Genomic insights into adaptive evolution of Island brown rats (Rattus norvegicus): evidence from Iturup Island in the Russian Far East

The global spread of Rattus norvegicus has been shaped by human activity and climate change, with island isolation driving unique evolutionary paths. Understanding population differentiation and adaptive evolution in invasive rodents informs island biodiversity conservation and invasive species management. Through whole-genome analysis of 38 R. norvegicus individuals from Iturup Island and adjacent mainland, we revealed their demographic history and genetic signatures of island adaptation. High-quality SNPs were obtained by mapping to the reference genome, followed by quality filtering for population genomic analyses. We initially investigated population structure and inferred demographic history. Subsequently, genetic diversity and load were compared across populations using sliding window. Divergent selection between mainland and island populations was detected, with identified candidate genes subjected to functional annotation. This study revealed that the island rat population formed a monophyletic, homozygous lineage diverging from Northeast Asian mainland population ~ 50,000 years ago, likely via glacial-era colonization over land bridges. Demographic reconstructions showed post-colonization expansion followed by a bottleneck, in contrast to mainland declines during glaciation. Island rats displayed reduced diversity, elevated linkage disequilibrium, and moderate inbreeding, suggesting they were undergoing a selective sweep. Despite high genetic load, genomic scans identified adaptive sweeps in genes related to olfaction, neural function, immunity, and metabolism, reflecting adaptations to ecological stressors. The R. norvegicus population on Iturup island illustrated evolution shaped by long-term geographic isolation. Insular conditions have led to inbreeding bottlenecks and increased genetic load, while driving unique local adaptations. Positive selection on genes related to olfaction, neural development, immunity, and metabolism might contribute to adaptation to the local environment. These results highlight pronounced genomic divergence and adaptive evolution in invasive island rodents, offering insights into evolutionary resilience in isolated ecosystems.