A flu virus found in a sturgeon hints that influenza viruses originated in fish around 600 million years ago and evolved with their hosts to infect a wide range of species today.
Influenza viruses cause illness in many vertebrates, including humans, birds, cats, whales and dolphins, but little is known about how they first evolved.
In 2018, researchers . By comparing its genetic sequence with other flu viruses to reconstruct a family tree, researchers found that the hagfish virus was probably closest to their ancestral form, suggesting these viruses may have originated in fish.
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Researchers have since found influenza viruses from more fish, including a spiny eel, a chum salmon and a type of tropical fish called the Siamese algae eater. It is still unclear whether the virus can replicate in living fish, how it is transmitted and how fish are affected by the infection.
Now, at the University of Sydney and her colleagues have found more evidence that influenza viruses first evolved in fish. To do this, they searched genetic data from fish for sequences similar to the previously identified hagfish-associated flu virus. In particular, they looked for a gene encoding a protein that is crucial for replicating the virus鈥檚 genome.
This revealed a version of the influenza virus that was 25 per cent genetically similar to the hagfish-associated virus, discovered in the intestine of a sturgeon. By comparing the genetic sequences of all known influenza viruses, the team found that the sturgeon virus appears to be most like the ancestor of these viruses.
As this sturgeon recently lived in a fish farm in China, this isn鈥檛 the exact form of the virus that would have first evolved millions of years ago, but it looks highly similar to what you would expect the real ancestral form to have looked like, says Petrone.
鈥淯nderstanding the evolution of the flu virus is really important because it could give new insights into its ability to jump between hosts, and help with detecting the next virus with pandemic potential,鈥� says Petrone.
鈥淭he study provides compelling evidence that influenza viruses have an aquatic origin,鈥� says at the University of Glasgow, UK.
However, he says it is important to keep in mind that researchers have only sampled a small portion of influenza virus diversity, so we should remain open to the possibility that our understanding of how influenza evolved will change with new data.
BioRxiv
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