Four gene variants of major importance during the evolution of Baltic herring and its adaptation to spawning in brackish water. These include (i) LRRC8C2: a sperm-specific anion channel, (ii) ZPBA1: a structural protein present in the egg envelope, (iii) FTG: an enzyme cross-linking egg envelope protein which produces a harder egg envelope that prevents egg swelling in brackish waters, and (iv) HE1C: an enzyme expressed in hatching embryos that degrades the reinforced egg envelope. Credit: Sarah Sheppard. (License CC BY-NC-ND).

When the Atlantic herring colonized the Baltic Sea thousands of years ago, it needed to adapt to the low salinity. Genes with a vital role in the functioning of sperm, eggs and embryos were crucial to this adaptation.…