Determining the mutation rate of cetaceans is not an easy task, as it is difficult to locate and sample them. Photo: Julia Hager
Before commercial whaling began, the number of humpback whales in the North Atlantic was apparently 86 percent lower than previously thought. The new estimate is based on recalculating the mutation rate using the DNA of family groups.
By measuring the mutation rate, it is possible to understand evolutionary processes and estimate the size of historical populations. An international research team led by the University of Groningen and the Center for Coastal Studies in Massachusetts, USA, has now used the pedigree method to calculate the mutation rate of humpback whales, fin whales, blue whales and bowhead whales in the North Atlantic for the first time. The results, published in the journal Science, show that the mutation rates of the whales are much higher than the rates previously determined using the less accurate phylogenetic method and they are comparable to those of smaller mammals such as dolphins, apes or humans.
From the newly calculated, higher mutation rates, the scientists were able to infer the historical population sizes of the whales in the North Atlantic. They found that the number of humpback whales in the region before commercial whaling was closer to 20,000 individuals. Previous estimates based on the phylogenetic method, which relies on fossil records, were 150,000 animals. The North Atlantic Marine Mammal Commission (NAMMCO) estimates the current humpback whale population in the North Atlantic at around 35,000 animals.
The newer pedigree method provides much more accurate results because it is based on very few assumptions and is therefore very suitable for comparing mutation rates of different species. To determine the rates, the researchers needed DNA samples from a parent pair and its offspring to identify new mutations in the offspring.
For the current study, the team, which consists of scientists from the Netherlands, the USA, Greenland, Denmark, Canada and the United Kingdom, was able to draw on skin samples from whales collected as part of a collaboration that has existed for more than 30 years. From all these samples, the researchers identified trios of both parents and their calf by using microsatellite markers in the DNA to create genetic fingerprints of the individuals. “I sifted through the microsatellite data to find individuals that were related as mother and calf. Next, I looked for possible fathers in the database,” Marcos Suárez-Menéndez, a researcher at the University of Groningen and lead author of the study, explains in a university press release.
Suárez-Menéndez was able to identify 212 parent-offspring trios in the four different whale species. The researchers sequenced the genome of eight of the trios and then estimated the number of new mutations in the calf and the average mutation rate in the whales. “And just like in humans, most new mutations originate from the father. So in that respect, whales are very similar to us,” Suárez-Menéndez said.
In addition, the team also analysed DNA from the mothers’ mitochondria: “Our study revealed that the mutation rate in whale mitochondrial DNA is also much higher than earlier estimates based on the phylogenetic method,” Suárez-Menéndez further explained.
Along with the important finding that the pedigree method makes it possible to determine the mutation rate in wild animal populations as well — until now it has only been applied to a few wild animals and zoo animals — the new estimate of the number of humpback whales in the North Atlantic is of particular importance: This information contributes to a better understanding of the state of the oceans before whaling.
Julia Hager, PolarJournal
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