The phylogeny of polar fishes and the structure,function and molecular evolution of hemoglobin

Fishes thriving in polar habitats offer many opportunities for comparative approaches to understanding protein thermal adaptations. Investigations on the remarkable evolutionary adaptations to these environments of basic proteins such as hemoglobin, the oxygen carrier, can provide new insights into the mechanisms studied in temperate organisms and can shed light on convergent processes evolved in response to thermal adaptations. At the molecular level, hemoglobins are one of the most intriguing systems for studying the relationships between environmental conditions and adaptations. This review summarizes the current knowledge on molecular structure, biological function and phylogeny of hemoglobins of fish species living in both polar habitats but having different evolutionary histories. In benthic, non-migratory, cold-adapted fishes, the stability of thermal conditions may have generated no or few variations in selective pressures on globin sequences through evolutionary time, so that sequences retain the species phylogenetic “signal”. In pelagic, migratory, cold-adapted or temperate fishes, variations in selective pressures on globin sequences caused by variations in temperature accompanying the dynamic life style may have disrupted the phylogenetic “signal” in phenetic trees.