A new study reconstructing the evolutionary tree of flu viruses challenges conventional wisdom and solves some of the mysteries surrounding flu outbreaks of historical significance.
Zoonotic infectious diseases such as influenza continue to pose a grave threat to human health1. However, the factors that mediate the emergence of RNA viruses such as influenza A virus (IAV) are still incompletely understood2, 3. Phylogenetic inference is crucial to reconstructing the origins and tracing the flow of IAV within and between hosts3, 4, 5, 6, 7, 8. A group of scientists now show that explicitly allowing IAV host lineages to have independent rates of molecular evolution is necessary for reliable phylogenetic inference of IAV and that methods that do not do so, including ‘relaxed’ molecular clock models9, can be positively misleading. A phylogenomic analysis using a host-specific local clock model recovers extremely consistent evolutionary histories across all genomic segments and demonstrates that the equine H7N7 lineage is a sister clade to strains from birds—as well as those from humans, swine and the equine H3N8 lineage—sharing an ancestor with them in the mid to late 1800s. Moreover, major western and eastern hemisphere avian influenza lineages inferred for each gene coalesce in the late 1800s. On the basis of these phylogenies and the synchrony of these key nodes, which in turn suggests that the internal genes of avian influenza virus (AIV) underwent a global selective sweep beginning in the late 1800s, a process that continued throughout the twentieth century and up to the present. The resulting western hemispheric AIV lineage subsequently contributed most of the genomic segments to the 1918 pandemic virus and, independently, the 1963 equine H3N8 panzootic lineage. This approach provides a clear resolution of evolutionary patterns and processes in IAV, including the flow of viral genes and genomes within and between host lineages.
The new 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.
"We now have a really clear family tree of theses viruses in all those hosts – including birds, humans, horses, pigs – and once you have that, it changes the picture of how this virus evolved," said Michael Worobey, a professor of ecology and evolutionary biology at the University of Arizona, who co-led the study with Andrew Rambaut, a professor at the Institute of Evolutionary Biology at the University of Edinburgh. "The approach we developed works much better at resolving the true evolution and history than anything that has previously been used."
Worobey explained that "if you don't account for the fact that the virus evolves at a different rates in each host species, you can get nonsense – nonsensical results about when and from where pandemic viruses emerged."