Source: Deutsche Nachrichten
With water temperatures as low as minus 1.9 degrees Celsius, the Southern Ocean is a place of extreme living conditions. A new study led by Senckenberg researchers shows how fish have adapted to Antarctic temperatures. The researchers used the Antarctic spiny plunderfish Harpagifer antarcticus as a model organism and, for the first time, analyzed the maternal and paternal copies of its genome separately. This allowed them to uncover mechanisms that contribute to this group of fishes’ adaptation to extreme cold. The results, published in the journal “Genome Biology,” show that survival in extreme cold depends on the interplay of various genetic and evolutionary adaptation mechanisms.
Minus 1.9 degrees Celsius – that’s how cold the water is in the Southern Ocean surrounding Antarctica. This poses a challenge for the fishes living there: In cold temperatures, proteins can lose their function, and the water’s high oxygen content can put additional stress on the cells. “Over the course of their evolution, around 140 notothenioid fish species have developed special strategies to survive in the icy world of Antarctica,” explains Dr. Iliana Bista, the study’s last author from the Senckenberg Research Institute and Natural History Museum in Frankfurt, and she adds, “The Antarctic spiny plunderfish Harpagifer antarcticus demonstrates just how well these fishes have adapted to the extreme conditions. This small, scaleless fish lives in the shallow waters of the Southern Ocean on rocky bottoms and has, among other things, developed special antifreeze proteins that prevent ice crystals from continuing to grow.”
Working with an international research team that included researchers from the British Antarctic Survey (BAS), the Wellcome Sanger Institute, the University of Cambridge and the University of Colorado, Bista has now succeeded for the first time in assembling the genome of Harpagifer antarcticus in such a way that the two genetic variants inherited from the mother and father – known as haplotypes – can be analyzed separately. “This distinction is scientifically significant because differences between maternal and paternal genetic material can remain hidden in conventional genome sequencing methods. By analyzing the haplotypes separately, these differences can be examined more closely – for example, in gene sequences, in inserted or deleted DNA segments, and in genomic regions where certain genes occur multiple times,” says Bista.
The results surprised the research team: The maternal and paternal genomic variants of the Antarctic spiny plunderfish were remarkably similar overall – one variant contains 17 copies, the other 16. However, the researchers found significant differences in repetitive DNA segments and in the length of the genes responsible for antifreeze proteins. “The length of the gene segments differs significantly. Since larger antifreeze proteins can inhibit the growth of ice crystals particularly effectively, these differences could help the fishes adapt to varying conditions in Antarctica,” explains the Frankfurt-based researcher.
However, adaptation to extreme cold is not solely determined by the genes for antifreeze proteins. According to the new study, approximately 43 percent of the genome of the fishes examined consists of repetitive DNA segments. Among these, so-called LINE-L2 transposons are particularly common. These mobile DNA segments can change their position within the genome or create new copies of themselves – which is why they are also referred to as “jumping genes.” The researchers found evidence that these elements multiplied particularly rapidly at certain times during the evolution of Antarctic fishes. As a result, they may have contributed to the expansion of the genome and the emergence of new genetic variants. In addition, the team discovered evidence of adaptations in genes involved in, among other things, defending against harmful oxidative processes, fat metabolism, the proper folding of proteins, and the maintenance of important cellular functions – all processes that are crucial for survival under extremely cold conditions.
“Our results show that Antarctic fishes have adapted to extreme cold through the interplay of various evolutionary processes. By comparing different species and analyzing maternal and paternal genetic variants separately, we were able to identify genetic differences that might have gone undetected using conventional methods. Our study illustrates how genetic changes help Antarctic fishes to survive even under the extreme conditions of the Southern Ocean,” adds Bista in summary.