Population Genomics Reveals Loss of Odorant Receptor Gene Repertoire during Polar Bear (Ursus maritimus) Evolution

Date of Award


Degree Type


Degree Name

Master of Science in Biology



Chief Instructor

John G. Gibbons

Second Reader

Nathan Alhgren

Third Reader

Deborah Robertson


The polar bear (Ursus maritimus) and brown bear ( Ursus arctos) are a recently diverged species pair but are morphologically, behaviorally, and physiologically distinct. These phenotypic differences reflect adaptation to local environments. Previous research aimed to identify the genetic underpinnings of adaptive traits focused mainly on single nucleotide polymorphisms (SNPs), while ignoring copy number variation (CNV). CNV refers to loci that vary in their number of copies between individuals due to duplication or deletion. Here, we computationally predicted whole genome copy number profiles across 17 polar bear and 9 brown bear genomes using FREEC. We identified hundreds of genes overlapping copy number variable regions, with deleted genes being more common than duplicated genes. We identified 50 genes with highly divergent copy number profiles between the polar and brown bear individuals. The vast majority of these copy number divergent genes were present at a lower copy number in polar bears, and 34 of these 50 genes encode odorant receptors. Comparing the copy number profiles of these genes to the black bear genome suggests odorant receptor gene loss in the polar bear. Our results suggest that loss of odorant receptor gene copy number variation accompanied polar bear evolution.

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