In the Atlantic Ocean, 2,000 km from the Cape of Good Hope, lies a tiny island called Bouvet, known as the most remote island in the world. With a thick layer of ice that ends abruptly in sharp cliffs with black volcanic sand beaches, and an average temperature below the freezing point of water, it is not that it is a pretty vacation spot. Disembarking is not easy and the best way to do it is from a helicopter.
In 1928 the Norwegian ship Norvegia landed on the island with the aim of turning it into a refuge and store of provisions for shipwrecked sailors . Sailors from the Norvegia were there for a month. Among them was the ship’s biologist, Ditlef Rustad, a zoology student, who captured a curious fish : large eyes, a large jaw full of teeth, long spines on the pectoral and tail, and most surprisingly, it gave the impression look at it to be transparent. Upon closer examination, he discovered that its “white crocodile fish” appearance was due to the fact that its blood had no color at all.
The challenge of living in very cold waters
The Champsocephalus gunnari , like many other fish that live in the cold Antarctic waters, does not have red blood cells, present in all vertebrates on the planet and whose characteristic color is given by the presence of hemoglobin, which fixes oxygen inside and transported to the cells of the body. The study of their DNA has discovered that the two genes that code for the globin part of the molecule have disappeared . One is a simple molecular fossil, a memory stored in its genome of “something” that fulfilled a function tens of millions of years ago and that has been eroding like a fossil exposed to the elements. The other, which is usually right next to the previous one, has completely disappeared.
The fish that live in the Antarctic Ocean, to reduce the increase in blood viscosity due to the very low water temperatures, must reduce the density of red blood cells in the blood. In this way, if we have a hematocrit of 45%, they have lowered it from 15 to 18%. This fish has taken the reduction to the extreme, so that its blood only carries 1% of cells , and all of them are white blood cells. Ice water literally runs through his veins .
Antifreeze in the blood
The heart of this fish, larger than the rest, has a pale color, not at all like the pinkish color of the rest of the vertebrates and that is due to the presence of a protein called myoglobin. In the case of the ‘ice-fish’, 5 letters have been inserted in the gene that encodes this protein and that completely alters it, turning it into a fossil gene. But the most interesting thing is that one of the processes of adaptation to such a freezing environment consists of the appearance of genes that create antifreeze proteins , which prevent the poor fish from turning into an ice statue. Where have they come from? The answer was found by Cheng, DeVries, and their colleagues at the University of Illinois in 1997: everything originated from a 9-letter gene that encoded a digestive enzyme , was reinstalled elsewhere in the genome, and the code evolved into the one that encodes much-needed antifreeze.
An example of the evolutionary process
This Antarctic fish is a clear example of the evolutionary process. To survive in an extreme environment, he invented an antifreeze, enlarged the heart, changed his blood and got rid of parts that had been present in all types of fish for 500 million years. As biologist Sean B. Carroll says, it’s “like completely changing the engine while the car is running.”
The DNA of all species is proof of how the evolutionary process discovered by Darwin has worked. As in the case of the Antarctic fish, the new elements do not appear by magic, designed from scratch, but the materials that are available are used. With the weapons of molecular biology we are capable of following the traces that millions of years of evolution have left in the genetic heritage of all species . Some of these steps are very small, like the change of a single letter in the code of a gene; others are larger, with entire genes missing. The genome is not simply an inventory of all the genes that operate in our body; it is also a window to our most distant past.
Reference:
Carroll, A. B. (2005) Endless forms most beautiful, Norton & Co.
