The polar bear: from children’s books to grim tales of climate change, it’s a species that captures the hearts and imaginations of humans. But where did these white, hardy bears come from? And how do they live on an extreme diet in an extreme environment?
By analysing the DNA of polar bears, collaborative research involving Maynooth University and an international team of scientists have discovered that polar bears are a relatively new species that have genetic adaptations that help them to thrive on a high-fat lifestyle.
“This is the most comprehensive genomic study of a non-human mammal that has ever been carried out,” says researcher Prof James McInerney of the international study, which is being published in the journal Cell.
Prof McInerney, who co-directs the Bioinformatics and Molecular Evolution Unit in the Department of Biology at Maynooth University, was approached to take part in the study by the Beijing Genomics Institute, which has an interest in the genetics of animal species that live in extreme environments.
“They are looking at a range of animals in order to understand what happens to gene sequences in environments at the limits,” he says. “And they asked us to do evolutionary analysis on the genetic information from Polar bears.”
The population-based study involved sifting through DNA data from almost 70 Polar bears that span territories from Greenland and Alaska through to Northern Russia, and also from 10 brown bears. Getting these numbers of animals into the study gave it more strength, notes Prof McInerney. “As you sequence various members of the species you improved the overall understanding of each individual gene, they reciprocally illuminate each other,” he says.
The resulting data allowed the researchers to look at how diverse the bears were, at the genes that seemed to adapt positively to extreme environments and to figure out what genetic sleight of hand could support the Polar bear’s extraordinary body fat.
Younger than your average bear
One of the reasons that the study looked at the diversity of genes in the Polar bear populations was to get a handle on when the species arose, explains Prof McInerney. “The older a species is the more genetic diversity you tend to see in that species,” he says, noting that the analysis found Polar bears are relatively new to the scene. “Previous estimates had thought that Polar bears have existed for maybe as long as four million years, but we have reduced this estimate considerably to approximately 400,000 - 600,000 years.”
The research points to Polar bears diverging from brown bears at a time when ice covered far more of the planet than it does today, and Prof McInerney surmises that the white bear would have had some advantage for camouflage. “A Polar bear hunts by blending in, it creeps along on its belly to an unsuspecting seal,” he says. “On ice a white bear will do better than a black bear, and so the Polar bear and its offspring will tend to do better.”
Another crop of genes that seem to have helped the Polar bear adapt to the chilly climes of the Arctic have to do with fat metabolism. “Polar bears thrive on fat in their diet, and their body is 50 per cent fat,” says Prof McInerney. “If you are 50 per cent fat as a human, you are really unhealthy, but if you are 50% fat as a polar bear you are in great shape. So how does that become normal - what genes are affected and what types of gene variants make this physiology and body shape and mass fine and healthy?”
The analysis of the Polar bear genome spotted that genes involved in fat metabolism look radically different from other species, and the variants seem to have been selected to make the Polar bears better equipped for Arctic life. “These genes popped out of the analysis - the polar bear has varieties of these genes that we knew nothing about before,” says Prof McInerney. “Evolution has done the experiments on how to handle fat, and it has done them in the polar bear.”
Understanding how the Polar bear manages fat could eventually have implications for human health too, notes Prof McInerney. “If we can understand how the Polar bear stays healthy and protected from heart disease despite such high fat and cholesterol levels, we might get insights into developing protection in a human system. But this is far down the line - at the moment, what we are doing is fundamental research that is pointing us to interesting gene adaptations.”
Scaffold for future knowledge
The genomic collection and analysis, which was led by Eske Willerslev in Denmark and involved collaborations with The Beijing Genomics Institute, the University of California at Berkeley and Dublin City University has now built a “scaffold” for Polar bear genetics that can be enriched as more information about bears gets collected.
“Bears from zoos or animals that have been recovered or found can be added to our sample for a relatively small amounts of money - and this will build up a better overall understanding,” says Prof McInerney. “This means science will be able to look more closely at diversity and threats to the Polar bear population. It is iconic for climate change and it is a species we keep a close eye on.”