The genomic basis and environmental correlates of local adaptation in the Atlantic horse mackerel (Trachurus trachurus)
Understanding how populations adapt to local environments is increasingly important to prevent biodiversity loss due to climate change. Here we examined whole-genome variation of twelve Atlantic horse mackerel samples from the North Sea to North Africa, and the western Mediterranean Sea. This marine migratory benthopelagic fish is one of the most widely distributed and commercially important species in the eastern Atlantic. We found low population structure at neutral loci, but high differentiation at adaptive loci distinguishing the western Mediterranean and the North Sea populations from other Atlantic locations. Candidate genes distinctive of the Mediterranean include a green-sensitive-opsin harbouring two missense mutations that might fine-tune the spectral sensitivity to blue-green light conditions. Candidate genes characteristic of the North Sea could play a critical role in cold tolerance (energy metabolism and cell membrane structure) and increased sensitivity to odours, presumably to compensate reduced visibility in turbid waters. We also discovered a putative chromosomal inversion (9.9 Mb) that follows a climate-related latitudinal cline with a break near mid Portugal. Genome-environment association analysis indicated that seawater-dissolved oxygen concentration and temperature are likely the main environmental drivers of local adaptation. Our genomic data broadly supports the current stock divisions, but recommends revision of the western and southern stock boundaries. We developed a reduced SNP panel that genetically discriminate the North Sea and North Africa from neighbouring populations. Our study highlights the importance of life history and chromosomal inversions in adaptation with gene flow, and the complexity of evolutionary and ecological processes involved in local adaptation.