Genomic evidence for large inversions in the polar cod Boreogadus saida at similar chromosomal locations as found in Atlantic cod Gadus morhua

Abstract Chromosomal structural rearrangements such as inversions are large scale genomic mutations. Inversions suppress recombination in heterozygotes and are often referred to as supergenes if they combine adaptive gene complexes within their genomic islands of differentiation. Such islands are known in the Atlantic cod ( Gadus morhua ), maintaining genomic divergence that define known ecotypes. Little is known about the genome architecture of the related polar cod ( Boreogadus saida ) although its panmictic distribution suggests small, if any, genetic diversity and population divergence. However, previous observations show seasonal and spatial variation in biological and ecological characteristics that strongly indicate distinct populations, although genetic analyses have not yet been conclusive. Here we use four approaches and find genomic evidence for inversions in a sample of polar cod from north-eastern Greenland at the same chromosomal locations as are found in Atlantic cod. The inversions on chromosomes 1 and 7 in the Atlantic cod, both linked to temperature adaptation, are also present in the polar cod. There is a geographical separation among these different inversions. The polar cod is a key player in the Arctic food-web, transferring a large share of energy from plankton to mammals and birds. With warming temperatures, cold adapted gadid species such as the polar cod are under threat, as is the whole Arctic ecosystem. As the habitat of polar cod is already shrinking, these results may aid our understanding of polar cod’s future success or demise.