Reconstructing the evolutionary tree of influenza viruses

Reconstructing the evolutionary tree of influenza viruses.

The work is a collaboration between Andrew Rambaut in Edinburgh and his collaborator at the University of Arizona, Michael Worobey.  The new study reconstructs the evolutionary tree of flu viruses and both challenges conventional wisdom, and solves some of the mysteries, surrounding flu outbreaks of historical significance.

Left:  Data demonstrating the ability of the new model to predict current evoluiotn of influenza viruses (from Nature);  Right: Electron micrograph of purifiued influenza virus particles

The paper provides the most comprehensive analysis to date of the evolutionary relationships of influenza viruses across different host species over time. In addition to dissecting how the virus evolves at different rates in different host species, the study challenges several tenets of conventional wisdom, for example the notion that the virus moves largely unidirectionally from wild birds to domestic birds rather than with spillover in the other direction.  It also helps resolve the origin of the virus that caused the unprecedentedly severe influenza pandemic of 1918.

This research is likely to change how scientists and health experts look at the history of influenza virus, how it has changed genetically over time and how it has jumped between different host species. The findings may have implications ranging from the assessment of health risks for populations to developing vaccines

The team analyzed a dataset with more than 80,000 gene sequences representing the global diversity of the influenza A virus and analyzed them with their newly developed approach.  Using the new family tree of the flu virus as a map, the researchers showed which species moved to which host species and when. The map revealed that for several of its genomic segments, avian influenza virus is not nearly as ancient as was often assumed.

With regard to humans, the research sheds light on a longstanding mystery.  Ever since the influenza pandemic of 1918 it has not been possible to narrow down even to a hemisphere the geographic origins of any of the genes of the pandemic virus.  However the new study shows that  in fact most of the influenza virus genome jumped from birds very close to 1918 in the Western Hemisphere, and there is a suggestion that it was North America in particular.

The results also challenge the accepted wisdom of wild birds as the major reservoir harbouring the flu virus from where it jumps to domestic birds and other species, including humans.

Professor Michael Worobey, Department of Ecology and Evolutionary Biology, University of Arizona, said:

People tend to think of wild birds as the source of everything, but we see a very strong indication of spillover from domestic birds to wild birds.  It turns out the animals we keep for food and eggs may be substantially shaping the diversity of these viruses in the wild over time-spans of decades".

Source University of Arizona

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