Skip to main content

POLAR BEAR RESPONSE TO GLOBAL WARMING: Insights from shotgun sequencing of the Polar Bear Genome

Final Report Summary - POLAR BEAR GENOME (POLAR BEAR RESPONSE TO GLOBAL WARMING: Insights from shotgun sequencing of the Polar Bear Genome)

The research carried out during my (Eline Lorenzen) Marie Curie IOF resulted in a seminal paper on polar bear evolution, which was published in the journal Cell in May 2014 as the cover story. The paper was the result of a large collaboration between UC Berkeley, BGI-Shenzhen and University of Copenhagen. In the study, we generated 80 complete polar bear genomes and ten high-coverage brown bear genomes, which we analyzed within a population genomic framework to address (i) the age of the polar bear as a species; (ii) the joint demographic history of polar bears and brown bears since the two species diverged; and (iii) what genes have been under strongest positive selection in the polar bear lineage, enabling the species to survive a the extremities of life in the High Arctic, in some of the harshest climates and most inhospitable conditions on Earth.
In our paper, we demonstrate that the polar bear is a very young species, which diverged from the brown bear less than half a million years ago, 479–343 hundred thousand years ago. Importantly, our work finally settles the long-standing debate on the age of the polar bear as a species; previous genomic estimates have ranged from 600.000 years old to 4–5 million years old, based on genomic data. Estimating a correct age is fundamental to understanding polar bear adaptations – did the species adapt to the High Arctic over hundreds of thousands of years, or over millions of years? And what extrinsic events triggered speciation?
Our divergence date estimate coincides with the end of Marine Isotope Stage 11, which was the longest interglacial in the preceding half million years. Likely, brown bear populations were able to expand northwards, but as conditions shifted rapidly towards glacial climates, a population marooned in the north was forced to adapt very quickly and diverged from the rest – ultimately becoming the polar bear we know today.
Polar bears differ fundamentally from brown bears in ecology, behaviour and morphology, reflecting adaptations to different ecological niches. Their recent divergence represents an unprecedented timeframe for rapid evolution in a large mammal. The polar bear has since undergone drastic physiological and phenotypic changes in response to life out on the Arctic sea ice. In cold Arctic climates, energy is in high demand, and the life of a polar bear revolves around fat and a high-caloric diet.
A major finding of our study is our determination of the top-20 genes that show the strongest signal of positive selection in the polar bear genome. Ultimately, these are prime candidates underlying polar bear-specific traits and adaptations. Incredibly, we find that the genes are associated with the development of adipose tissue and cardiovascular function, traits that are fundamental in a species whose survival revolves around fat. Individuals have substantial adipose deposits under the skin and around organs, which can comprise up to 50% of the body weight of an individual, depending on its nutritional state. Cubs nurse on milk containing ~ 27% fat and adults feed primarily on seal blubber. Consequently, cholesterol levels in polar bear blood plasma are extreme; in humans, elevated cholesterol levels are a major risk factor for the development of cardiovascular disease. Until our work, it had remained an enigma how polar bears are able to deal with such lifelong elevated levels of cholesterol, which would put extreme evolutionary pressure on the species’ cardiovascular system.
In summary, our study has pin-pointed polar bears as an unlikely model organism for studying rapid adaptation to a novel environment and a high-caloric diet, and holds great potential for future research. The polar bear research has since been incorporated into several textbooks, including Feldhamer’s Mammalology (4th edition, by Carey Krajewski and others) and Evolution: Making sense of Life (2nd edition, by Carl Zimmer and Doug Emlen). Furthermore, the research has become a vantage point for an outreach programme on selection and adaptation by the Education Department at the Natural History Museum in Copenhagen.

**** Note to REA: The following text is not for public viewing, as it discusses furture research, which is in direct conflict with other groups working on polar bear genomics ****

I (Eline Lorenzen) am continuing my analyses of the polar bear data, and expect to have additional work ready for submission during 2016. This includes my continuous analysis of twenty 10,000-year old polar bear genomes, where I am focusing on the past 20,000 years of polar bear evolution, and investigating responses to recent climate change events. I have had some difficulties in the analysis of the data, as I have so far been unable to confidently infer the most recent evolutionary history of the species. However, I is actively collaborating with colleagues in the US and Europe, who have developed the available software for recent demographic analysis of whole-genome data, and hope to have the reliable results available during 2016.
My on-going work on polar bear also includes a worldwide biogeographic study of the species, including estimates of population subdivision, migration, gene flow. The latter includes a compilation of all the genomic data published so far from Greenland and Siberia (my data), and Alaska, Canada and Svalbard. Furthermore, I am working on an in-depth analysis of the genomic data of the Greenlandic polar bear data previously published to infer the joint demographic history of the two populations in east and west; the genomic data will be contrasted with demographic information that I am now gathering using non-genomic methods (contaminant load, age and sex of individuals harvested in different years) from the collection of >800 historic Greenlandic polar bear skulls spanning the past 250 years, which is housed at the Natural History Museum in Copenhagen (where I am now faculty).
In addition to working on polar bear, I was involved in many other research collaborations, both large and small, during my Marie Curie IOF. One encompassed an invited review of my previous work on biogeography of African savannah herbivores (Lorenzen et al. 2012), and another encompassed a study on 50-thousand years of Arctic vegetation change using ancient DNA retrieved from sediments and the gut content of mummified remains of extinct Ice Age mammals (Willerslev*, ... Lorenzen* et al. 2014 Nature).