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Scientists Trace Black Bear Lineage
In a first-ever genomics study of black bears across their North American range, researchers have determined where they come from and how closely different populations are related to one another.
As part of the study, published in Molecular Biology and Evolution, researchers at the University of Missouri examined the genetic diversity of American black bears (Ursus americanus) in North America to determine the bears’ lineages and how they ended up in locations where they exist today — information that could prove useful in managing and maintaining healthy American black bear populations.
According to Emily Puckett, the lead author of the study who recently received her doctoral degree from University of Missouri, the purpose of the study was to determine what the bears’ continental population structure is as well as how glacial history has influenced their populations and distribution before and after the Last Glacial Maximum, around 18,000 to 21,000 years ago.
“As technology has improved, we can now better investigate the nuclear genome,” Puckett said, adding that past studies on the mitochondrial DNA of the species only includes information from the mother. “The nuclear genome which includes information from the mother and father has different pieces of information.” Puckett said that while researchers have a good understanding of black bears’ mitochondrial biogeographic history, this is the first study to look at their nuclear genome across their entire range.
Puckett and her team isolated DNA from blood and tissue samples from over 500 black bears in the continental U.S., Alaska and Canada. The samples were collected by bear managers from 32 state and provincial wildlife agencies, universities and private game outfitter partners — a collaborative effort that Puckett appreciates.
The researchers then sequenced the DNA of 94 bears and traced the inheritance of small variations in the DNA code called single nucleotide polymorphisms (SNPs), which showed the relation of bear populations to one another as well as patterns of admixture, where previously diverged populations come back into contact. Next, they modeled where the black bears were likely to have lived before and after the Ice Age based on climate data.
Puckett and her team’s analysis showed there are currently three distinct lineages of black bears including two that occupy the western and eastern ranges of the North American continent and one lineage in Alaska. The researchers were surprised to find that black bears in the Alaskan lineage are more genetically related to bears in the Eastern lineage than bears in the Western one.
“I personally thought it was surprising that the Alaskan cluster were more closely related to the eastern lineages than to the western lineages,” Puckett said. “You would expect that populations that are geographically close should be genetically close.” But Puckett said the reasoning might be the presence of glacial refugium or pockets of ice-free habitat during the Ice Age as well as the migration patterns that came about after the ice began to recede. The researchers identified four refugia for black bears in the Southeast, Southwest, Pacific Northwest and Alaska, that they believe explain the distribution of black bear populations today. They also were able to date the lineages of the three bear lineages and found that bears from the western lineage split off about 67,000 years ago, and the eastern and Alaskan clusters split off about 31,000 years ago.
This information is not only new and interesting, it provides important information for bear managers, Puckett said. Specifically, the data can inform managers on how a population is related to other populations within their geographic region and how genetic diversity differs across populations, which is of particular interest as bears are naturally repopulating in the East and West. Further, this information can potentially aid conservation of black bears, Puckett said. “With subpopulations with known low genetic diversity that are similar to one another, managers can make more informed decisions on potential populations for translocations if they wanted to manage for low genetic diversity,” she said.