Blue whale genes show less inbreeding and more crossing than expected
In 2014, nine blue whales died after becoming stuck in the ice near Newfoundland, Canada. Two of their carcasses washed ashore and began to swell as they decomposed, raising concerns of an impending explosion and possibly inspiring a sketch on “Saturday Night Live” that ended with Charlize Theron covered with whale intestines. More recently, the colossal corpses of these unfortunate mammals have made important contributions to science.
Nearly a decade after their deaths, researchers analyzed the beached whales' DNA – along with samples collected from 26 other blue whales – to create the best results. to date provides an accurate picture of the structure of the North Atlantic population. The results, published this month in the journal Conservation Genetics, reveal a ticking time bomb in blue whale demographics, peculiar migration patterns and even clandestine matings between species.
The world population of blue whales was severely damaged by industrial whaling in the early 20th century. A declining population can increase inbreeding rates, which can reduce variation in a population's gene pool and increase the risk of species extinction.
With fewer than 3,500 adult blue whales left in the North Atlantic, scientists expected to find such a genetic bottleneck. At first they were surprised when their genomic analysis showed no signs of this. They say this could be a result of the time it takes blue whales to reproduce – about 30 years to go from one generation to the next.
“It's probably not been enough time to really see a bottleneck effect,” said Mark Engstrom, a genomicist at the Royal Ontario Museum in Toronto, who oversaw the research effort.
That blue whales in the North Atlantic have “maintained their genetic variation despite about a century of industrial whaling” is good news, says Axel Janke, a genomicist at Goethe University Frankfurt in Germany, who was not involved in the study.
But if the population doesn't grow over the next century, Dr. Engstrom says, he expects a bottleneck to emerge — to the detriment of the whales.
Whalers historically thought that the North Atlantic blues were split into distinct eastern and western populations. But the whales' DNA tells a different story. The team of Dr. Engstrom found ample evidence for mixing between east and west.
“There are differences,” he said, “but that is a fairly recent split.” Typically, genes flow from western blue whales to those in the east. The team speculates that this movement may reflect the North Atlantic Current, in which seawater flows from west to east. Western whales may follow their favorite food, krill, as the waves wash the prawns east, where they encounter another population of whales.
Even more surprising, genes cross both evolutionary and geographic boundaries.
It has long been known that blue whales mate and reproduce with fin whales, even though the two are not particularly close relatives – and blue whales can be over 7 meters longer and as much as 85 tonnes heavier. Many hybrid animals (like mules for example) are infertile. But armed with their new data set, Dr. Engstrom evidence of persistent mixing between species. All told, about 3.5 percent of the blue whale's genome comes from fin whales.
“This is fantastic,” said Dr. Janke. He had previously looked for blue whale DNA in fin whale genomes but didn't find it. The new data, coupled with an analysis that Dr. Engstrom has conducted, suggesting that the hybrids, although not sterile, only mate with blue whales.
“What traits are transferred from the fin whale to the blue whale?” asked Doctor Janke. He wonders whether these genes benefit blue whales in any way.
Dr. Engstrom plans to collect more blue whale genomes, of which Dr. Janke hopes it can help scientists better understand populations in other parts of the world. It's a big ocean, and with dwindling numbers of blue whales, knowledge is power, he said.
“There's kind of a motto that we have,” said Dr. Janke. “You can only protect what you know.”