Hemoglobin polymorphisms in Atlantic cod--a review of 50 years of study.

Hemoglobin is one of the most studied proteins in nature, and evolutionary modifications of the interacting subunits seem to have refined the oxygen binding properties in the wide range of land- and/or water-living vertebrates. The adaptation of fish to varying environments seems to involve multiple hemoglobins, and polymorphic variants may further increase the diversity of functional properties. The pioneering study of Knud Sick on the hemoglobin polymorphisms in Atlantic cod fifty years ago was accompanied by multiple population genetic, physiological and behavioral studies before the recent identification of the genetic basis of the protein variants. The Met-Lys and Val-Ala substitutions in the cod β1 globin subunit provided the link between genotype and physiological functions, and the geographical distribution of the variants in temperate and Arctic waters strongly indicate that hemoglobin is under adaptive evolution in Atlantic cod. The structural and regulatory polymorphisms of the cod β1 globin highlight the relationship between temperature and functional molecular variation in the hemoglobin system.