Polar Bears Evolutionarily Five Times Older and Genetically More Distinct
Hailer, F., V. E. Kutschera, et al. (2012). "Nuclear Genomic Sequences Reveal that Polar Bears Are an Old and Distinct Bear Lineage." Science 336(6079): 344-347.
ScienceDaily (Apr. 20, 2012) — A study appearing in the current issue of the journal Science reveals that polar bears evolved as early as some 600,000 years ago. An international team led by researchers from the German Biodiversity and Climate Research Centre (BiK‐F) shows the largest arctic carnivore to be five times older than previously recognized. The new findings on the evolutionary history of polar bears are the result of an analysis of information from the nuclear genome of polar and brown bears, and shed new light on conservation issues regarding this endangered arctic specialist.
Polar bears are uniquely specialized for life in the arctic. This fact is undisputed, and supported by a range of morphological, physiological and behavioural evidence. However, conducting research on the evolutionary history of polar bears is difficult. The arctic giant spends most of its life on sea ice, and typically also dies there. Its remains sink to the sea floor, where they get ground up by glaciers, or remain undiscovered. Fossil remains of polar bears are therefore scarce. Because the genetic information contained in each organism carries a lot of information about the past, researchers can study the history of the species by looking at the genes of today's polar bears.
Analysis of the genetic information in the cell nucleus
Recent studies had suggested that the ancestor of polar bears was a brown bear that lived some 150,000 years ago, in the late Pleistocene. That research was based on DNA from the mitochondria ‐ organelles often called the 'powerhouses of the cell'. Researchers from the German Biodiversity and Climate Research Centre (BiK‐F), together with scientists from Spain, Sweden and the USA, now took an in‐depth look at the genetic information contained in the cell nucleus. Frank Hailer, BiK‐F, lead author of the study explains: "Instead of the traditional approach of looking at mitochondrial DNA we studied many pieces of nuclear DNA that are each independently inherited. We characterized those pieces, or genetic markers, in multiple polar and brown bear individuals."
Polar bears had much more time for adaptation and speciation than previously assumed
This genetic survey was well worth the effort ‐ the information obtained from nuclear DNA indicates that polar bears actually evolved in the mid Pleistocene, some 600,000 years ago. This provides much more time for the polar bear ancestors to colonize and adapt to the harsh conditions of the arctic. Based on studies of mitochondrial DNA, polar bears had earlier been considered an example of surprisingly rapid adaptation of a mammal to colder climates. The polar bear's specific adaptations, including its black skin, white fur, and fur-covered feet now seem less surprising. "In fact, the polar bear genome harbours a lot of distinct genetic information," says Hailer, "which makes a lot of sense, given all the unique adaptations in polar bears."
Maternally inherited (mitochondrial) DNA was showing a biased picture
Previous studies of mitochondrial DNA had indicated that polar bears are much younger as a species. The authors of the new paper in "Science" explain this apparent discrepancy with past events of hybridization between polar and brown bears ‐ a process recently observed in the Canadian arctic. After their initial speciation, polar bears and brown bears came into contact again, maybe due to past climatic fluctuations. The mitochondrial DNA found in polar bears today was probably inherited from a brown bear female that hybridized with polar bears at some point in the late Pleistocene. It appears that much of the nuclear genome remained unaffected by hybridization, so polar bears retained their genetic distinctiveness.
"Each part of the genome tells its own story. In our study we analysed nuclear DNA that is inherited from both parents. It provides a more detailed and accurate picture of the evolutionary history of a species than mitochondrial DNA that is inherited only from the mother," says Axel Janke, BiK‐F, senior author on the study who also headed the recent sequencing of the brown bear genome. He goes on to say: "Inferring a species' evolutionary history based on mitochondrial DNA alone is like solving a puzzle with only a few of the many available pieces. You need to study many genetic markers (loci) to put together the full picture."
Genome carries evidence of past climate fluctuations
The new genetic data indicate that polar bears went through tough times over the course of their 600,000 year‐old evolutionary history. Polar bears show much less genetic diversity than brown bear. This is probably due to dramatic reductions in population size in the past. Maybe those times coincided with phases of climatic warming. Whether polar bears will be able to survive the current phase of sea ice melting is not clear. Firstly, human impacts are accelerating the rate of climate change, and the arctic could reach higher temperatures than in previous interglacial warm phases. In addition, numerous human‐related issues are threatening the polar bear today.
Polar bears colonizing coastal regions due to sea ice melting frequently encounter human habitat, and many bears are killed. Besides persecution, polar bears are also facing other ‐ evolutionarily novel ‐ threats, including pollution by persistent chemicals in the food chain. "If we were to lose polar bears in our era, we would have to ask ourselves what role we played in pushing them over the edge. They clearly were able to survive previous warm phases," Hailer concludes upon the wider implications of the study.
Recent studies have shown that the polar bear matriline (mitochondrial DNA) evolved from a brown bear lineage since the late Pleistocene, potentially indicating rapid speciation and adaption to arctic conditions. Here, we present a high-resolution data set from multiple independent loci across the nuclear genomes of a broad sample of polar, brown, and black bears. Bayesian coalescent analyses place polar bears outside the brown bear clade and date the divergence much earlier, in the middle Pleistocene, about 600 (338 to 934) thousand years ago. This provides more time for polar bear evolution and confirms previous suggestions that polar bears carry introgressed brown bear mitochondrial DNA due to past hybridization. Our results highlight that multilocus genomic analyses are crucial for an accurate understanding of evolutionary history.
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