The role of myoglobin in the evolution of mammalian diving capacity - The August Krogh principle applied in molecular and evolutionary physiology.

After the Devonian tetrapod land invasion, groups of terrestrial air-breathing and endothermic mammals repeatedly went back to live in the sea, relying on air intake at the surface for extended breath-hold dives to forage underwater, often at great depths and even in the coldest oceans. Studies on the physiological mechanisms behind prolonged breath-hold diving have a long history, including August Krogh's estimates of the maximal dive duration of the blue whale. Yet the molecular underpinnings of such extreme physiological adaptations are only beginning to be understood. The present review focuses on the molecular properties of the respiratory protein myoglobin that has repeatedly evolved an elevated net positive surface charge in several distantly related groups of diving mammals. This has enabled substantial increases of maximal myoglobin concentration in muscle cells, and hence muscle oxygen storage capacity and maximal dive duration. Using myoglobin net surface charge as a marker has allowed unprecedented insights into the evolution of mammal diving capacity and into the general mechanisms of adaptive protein evolution. From these findings it is argued, in an extension of the August Krogh principle, that for a large number of problems in molecular and evolutionary physiology there will be some protein of choice, or a few such proteins, on which it can be most conveniently studied.