Call it the evolutionary penguin walk.
Over 50 million years ago, the adorable tuxedo-clad birds began to leave their avian parents on the shore, waddling to the water’s edge and diving in search of seafood.
Webbed legs, fin-shaped wings, and unique feathers have all helped the penguins adjust to their underwater excursions. But new research from the University of Nebraska-Lincoln has shown that the evolution of diving is also in their blood, which has optimized its capture and release of oxygen to ensure that the penguins do not waste their breath while holding it back.
Compared to land birds, penguin blood is known to contain more hemoglobin: the protein that scavenges oxygen from the lungs and carries it through the bloodstream before depositing it in various tissues. This abundance could in part explain the underwater skills of, for example, the Emperor Penguin, which dives deeper than any bird and has been documented holding its breath for over 30 minutes while preying on krill, fish and squid.
Still, the peculiarities of their hemoglobin – and how much it actually evolved to help penguins become torpedoes of fish that spend half their life underwater – have remained open questions. Nebraska biologists Jay Storz and Anthony Signore, who often study hemoglobin in birds that survive miles above sea level, have decided to investigate which birds are most apt to dive below.
“There just wasn’t a lot of comparative work on the transport of oxygen in the blood with respect to the physiology of diving in penguins and their non-diving parents,” said Signore, postdoctoral researcher in the Storz laboratory.
Answering these questions meant sketching out the genetic patterns of two ancient hemoglobins. One belonged to the common ancestor of all penguin species, which began to branch out from this ancestor around 20 million years ago. The other, dating to around 60 million years ago, resided in the common ancestor of penguins and their closest non-diving relatives – albatrosses, shearwaters and other flying seabirds. The thought was simple: because one hemoglobin arose before the emergence of line diving, and the other after, any major difference between the two would implicate them as important to the evolution of diving in penguins.
In fact, comparing the two was less straightforward. For starters, the researchers literally resurrected the two proteins by relying on models that take into account the genetic sequences of modern hemoglobins to estimate the sequences of their two ancient counterparts. Signore spliced these resulting sequences into E. coli bacteria, which produced the two old proteins. The researchers then carried out experiments to assess each person’s performance.
They found that hemoglobin from the common penguin ancestor captured oxygen more easily than the version found in the blood of the older, non-diving ancestor. This stronger affinity for oxygen would mean less chance of leaving traces in the lungs, a problem especially vital in semi-aquatic birds who must make the most of a single breath when hunting or traveling under the sea. water.
Unfortunately, the very strength of this affinity can present challenges when hemoglobin reaches the oxygen-deprived tissues it carries.
“Having a greater hemoglobin-oxygen affinity acts like a more powerful magnet to extract more oxygen from the lungs,” Signore said. “It’s great in this context. But then you’re lost when it’s time to let go.”
In other words, all of the breath retention benefits gained by scavenging extra oxygen can be negated if the hemoglobin has trouble loosening its iron hold and releasing its precious cargo. The likelihood of this happening is partly dictated by the acidity and carbon dioxide in the blood. Higher levels of either make the hemoglobins more likely to loosen.
As Storz and Signore expected, the hemoglobin of the penguin’s recent ancestor was more sensitive to its surrounding pH, with its biochemical hold on the release of oxygen in response to high acidity. And that, Signore said, made hemoglobin more biochemically responsive to the stress and oxygen requirements of the tissues it served.
“It really is a magnificent system, because hard-working tissues become acidic,” he said. “They need more oxygen, and the affinity of hemoglobin for oxygen is able to change in response to this acidity to provide more oxygen.
“If the pH drops by, say, 0.2 units, the oxygen affinity of the hemoglobin of the penguins will drop more than that of the hemoglobin of their non-diving parents.”
Taken together, the results indicate that when the penguins took to the sea, their hemoglobin evolved to maximize both collection and loss of available oxygen – especially when it was last inhaled five, 10, or even 20 minutes earlier. They also illustrate the value of resuscitating proteins that existed 20, 40, even 60 million years ago.
“These results demonstrate how experimental analysis of ancestral proteins can reveal the mechanisms of biochemical adaptation,” Storz said, “and also shed light on how the physiology of organisms has evolved in response to new environmental challenges.”
The researchers received support from the National Institutes of Health and the National Science Foundation.