It is capable of leaping 30 to 40 feet in a single bound and pogo-sticking along at speeds of more than 50 mph. But at a sub-Saharan riverside, where the skittish impala stills itself for a drink in 100-degree heat, that platinum-medal athleticism is forgotten. For the past hour, a Nile crocodile has subtly submerged itself in that same murky river. The infamous teeth of the unseen apex predator latch onto a hindquarter of the impala when it leaps from the water to capture it, clenching its jaws with 5,000 pounds of force. However, the deep-breathing reptile drags its prey into the deep end, drowning them, and it is the water itself that kills them.
The crocodile's ambush was successful due to the nanoscopic scuba tanks—hemoglobin—that passed through its bloodstream. These hemoglobin move oxygen from its lungs to its tissues at a steady and slow rate, allowing it to survive for hours without air. Some biologists have wondered why, out of all the jawed vertebrates in the world, only crocodiles discovered such an effective method for maximizing a breath because of its hyperefficiency.
Jay and colleagues at the University of Nebraska-Lincoln have discovered new explanations for why by statistically reconstructing and experimentally resurrecting the hemoglobin of an archosaur, the 240 million-year-old ancestor of all crocodiles and birds. The unique properties of crocodiles hemoglobin came from 21 interconnected mutations that are scattered throughout the intricate component of red blood cells, rather than just a few key mutations, as previous research suggested.
According to the researchers, this complexity and the numerous knock-on effects that a single mutation can cause in hemoglobin may have forged an evolutionary path that was so complicated that nature was unable to retrace it even over tens of millions of years.
According to Story, a senior author of the study and the Willa Cather Professor of biological sciences at Nebraska, "everyone would be doing it if it was such an easy trick -- if it was that easy to do, just making a few changes."
Before swimming through the bloodstream and eventually delivering oxygen to the tissues that are dependent on it, all hemoglobin forms a bond with oxygen in the lungs. Organic phosphate molecules, which can coax hemoglobin into releasing its valuable cargo by attaching themselves to it, control the majority of vertebrate hemoglobin's affinity for capturing and holding oxygen.
However, a molecule called bicarbonate, which is created when carbon dioxide is broken down, has taken over the role of organic phosphates in crocodiles—crocodiles, alligators, and other crocodile-related animals. Hardworking tissues also indirectly produce a lot of bicarbonate because they produce a lot of carbon dioxide. This makes it easier for hemoglobin to deliver oxygen to the tissues that need it most.
Story stated, "It's a super-efficient system that provides a kind of slow-release mechanism that enables crocodiles to effectively exploit their onboard oxygen stores." They're able to stay underwater for so long because of it.
Chandrasekhar Natarajan, Tony Signore, and Bautista had already contributed to the understanding of the crocodile hemoglobin as postdoctoral researchers in Story's lab. Story's team decided to conduct a multidisciplinary investigation into the origins of the oxygen-ferrying marvel with colleagues from Denmark, Canada, the United States, and Japan.
In previous attempts to comprehend its evolution, known mutations were incorporated into human hemoglobin and functional changes were typically scant. Story was convinced that the approach was flawed by recent lab results. After all, there were a lot of differences between human hemoglobin and that of the ancient reptiles, from which crocodiles today evolved.
Story stated, "What's important is to understand the effects of mutations on the genetic background in which they actually evolved. This means making vertical comparisons between proteins from ancestral and descendant species rather than horizontal comparisons between proteins from contemporary species." You can ascertain the truth by employing that strategy."
Therefore, the team set out to reconstruct blueprints of hemoglobin from three sources with the assistance of biochemical principles and statistics: the archosaur ancestor, who lived 240 million years ago; the last bird with a common ancestor; as well as the 80 million-year-old common ancestor of modern crocodiles. The team found that only the hemoglobin of the direct crocodiles ancestor had bicarbonate sensitivity and lacked phosphate binding after testing all three of the resurrected hemoglobin in the laboratory.
The archosaur and crocodile ancestors' blueprints for hemoglobin were compared, and changes in amino acids, or the joints of the hemoglobin skeleton, that might have been important were discovered. Storz and his colleagues began introducing crocodile-specific mutations into the ancestral archosaur hemoglobin to test those mutations. The team was able to piece together the changes that were responsible for those distinct, crocodiles specific properties by locating the mutations that altered archosaur hemoglobin to behave more like that of a modern crocodile.
Story and his colleagues found that, contrary to conventional wisdom, evolved changes in hemoglobin's response to phosphates and bicarbonate were caused by different sets of mutations, so that the gain of one mechanism did not require the loss of another. In addition, their comparison revealed that while a few mutations were sufficient to eliminate the phosphate-binding sites, numerous additional mutations were required to completely eradicate phosphate sensitivity. Similar to this, two mutations appeared to be the direct cause of bicarbonate sensitivity, but only when they were combined with or preceded by other, easy-to-miss mutations in distant hemoglobin regions.
According to Story, the results demonstrate that a combination of mutations may result in functional changes that are greater than the sum of their individual effects. In any number of ways, a mutation that does not produce any functional effects on its own could open the door to other mutations that have clear, immediate effects. In a similar vein, he asserted, those subsequent mutations may have little effect without the appropriate stage-setting predecessors. Additionally, the setting in which they take place has the potential to either accelerate or impede their development.
Story stated, "It suggests that certain evolutionary solutions are only accessible from certain ancestral starting points when you have these complex interactions." With the ancestral hemoglobin of the archosaur, you have a genetic background that allows you to evolve the distinctive properties we see in hemoglobin of modern crocodiles. On the other hand, using the mammalian ancestor as a starting point, you might be able to evolve the same property, but you would need to use a completely different molecular mechanism because you would be working in a completely different structural context."
Story stated that the study also contributes to the explanation of the difficulty of engineering a human hemoglobin that can approximate the performance of the crocodile.
We cannot simply state, "OK, it's mostly because of these five mutations." Storz stated, "We'll have one with these exact properties, and we'll be able to stay underwater for two hours, too, if we just take human hemoglobin and introduce those mutations." That is not the case, as it turns out.
"The tree of life is full of problems that can't be solved by going here."
Together with Signore, who is now at the University of Manitoba, Story, Natarajan, and Bautista carried out the study; Angela of Aarhus University; Federico Hoffmann, a student at Mississippi State University; and Jeremy Tame of Yokohama City University. The National Science Foundation and the National Institutes of Health supported the research, which was published in the journal Current Biology.
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