pseudogene.
“That, I think, is one of the most interesting results,” Dr. McGowen said.
In addition to size, E.G.F. is also important for the development of teeth — which baleen whales don’t have. It’s not clear when E.G.F. became a pseudogene in the ancestor of baleen whales, Dr. Nery said, but “we can infer it was related to the emergence of lunge-feeding,” the way baleen whales scoop up huge mouthfuls of plankton. Being as big as a whale has obvious advantages, like scaring off predators, but there should be downsides, too, like an increased risk of cancer.
More cells in a body means more cell divisions, which means more chances for cancer to develop.