Brown bears on an Alaskan archipelago are the descendants of an ancient polar bear population rather than being the ancestors of modern polar bears, new research published in PLOS Genetics shows.
Scientists have long struggled to understand the exact nature of the evolutionary relationship between brown bears and their arctic descendants. It is known that these two species can mate successfully in captivity and in the wild, but how much of their genetic histories are the result of past interbreeding has remained a puzzle.
Previous analyses of DNA sequences have yielded conflicting results on this question. At the center of the controversy are a group of brown bears that live on the Admiralty, Baranof and Chichagof (ABC) Islands of southeastern Alaska. These bears – clearly brown bears in appearance and behavior – inexplicably carry mitochondrial DNA that match polar bears more closely than other brown bears. This observation led some researchers to conclude that the ABC Islands brown bears gave rise to modern day polar bears.
"The key to solving this mystery was to analyze DNA from the ABC Islands bears' nuclear genomes, and in particular their X-chromosomes," explained Beth Shapiro, Associate Professor at UC Santa Cruz, who led the research. "Focusing on the X gave us a surprising result."
The team compared the X chromosomes of the ABC Islands brown bears to the X chromosome of brown bears from the Alaskan mainland. They found that around 6.5% of the X chromosomes of the ABC Islands bears had recently come from polar bears. In contrast, only about 1% of the rest of the genome of the ABC Islands brown bears had come from polar bears.
The team simulated various scenarios to determine the most likely evolutionary history for the ABC Islands bears. The results suggested a situation that differs considerably from any previously imagined for these bears. They concluded that the ABC Islands bears descended from polar bears that were gradually converted into brown bears through hybridization with male brown bears dispersing from the Alaskan mainland.
"During a previous ice age, polar bears ranged much farther to the south than they do today, reaching the present-day ABC Islands and Alexander Archipelago," said Ian Stirling, an Adjunct Professor at the University of Alberta in Edmonton, Canada. "As the climate warmed and ice began to retreat, it is possible that some of the animals began to spend progressively longer on land with reduced access to ice. We see the same sort of thing happening today with polar bears in areas such as western Hudson Bay or the Russian coast, in response to continued climate warming and loss of ice."
"One of the most exciting results from this study is that we now finally understand what happened between the polar and brown bears on the ABC islands," said Stirling. "But, for the moment at least, we are back to not being certain where in the Arctic polar bears first diverged from brown bears."
FINANCIAL DISCLOSURE: Funding supporting this research was from the National Science Foundation ARC-09090456. F Jay and M Slatkin were supported in part by NIH grant R01-GM40282. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
COMPETING INTERESTS: The authors have declared that no competing interests exist.
CITATION: Cahill JA, Green RE, Fulton TL, Stiller M, Jay F, et al. (2013) Genomic Evidence for Island Population Conversion Resolves Conflicting Theories of Polar Bear Evolution. PLoS Genet 9(3): e1003345. doi:10.1371/journal.pgen.1003345
PLEASE ADD THIS LINK TO THE FREELY AVAILABLE ARTICLE IN ONLINE VERSIONS OF YOUR REPORT (the link will go live when the embargo ends): http://www.plosgenetics.org/doi/pgen.1003345
Beth Shapiro: +1 (814) 321-8389
Richard Green: +1 (831) 502-7394
James Cahill: +1 (831) 450-3009
Image 1: Polar bears. Credit: Paul Nicklen, BBC Wildlife photographer of the year.
Image 2: Brown bear from the ABC islands population studied in this paper. Credit: Michael Dobson.
Image 3: Brown bear from the ABC islands population studied in this paper. Credit: Michael Dobson.
This press release refers to an upcoming article in PLOS Genetics. The release is provided by journal staff, or by the article authors and/or their institutions. Any opinions expressed in this release or article are the personal views of the journal staff and/or article contributors, and do not necessarily represent the views or policies of PLOS. PLOS expressly disclaims any and all warranties and liability in connection with the information found in the releases and articles and your use of such information.
About PLOS Genetics
PLOS Genetics (http://www.plosgenetics.org) reflects the full breadth and interdisciplinary nature of genetics and genomics research by publishing outstanding original contributions in all areas of biology. All works published in PLOS Genetics are open access. Everything is immediately and freely available online throughout the world subject only to the condition that the original authorship and source are properly attributed. Copyright is retained by the authors. The Public Library of Science uses the Creative Commons Attribution License.
PLOS is a non-profit organization of scientists and physicians committed to making the world's scientific and medical literature a freely available public resource. For more information, visit http://www.plos.org.
Everything published by PLOS Genetics is open access, allowing anyone to download, reuse, reprint, modify, distribute, and/or copy articles, so long as the original authors and source are cited. Please mention PLOS Genetics in your report and use the link(s) below to take readers straight to the online articles. Thank you.
AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert! system.