Fresh Evidence Suggests Hb Adaptation to Hypoxia in High-altitude Fishes

As one of the most extreme living conditions on earth, high-altitude environments areas with the lower partial pressures of oxygen as well as the colder temperatures relative to low-altitude areas pose a number of unique physiological challenges to the native organisms. Relatively, the organisms have typically evolved multiple hierarchical levels of compensatory physiological adjustments to cope with the survival challenges of the reduced environmental O2 availability. Dissecting the genetic basis underlying hypoxia adaptation has received a great deal of attention in the studies of evolutionary biology.   

A research group led by Prof. HE Shunping from the Institute of Hydrobiology (IHB) of the Chinese Academy of Sciences provided evidence that the genetically based modification of hemoglobin (Hb)-O2 affinity in schizothoracinae fishes might facilitate the evolutionary adaptation to Tibetan aqueous environments. The research was published in the International Journal of Biological Macromolecules   

Prof. He’s team first investigated the association between Hb concentration and altitude across four fish groups (Lhasa, Sichuan, Yunnan, and Hubei). The results indicated that fishes carry species-specific Hb concentrations which are not positively correlated with the living altitude, and the long-term maintenance of high Hb concentration may be a disadvantage for physiologically acclimating to high altitude hypoxia.   

Researchers then identified the expression patterns of Hb in these fishes. They found that 6–9 distinct Hb genes, including 3–5 α-globin genes and 5–6 β-globin genes, were identified in each species. Meanwhile, they also identified that five Hb isoforms are co-expressed in these species by two-dimensional (2-DE) gel electrophoresis and LA-MS/MS analyses.    

To better understand the dynamics of Hb genes evolution associated with O2 adaptation in schizothoracinae fishes, researchers analyzed the selective pressure acting on these genes. The result of the clade model showed that four out of seven Hb genes in schizothoracinae lineage exhibited evolutionary patterns that were significantly divergent from the low-altitude relatives. It can be safely assumed that positively selected sites likely facilitated the functional divergence of Hb isoforms.   

This study revealed that the modifications of Hb-O2 affinity rather than Hb concentration promote the evolutionary adaptations for permanent hypoxia of schizothoracinae fishes. It provided important insights into hypoxic tolerance mechanisms of physiological evolution of aquatic species.