Narwhals are thriving despite extremely low genetic diversity

Narwhals
Narwhals are famous for their tusks

Mads Peter Heide Jorgensen

The narwhal gene pool is not very diverse. That is normally a problem for animal populations, making them less able to adapt to changing circumstances. But narwhals are doing pretty well – which suggests genetic diversity might not be as vital as we think.

Narwhals are toothed whales that live in the Arctic, and are famous for having an enlarged tusk that protrudes from their upper jaw.

Michael Westbury of the Natural History Museum of Denmark and his colleagues sequenced the genome of a narwhal from west Greenland.

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The genome of a single individual can tell biologists a lot about the genetic diversity of the species, not only now but deep in the past. For each section of the genome, every individual has two copies: one from their mother and one from their father. By comparing how similar these sequences are, and how areas of high and low similarity are spread throughout the genome, geneticists can piece together how closely related the animal’s parents were, and make inferences about ancestral populations.

The analysis found much lower levels of diversity throughout the narwhal genome compared with other Arctic marine mammals, like the beluga whale, the bowhead whale and the walrus.

Narwhals from above

Carsten Egevang

Low diversity is often the result of inbreeding, or a small population size. But neither of these explanations fits with the narwhal data. The global population is estimated at 170,000 – enough to change their official conservation status from “near threatened” to “least concern” recently.

Their genetics suggest the narwhal population has grown rapidly since the start of the last glacial period around 115,000 years ago, but before this had been slowly declining for about a million years.

This gradual decline might explain why the lack of diversity does not seem to be a problem. While a rapid population crash can mean important genetic variations are lost, the narwhals’ slow decline could have enabled them to preserve diversity where it matters.

A thriving population with low genetic diversity is unusual, but there are other examples, like the brown hyena, says Westbury.

We need to understand why a species lacks diversity before we consider it a problem, he says. “It seems that it’s most likely not a problem [for the narwhal], as there’s been a lot of climatic changes through time and they’ve been able to maintain the species despite the low diversity.”

However, the narwhal may yet be threatened by climate change in the future – especially if it proves to be more drastic than previously experienced. “It depends how quickly the environment changes,” says Westbury.

Journal reference: iScience, DOI: 10.1016/j.isci.2019.03.023

More on these topics:

https://www.newscientist.com/article/2201289-narwhals-are-thriving-despite-extremely-low-genetic-diversity/?utm_campaign=RSS%7CNSNS&utm_source=NSNS&utm_medium=RSS&utm_content=home

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