Leif Andersson

The Atlantic herring is one of most abundant vertebrates on earth and sustains one of the most important marine fisheries in the world. This species has developed into a gold mine for exploring the genetics of ecological adaptation in vertebrates because of the following reasons. Firstly, it is subdivided into many subpopulations adapted to different climate conditions, spawning time, migration and feeding behavior. Secondly, the huge population size and gene flow between populations essentially eliminate genetic drift even between geographically distant populations. Thirdly, the herring deposit sticky eggs on the sea floor or on vegetation which means that the developing embryo is exposed to the environmental conditions at the spawning site. Thus, herring is exposed to much more diverse environmental conditions during the sensitive early stage of development compared with a pelagic spawner, or a bird, or a mammal. Fourthly, the high fecundity leaves plenty of room for natural selection to act. Fifthly, the herring has recently (within the last 8,000 years) adapted to a new environment, the brackish Baltic Sea which is extreme as regards salinity in which this marine species is able to reproduce. Here, I will review our current knowledge concerning how herring have been able to adapt to the Baltic Sea. I will discuss the genetic architecture underlying adaptation, the importance of changes in protein sequence and gene regulation, and high-light some of the genes that have been particularly important for the successful colonization.