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Much of Polar Bear Y-Chromosome Sequence Is Identified; Study Shows That Two Male Lineages Diverged Over 100,000 Years Ago

For the first time, a team of scientists, led by Professor Axel Janke of the Senckenberg Research Center for Biodiversity and Climate in Franfurt, Germany, has reconstructed part of the male chromosome in polar bears (Ursus maritimus). The scientists were able to assign 1.9 million base pairs specifically to the polar bear Y chromosome. In their study, published online on May 27, 2015 in an open-access article in “Genome Biology and Evolution,” they show that more than 100,000 years ago, the male polar bear lineages split and developed in two separate genetic groups. The article is titled “Genome-Wide Search Identifies 1.9 Megabases from the Polar Bear Y Chromosome for Evolutionary Analyses.” The polar bear is the world’s largest land-dwelling predator and is hard to miss. Nevertheless, it is difficult to study the evolution of this Arctic resident. Polar bears live and die on the frozen sea, and their remains are seldom found. “In order to gain insights into the evolutionary development of Ursus maritimus, we use genetics instead of fossils,” explains Professor Janke. For the first time, the evolutionary geneticist and his Ph.D. student Tobias Bidon, have reconstructed large parts of the polar bear Y chromosome. “In this age of biological revolution, it is possible to sequence the entire genome of an organism rather quickly and cost-effectively,” says Dr. Janke. However, to date, such comprehensive genome projects have mostly been limited to female animals, neglecting the males’ special chromosome – the Y chromosome. Mr. Bidon commented that “this is quite surprising, because the Y chromosome is an important part of the mammalian genome. It is the only genetic material that is passed on from male to male, thereby offering unique insights into the evolutionary history and population dynamics.”

Studying billions of small anonymous sequence segments, the Frankfurt research team has now been able to identify those puzzle-bits that are associated with the polar bear Y chromosome. Their efforts were made possible by the fact that there is only one copy of the Y chromosome present in males, while females are missing it. “Using this property of the mammalian genomes and bioinformatics, we were able to identify 1.9 million base pairs in the genetic material of male polar bears,” adds Dr. Janke.

In their evolutionary analyses of the polar bears’ almost two million Y chromosome nucleotides, Senckenberg scientists identified two genetic groups of male polar bear lineages, which, according to the teams’ calculations, started to evolve separately more than 100,000 years ago.

“Today, individuals from both genetic groups can be found in various Arctic regions – from Alaska to Spitsbergen,” says Mr. Bidon. This confirms the idea that polar bears roam across vast distances and distribute their genetic material throughout the entire Arctic.

Dr. Janke offers the following preview for additional projects: “We will to use our bioinformatics approach to reconstruct large parts of the Y chromosome from other male genomes, in order to study paternal inheritance and migration from the male perspective.”

[Press release] [Genome Biology and Evolution article]