Oxygenation Properties of Hemoglobin Facilitate Adaptive Evolution of Schizothoracinae Fishes in Qinghai-Tibetan Plateau

As one of the most extreme living conditions on earth, high-altitude environments pose a number of physiological challenges to the native organisms. In addition to the characteristically cold temperatures, high altitude environments are also characterized by lower partial pressures of oxygen (O2).
Since the reduced partial pressures of oxygen will typically result in a concomitant reduction in the O2 saturation of arterial, the organisms have evolved multiple hierarchical levels of compensatory physiological adjustments to cope with the survival challenges of O2 supply. However, the genetically based changes that have evolved under the influence of natural selection are the most common evolutionary adaptations for permanent hypoxia. Dissecting the genetic basis underlying hypoxia adaptation can provide important insights into mechanisms of physiological evolution.

In a study published in International Journal of Biological Macromolecules, 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 researchers first investigated the association between Hb concentration and altitude across four fish groups from regions at different altitudes. 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.

Then they 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 found 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, the researchers analyzed the selective pressure acting on these genes. The result of evolutionary dynamic analysis 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 facilitate 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.