Fish survive in very cold waters by producing antifreeze proteins. Two unrelated fish groups – one in the Arctic, one in the Antarctic – stumbled on the same protein through different evolutionary paths, one of which has astonished geneticists, as it does something most researchers thought impossible.
It's not unusual for unrelated species to evolve the same solution to specific challenges of nature. It's called convergent evolution, and is why dolphins and ichthyosaurs look alike, despite their vast separation in time and space.
The same thing can happen at the molecular level and has led Arctic cod and Antarctic notothenioid fishes to produce the same antifreeze, known as glycoproteins. The path the notothenioid fishes took has been known for some years, “But exactly how the codfish independently did it has remained a lasting puzzle,” said Professor Christina Cheng of the University of Illinois in a statement.
Genes usually evolve through replication and mutation of existing genes, so Cheng and her team searched other fish species for something that could have been the predecessor of the codfish glycoprotein gene. They found nothing.
Some codfish, living in warmer waters, don't produce the antifreeze, so Cheng compared their genomes with those that do make glycoproteins. In Proceedings of the National Academy of Sciences, she reports the ingredients from which the gene was made were found in the non-coding DNA of cod from more temperate waters.
Non-coding DNA, sometimes derisively known as junk DNA, doesn't produce proteins, which was once seen as DNA's whole job. On this basis, the majority of the genome was considered an irrelevant waste product. In recent decades we have learned non-coding DNA actually serves important purposes in living things, such as altering genes' expression to determine when and how much protein they produce.
Cheng's work highlights another role for non-coding DNA, acting as a raw material from which new genes can be constructed.
Making genes from junk is not easy. “Its development in these fishes that make their living in icy Arctic waters occurred as a result of a series of seemingly improbable, serendipitous events,” Cheng said.
The authors found a nine-nucleotide segment of non-coding DNA became duplicated several times, as sometimes occurs. In this case, the longer sequence codes amino acids that attach to ice crystals, preventing their growth to the point where they could harm the fish.
However, the antifreeze is useless as long as it remains inside the cells where it is produced. A random mutation caused the glycoprotein to be tagged for export from the cell to the blood, where it is needed, while another change caused the newly created gene to be transcribed into RNA, ensuring its continuance.
Cheng told IFLScience the emergence of functional genes from non-coding DNA; "May be more frequent than we know" because there is no easy way to determine function.
“Evolution is not that efficient,” Cheng said. “It's a make-do kind of thing.”
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