Recent Dominant Transposition Events Affect Gene Regulatory Regions, but Not Coding Sequences, in Polar and Brown Bear Genomes.

Transposable elements (TEs) are inserted into the genome and may change its properties; those occurring in or near regulatory regions may also alter gene expression. Given the challenges of detecting insertions in short-read sequencing, we analyzed structural variants in polar and brown bear genomes by a reciprocal alignment of one species' sample genomes to a reference sequence of the other species, thus inferring TE insertion as the other genome's "deletions". With this approach, we detected short interspersed elements (SINEs) belonging to the CAN SINE family as dominant fixed TEs. We observed a non-random distribution of CAN SINE insertion positions near both protein- and RNA-coding genes, where TEs often overlap UTRs or occur in their vicinity. In contrast, SINEs avoid coding sequences, suggesting TE insertions that would disrupt such sequences are under purifying selection. We used black bear as an outgroup and determined that most of the CAN SINE insertions in the polar bear genome were derived, since they are not present in black or brown bear, while there is no dominant trend for CAN SINE insertions in brown bear relative to the outgroup. Many of the genes with UTRs affected by CAN SINEs are potentially relevant to the differences between the species (body shape, size, etc.) or to Arctic-adaptation phenotypes such as fur color, metabolism, and the immune system. This supports a model that CAN SINEs have contributed to regulatory evolution in bears and provides further evidence of such events across carnivore genomes in the animal kingdom.