Comparison of Arctic and Antarctic teleost haemoglobins: primary structure, function and phytogeny

Organisms living in the Arctic and Antarctic are exposed to strong environmental constraints, especially temperature. Consequently, haemoglobin evolution has included adaptations with implications at the biochemical, physiological and molecular levels. The northern and southern polar oceans have very different oceanographic characteristics. Within the study of the molecular bases of cold adaptation in fish inhabiting polar habitats, and taking advantage of the information available on haemoglobin structure and function, we analysed the evolutionary history of the α and β globins of Antarctic and Arctic haemoglobins, under the assumption of the molecular-clock hypothesis, as a basis for reconstructing the phylogenetic relationships between species. Temperate fish, including two non-Antarctic notothenioids of special evolutionary interest, were also considered. Phylogenetic analysis was performed on the multiple sequence alignments constructed with the programme Clustal X. Tree topologies indicate that the chains of Antarctic major and minor haemoglobins cluster in two well separated groups and diverged prior to cold adaptation, forming a monophyletic group. In Arctic haemoglobins, the structure/function relationship reveals important differences in comparison with Antarctic ones, indicating a distinct evolutionary pathway. The Arctic ichthyofauna (unlike the Antarctic, dominated by one taxonomically uniform group) is characterized by high diversity, reflected in the phylogeny of a given trait. The constant physico-chemical conditions of the Antarctic waters are matched by a clear grouping of fish globin sequences, whereas the variability typical of the Arctic Ocean corresponds to high sequence variation, reflected in the trees by scattered intermediate positions between the Antarctic and non-Antarctic clades. The evolutionary history of the Root effect, an important physiological feature of fish haemoglobin, was investigated. Analysis of the fate of the residues of the β chains suggested to be correlated with the Root effect indicate that they should rather be regarded as ancestral characters, inherited by some species but not by others.