Insights into bear evolution from a Pleistocene polar bear genome

Area of Science:

• Paleogenomics
• Evolutionary Biology
• Climate Change Science

Background:

• Polar bears (Ursus maritimus) are vulnerable to climate change, making their evolutionary history crucial for understanding apex carnivore resilience.
• Genomic studies have explored bear speciation and admixture, particularly between polar and brown bears (Ursus arctos).

Purpose of the Study:

• To investigate the evolutionary history and population dynamics of polar bears using ancient and modern genomic data.
• To identify ancient admixture events between polar bears and brown bears and their potential impact.

Main Methods:

• Sequencing of a 130,000- to 115,000-year-old polar bear genome from Svalbard.
• Analysis of 10 new genomes from contemporary polar and brown bears in Alaska.
• Statistical modeling of admixture graphs to infer gene flow events.

Main Results:

• Demonstrated a significant decline in effective population size for the ancient polar bear lineage, with a slight recovery before extinction.
• Observed higher genetic diversity in the ancient polar bear compared to modern populations, indicating prolonged bottlenecks.
• Identified strong evidence for ancient introgression from brown bears into the polar bear ancestor, possibly predating 150,000 years.

Conclusions:

• Polar bears experienced severe genetic bottlenecks and population erosion.
• Ancient introgression from generalist brown bears may have provided adaptive variants to specialist polar bears during glacial periods.
• Understanding these evolutionary dynamics is vital for predicting polar bear responses to ongoing climate change.