Fish survival in the extreme cold aided by genomic adaptations

Below the sea ice of the Antarctic, which is largely isolated from the rest of the marine world by a circular current around the continent, a group of very special fish species lives in the icy water. The so-called notothenioids are remarkable in many ways. For example they have evolved antifreeze proteins which allow them to survive the water temperatures, which can reach -2°C (28 F) - a pretty hostile environment for most species. One subgroup of notothenioids called 'icefish' have lost their oxygen-binding hemoglobin proteins and that makes them the only vertebrates known to not have red blood. 

“Survival in such a harsh environment requires additional compensations of the organism”, Dr. Iliana Bista explains, “and these fish have developed special proteins that act as antifreeze to stop them from freezing.” Dr. Bista, a geneticist working with Naturalis Biodiversity Center in the Netherlands and Wellcome Sanger Institute in the UK, is lead author of the paper recently published in Nature Communications which helps explain the genomic evolution of the notothenioid fish. 

Split off

The international team of researchers from UK, the Netherlands, Norway, Switzerland, and the US have sequenced the genomes of 24 species of these fish. Using these new data they explore the evolutionary history of the notothenioids and the mechanisms that support adaptation to extreme cold. They show that the cold-resistant notothenioids split off from other species about 10.7 million years ago – more recent than was previously thought – and many new species started evolving rapidly approximately 5 million years ago. 

Several genomic characteristics have aided the survival and establishment of this group. They found that the size of the genome has doubled in the species that specialize in extreme cold, such as the family of Channichthyidae or 'icefish'. This expansion of the genome size was due to a large increase in the number of genomic elements known as transposons which have the ability to copy themselves into new positions within the genome and can potentially introduce new functions. 

White blood

At the same time functions that are normally considered essential for survival such as the production of hemoglobin, have been lost in the 'icefish'. “These fish are the only vertebrates known to have completely lost their hemoglobins, and as a result their blood looks white. This is remarkable because hemoglobins are needed to transport oxygen through the body. Their loss in icefish is only possible because oxygen dissolves better in water at very low temperatures, and because of additional genomic and physiological adaptations”, Dr. Bista explains.  

Senior author, Richard Durbin from the Wellcome Sanger Institute in Cambridge, explains why this sort of research is important: “Notothenioid fish live at the edge of viability.  Sequencing a broad collection of their genomes gives insights into how they have evolved to survive there, and supports our understanding of a critical ecosystem. This study is a great example of how advances in genomics are revolutionizing our ability to understand biodiversity across the world.”

More information

• The paper called 'Genomics of cold adaptations in the Antarctic notothenioid fish radiation' has been published in Nature Communications this week.
• This study constitutes an extensive new resource and all the data generated are freely accessible, which will support future studies by the wider community. 
• The Wellcome Sanger Institute is a world leading genomics research centre. Find out more at Sanger.ac.uk.
• Wellcome supports science to solve the urgent health challenges facing everyone. Find more information at Wellcome.org.

Text: Naturalis Biodiversity Center
Photos: Doug Allan (lead photo: blackfin icefish Chaenocephalus aceratus); Chris Gilbert