Mammoth Hemoglobin Offers More Clues to Its Artic Evolution
New York Times
For the first time in 43,000 years, a woolly mammoth has breathed again on earth.
Well, not the mammoth itself but its hemoglobin, the stuff in red blood cells that takes on oxygen in the lungs and offloads it in the tissues. By reconstructing the mammoth’s hemoglobin, a team led by Kevin L. Campbell of the University of Manitoba in Canada has discovered how the once-tropical species adapted to living in arctic temperatures.
Dr. Campbell’s work raises a somewhat astonishing possibility: that much of the physiology of extinct animals may one day be recoverable from the DNA extracted from their remains.
“It is a very exciting result and opens a new chapter in paleontology, a subject usually constrained to examining old bones and teeth,” said Adrian Lister, an expert on mammoth evolution at the Natural History Museum in London.
Mammoths, despite their association with the frozen north, originated in the tropics when they split apart from elephants some seven million years ago. To adapt to the cold of northern latitudes, they developed smaller ears, a thick fur coat and glands in their skin to keep the fur well oiled.
So much is clear from their remains. But other kinds of adaptation, which have not survived, would also have been necessary. Most arctic animals arrange their blood vessels so that the arteries going down a leg can transfer heat to the veins coming up. The blood reaching the toes is thus quite cold, and the animal conserves lots of heat while it stands on frozen ground.
But this arrangement raises a problem for the hemoglobin, the protein of red blood cells that takes on oxygen in the lungs and delivers it in the tissues. The offloading process becomes much less efficient at low temperatures. So animals like the arctic fox, whose foot temperature is just a degree or so above freezing, have changes in their hemoglobin genes that enable the protein to release oxygen more easily at very low temperatures.
Dr. Campbell set out nine years ago to see if the same was true of mammoths. His first problem was to figure out which of several globin genes were active in the species. Globin genes make the four globin chains from which the hemoglobin molecule is assembled. Humans have at least four globin genes — alpha, beta, gamma and delta. Most hemoglobin molecules in human blood consist of two alpha chains and two beta chains, but in 10 percent of the blood, delta chains substitute for the betas. The gamma globin gene is active only in the fetus.
Dr. Campbell figured that the hemoglobin system of living elephants would offer the best guide to how mammoth globin genes operate. After a frustrating effort to get permits to take samples from wild elephants, he acquired blood from an Asian elephant called Caesar at the Bowmanville Zoo in Ontario.
It turned out that way back in the elephant lineage, the beta and delta globin genes had swapped DNA to create a hybrid beta-delta chain. Elephant hemoglobin molecules are composed of two chains from the alpha globin gene and two from the fused beta-delta gene, and it is reasonable to assume mammoths had the same system.
With this knowledge, Dr. Campbell and his colleagues could construct the tools to fish out the alpha and beta-delta globin genes from the ancient DNA of three permafrost-preserved Siberian mammoths that lived between 25,000 and 43,000 years ago. They found the alpha chain differed in one of its amino acid units from that of Asian elephants, and the beta-delta chain differed in three units from its counterpart, they report in Nature Genetics.
The team’s next step was to synthesize copies of the mammoth’s two globin genes. Instead of doing that from scratch, Dr. Campbell used a technique for altering DNA units one by one and simply converted the Asian elephant’s two globin genes at the four differing sites to the mammoth version. The mammoth genes were then inserted into bacteria, which synthesized the two mammoth globin chains, inserted the required iron atoms, and assembled the chains into working hemoglobin molecules. With the mammoth hemoglobin in hand, Dr. Campbell could at last address the question of whether its genetic changes had been shaped by natural selection to help mammoths survive in the cold.
“It’s the same as if I went back 43,000 years in a time machine and took blood from a mammoth,” he said.
The answer was yes: In a chemical environment like that in red blood cells, the reconstructed mammoth hemoglobin let go of its oxygen much more readily at cold temperatures than did that of Asian elephants.
The DNA changes in the mammoth hemoglobin genes differ from those in other arctic animals, an instance of convergent evolution or attaining the same end by a different genetic route.
One species that did not modify its hemoglobin genes to cope with arctic temperatures is that of humans. “With our ability to make mitts and hats, we’ve not needed these sorts of changes,” Dr. Campbell said.
He is now reconstructing important proteins of other extinct species such as the mastodon, the woolly rhinoceros and the Steller’s sea cow, a huge dugong that lived in the arctic.
Two years ago, scientists atUniversity sequenced a large part of the mammoth’s genome from a clump of hair. They published the sequence along with the arresting suggestion that for just $10 million it might be possible to complete the sequence and use it to generate a living mammoth.
The suggestion was not as wild as it might seem, given that the idea came from George Church, a leading genome technologist at the Harvard Medical School. The mammoth’s genome differs at about 400,000 sites from that of the African elephant. Dr. Church has been developing a method for altering 50,000 sites at a time, though he is not at present applying it to mammoths. In converting four sites on the elephant genome to the mammoth version, Dr. Campbell has resurrected at least one tiny part of the mammoth.
Reconstructing the whole animal will take a little longer. “I’m 42 years old,” he said, “but I doubt I’ll ever see a living mammoth.”