Two genetically separate brown trout populations are studied following their release into an environment previously void of this species. We use a pooled sequencing approach to explore the genomic characteristics of the released stocks and populations established 30 years (4-5 generations) after the release in two lakes down-stream of the release site.. Genomic dynamics of brown trout (Salmo trutta) populations released to a novel environment

Exploring the genomic dynamics of population hybridization and establishment in novel environments is becoming increasingly topical in the context of climate change effects and human induced population translocations. Salmonid fishes represent a group of species experiencing several such large-scale perturbations expected to affect microevolutionary processes. Here, two genetically separate brown trout populations are studied following their release into an environment previously void of this species. We use a pooled sequencing approach to explore the genomic characteristics of the released stocks and populations established 30 years (4-5 generations) after the release in two lakes down-stream of the release site. We find that even though extensive hybridization has occurred, the released stocks have contributed unequally to the descendant populations. While most of the differences between the released stocks can be attributed to drift, we identify regions of the genome that appear to have been under selection in the stocks’ native environments. Most of these regions appear to be under balancing selection in the new environment. Genetic drift is the strongest force also in the new environment, but a few regions putatively under directional selection are identified, where genes from one of the released populations appear advantageous. The results demonstrate that hybridization, establishment, and adaptation can be rapid after release into novel environments. Such ongoing processes are important to follow for the conservation and management of populations. We show that they are possible to follow over microevolutionary time scales even for a species with relatively small local effective population sizes.