Parallel adaptive evolution of geographically distant herring populations on both sides of the North Atlantic Ocean.

Atlantic herring is an excellent species for studying the genetic basis of adaptation in geographically distant populations because of its characteristically large population sizes and low genetic drift. In this study we compared whole-genome resequencing data of Atlantic herring populations from both sides of the Atlantic Ocean. An important finding was the very low degree of genetic differentiation among geographically distant populations (fixation index = 0.026), suggesting lack of reproductive isolation across the ocean. This feature of the Atlantic herring facilitates the detection of genetic factors affecting adaptation because of the sharp contrast between loci showing genetic differentiation resulting from natural selection and the low background noise resulting from genetic drift. We show that genetic factors associated with timing of reproduction are shared between genetically distinct and geographically distant populations. The genes for thyroid-stimulating hormone receptor (TSHR), the SOX11 transcription factor (SOX11), calmodulin (CALM), and estrogen receptor 2 (ESR2A), all with a significant role in reproductive biology, were among the loci that showed the most consistent association with spawning time throughout the species range. In fact, the same two SNPs located at the 5' end of TSHR showed the most significant association with spawning time in both the east and west Atlantic. We also identified unexpected haplotype sharing between spring-spawning oceanic herring and autumn-spawning populations across the Atlantic Ocean and the Baltic Sea. The genomic regions showing this pattern are unlikely to control spawning time but may be involved in adaptation to ecological factor(s) shared among these populations.