What makes Antarctica cool

Cool adaptations to the cold

You really don't want to have to live there: in the Arctic Ocean around the South Pole, the water temperature is just under minus two degrees. People would have no chance of survival there, and that is too cold for most fish species: their blood would simply freeze, and ice crystals would burst their red blood cells - the erythrocytes. And yet there is a species of fish that feels comfortable under such hostile conditions and that reproduces there: the so-called ice fish from the family Nototheniidae.

An international team of scientists has now investigated which genetic adaptations are responsible for the fact that ice fishing does not mind even extreme cold, and has come across a number of characteristic changes. The geneticist Manfred Schartl, who holds the Chair of Physiological Chemistry at the Julius Maximilians University of Würzburg (JMU), was involved. In the current issue of the trade journal Nature Ecology & Evolution the researchers present the results of their investigations.

Life in sub-zero temperatures

In their study, Manfred Schartl and Hyun Park, together with John Postlethwait, who carried out research at the JMU Biozentrum as a Humboldt Prize winner in 2009, and other researchers from Korea and the USA, investigated the genome of the Antarctic black fin ice fish, Chaenocephalus aceratus, sequenced and searched for special changes that are responsible for the unique physiology. They also got insights into the development of this fish in the course of evolution. "Ice fish populations first appeared at the end of the Pliocene, after the surface temperatures of the Antarctic dropped by 2.5 degrees Celsius," explains Schartl. About 77 million years ago they had evolved away from the line of their ancestors - the sticklebacks - and then developed phenotypes that were better and better adapted to the cold.

Original nototheniids were red-blooded, but had no oxygen-binding proteins, so-called myoglobins, in their skeletal muscles. They also lived on the ocean floor and had no buoyancy-producing swim bladders. When the Antarctic cooled down and finally reached temperatures of just under minus two degrees Celsius around ten to 14 million years ago, new ecological niches opened up that ice fish were able to occupy thanks to special adaptations. Eight fish species from the notothenioid family, including the ice fish, also saw the opportunity to use the food available at a higher altitude - away from the sea floor - for themselves.

Blood without red blood cells

It is a combination of several factors that enables ice fishing to survive in extreme cold. The most striking of these: The animals lack red blood cells - and thus hemoglobin; their blood is therefore more or less transparent. Manfred Schartl explains that they still do not suffer from a lack of oxygen: "At the low temperatures, the oxygen saturation of the seawater and thus of all the fish's body fluids is so high that the oxygen transport by the auxiliary molecule hemoglobin is no longer necessary." the blood volume is twice as large as that of comparable fish species in temperate latitudes, their heart is enlarged and the blood vessels are also larger in diameter. The number of energy suppliers of the cells - the mitchondria - is increased in ice fish.

Another evolutionary achievement of this genus enables survival in freezing temperatures: ice fish produce special proteins that protect them from death by cold. While antifreeze glycoproteins prevent the formation of ice in the body of fish larvae and adult animals, ice-resistant egg chorion or zona pellucida proteins surround embryos and protect them from freezing.

Significant changes in the genome

All these changes have left clearly visible traces in the genome of the ice fish: "Our results show that the number of genes that are involved in protection against ice damage, including the genes that encode antifreeze glycoproteins, are greatly expanded in the ice fish genome" , explains Manfred Schartl. The high oxygen concentration both in the cold Antarctic waters and in the body of the ice fish has led to adaptations in the genetic make-up. Since oxygen radicals cause cell damage, the animals have more genes for enzymes that help to contain such damage.

At another point in the genome of the ice fish, the scientists came across changes that can be linked to the demanding habitat: they lack some important regulators that control the day-night rhythm of other animal species. The researchers suspect that the extremes of the almost permanent days of winter darkness and the long Antarctic summers may have diminished the usefulness of some of these regulators, and thus the evolutionary pressures to keep them. In order to finally answer this question, however, behavioral studies on Antarctic ice fish and other related species are necessary.

Model for a number of diseases

Their special properties make ice fish interesting for biomedical research. “Under natural conditions, they developed phenotypes that correspond to human diseases,” says Manfred Schartl. The lack of erythrocytes, for example, amounts to total anemia. In addition, in the course of their evolution, the animals have given up calcification in order to reduce their density. This had become necessary in order to be able to detach from the sea floor and swim in the open water again. To do this, they mainly had to reduce their body weight. Their bones or bones are therefore today in a condition that can be found in osteoporosis patients.

Antarctic blackfin icefish genome reveals adaptations to extreme environments. Bo-Mi Kim, Angel Amores, Seunghyun Kang, Do-Hwan Ahn, Jin-Hyoung Kim, Il-Chan Kim, Jun Hyuck Lee, Sung Gu Lee, Hyoungseok Lee, Jungeun Lee, Han-Woo Kim, Thomas Desvignes, Peter Batzel , Jason Sydes, Tom Titus, Catherine Wilson, Julian M. Catchen, Wesley C. Warren, Manfred Schartl, H. William Detrich III, John H. Postlethwait and Hyun Park. Nature Ecology & Evolution, https://doi.org/10.1038/s41559-019-0812-7

Contact

Prof. Dr. Manfred Schartl, T: 49 931 31-84149, [email protected]

By Gunnar Bartsch

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