Senckenberg Research

Genomes reveal the history of brown bears and polar bears

 

 Polar bear with cub.
Polar bear with cub. Photo: Alan Wilson.

 

 

Brown bears and polar bears have adapted to very different habitats — but they remain very closely related. By means of genome analysis, researchers at the Biodiversity and Climate Research Centre (BiK-F) have found out that they went their separate ways as species only as recently as 800,000 to 480,000 years ago. However, during warm periods their paths crossed again.

Brown bear in Finland.
Brown bear in Finland. Photo: Alexander Kopatz.

Polar bears and brown bears, the largest terrestrial predators of our era, fascinate us with their strength and elegance. Even non-experts can infer that they are closely related to each other on account of their similar appearance. However, whereas the brown bear is something of a generalist, preferring temperate climate regions and living from fish, hares, cadavers, berries and roots, the polar bear has not merely managed to adapt to the inhospitable Arctic, but adopted it as its preferred habitat. There is not a great deal of choice in terms of food there, so the polar bear has become a specialist hunter of seals and smaller whales.

An intriguing question arises: How and when did the polar bear diverge from its cousin and adapt to the Arctic? These days, many people are also wondering whether the polar bear will be able to survive the expected climate change.

The history of the polar bears is only vaguely documented by the fossil record. They spend most of their lives on the Arctic ice, so when they die their bones disappear into the depths of the ocean. In the 1960s, the Finnish / Swedish palaeontologist Björn Kurtén used morphometric methods to determine the point at which the polar bears and brown bears went their separate ways, but was only able to make a rough estimate of one million years ago. The scientific community therefore impatiently awaited genetic methods to obtain more accurate data about their kinship and the time of their evolutionary divergence.

The polar bears appear to be part of the brown bear population.
The polar bears appear to be part of the brown bear
population in the mitochondrial (mtDNA) tree,
with a divergence time 150,000 years ago. ABC shows
the relationship of the ABC island brown bears.

The first genetic studies on the bears’ evolution were conducted in the 1990s and involved analyses of DNA sequences of the mitochondrial genome (mtDNA ). Organisms with cell nuclei (animals, plants and fungi) have a small additional genome in the mitochondria – cell organelles – which are known to function as the cells’ ‘power stations’. Their numerous copies of the mtDNA per organelle, simplified the isolation and analysis with the early molecular techniques. For this reason, the mtDNA became the molecule of choice for evolutionary studies in the 1990s.

The hereditary analysis of parts of the mtDNA led to a great surprise, as it suggested that polar bears do not represent a distinct species. According to the data gained, the polar bears’ nearest relatives appeared to be a group of brown bears now living on the Admiralty, Baranof and Chichagof Islands off the Alaskan coast – the so-called ABC Islands. Also, the genetic dating yielded an unexpectedly recent date for the separation of the polar bears and brown bears: approximately 100,000 to 150,000 years ago.

In 2010, we set ourselves the task of studying adaptation to extreme climatic conditions, and to this end we chose the disparate cousins brown bear / polar bear for genetic analyses. The possibility that the polar bears had adapted to their extreme habitat in just 150,000 years promised to deliver some exciting results. A full genetic analysis entails the comparison of the entire genome and its three billion characters (nucleotides) that code for all the hereditary information and evolutionary history of an organism. The polar bear genome was sequenced in 2011, but at the time, nobody seemed to be interested in comparing it with that of the brown bear. However, in the era of genome analysis even our small research group (with Dr. Frank Hailer, Dr. Björn Hallström, Verena Kutschera and Vikas Kumar), in cooperation with BGI (China) and BioForsk (Norway), managed to decode the entire brown bear genome in the same year. This was the first de novo German genome project involving a wild mammal. Comparison of the approximately 24,000 genes is expected to show which ones are important to the bears with respect to adapting to their habitats.

Polar bears and brown bears are two distinct species

Figure: Multi-locus gene analyses show the polar bear as a separate species, with a divergence time of 600,000 years.
Multi-locus gene analyses show the polar bear as a separate
species, with a divergence time of 600,000 years. ABC – see upper
Figure.

The initial analyses of just a few genes produced a great surprise: The DNA of polar bears and brown bears differed in more places than was to be expected in the course of 150,000 years of evolution. In order to investigate this startling result more closely, we sequenced further gene locations for 45 bears. This analysis brought some sensational findings: Polar bears and brown bears are definitely two different species, and they separated as early as 600,000 years ago. Even experts were surprised at this result, which was taken on as the cover story in the April 2012 issue of SCIENCE magazine.

The conflicting results for the DNA and mtDNA analyses can be explained by hybridization between brown bears and polar bears during the Eemian interglacial stage that took place about 125,000 years ago. During this period, female brown bears paired with male polar bears and passed the mtDNA, which is transmitted through the maternal line, on to their offspring. The brown bear mtDNA finally spread to all of today’s polar bears. It was just this crossbred gene material that was analysed in the earlier mtDNA studies and produced the misleading results. Today polar bears and brown bears still mate, and their offspring are fertile. One suspects that the increasingly late formation of ice in the Arctic forces polar bears to stay on land longer and thus increase the likelihood meeting and mating with female brown bears. These cross-breeds (hybrids) are informally known as prizzly bears (polar bear / grizzly bear) or grolars (grizzly bear / polar bear).

To date, our comparison of the brown bear and polar bear genomes has revealed about 150 genes that show signs of adaptation (positive selection). We are now investigating the function of these genes and how they are spread amongst bears in order to find out more about the heritability of adaptations. Other research partners (Örebro University Hospital, Sweden) are interested in bear genes that enable bears to hibernate. During hibernation, brown bears lie practically motionless in caves for several months. Nevertheless, bears show no signs of muscle or bone reduction, whereas astronauts and people confined to bed show a significant loss of muscle tissue and bone density after just two weeks. Another extreme that is observed in bears is that prior to hibernation they can increase their body fat weight by up to 50 % and have cholesterol counts that would be dangerous for humans. Despite this, even old bears show no signs of artherosclerosis (thickening of the artery walls). This makes the comparison of bear genomes an interesting area of research for both medicine and astronautics.

Figure: Temperature curve of the last two million years based on stable isotope analyses.
Temperature curve of the last two million years based on stable isotope analyses.

According to our calculations, the brown bears and polar bears probably formed distinct species during the Cromer interglacial stage between 800,000 and 480,000 years ago. This would mean that polar bears have survived three warm interglacial periods. However, this was at the cost of lowering the degree of genetic variability, which is less than that of the brown bear. It is likely that in each interglacial stage the shrinking Arctic habitats reduced the number of polar bears and therefore also the genetic variability. Genetic bottlenecks like this represent a hazard for a species, because a lower degree of genetic variability also means less capacity to adapt to environmental change (e. g. temperature, nutrition, parasites). Today, polar bears face additional threats of their survival, such as the accumulation of environmental toxins, hunting and further habitat encroachment caused by human exploitation of the Arctic.

 

 Author

Dr. Axel Janke

Prof. Dr. Axel Janke came from the University of Lund (Sweden) to join BiK-F and the Goethe University (Frankfurt) in 2010. As a geneticist, he is interested in the evolution of vertebrates, especially that of mammals. In 1994 he published the first phylogenetic analysis based on complete mitochondrial genomes to understand mammalian evolution. For the same question he and Björn Hallström published the first phylogenomic analysis in 2007, analyzing 3000 orthologous genes from 13 genomes. Today his team around Frank Hailer studies the genetics of arctic adaptation. The analysis of genomic sequences revealed that polar bears are an old and distinct bear lineage.

 

 

 

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