Jumping between species helps viruses evolve

Researchers from the University of Sydney have discovered that viruses jump between species more often than previously thought — an action which plays a significant role in virus evolution.

Scientists had previously hypothesised that viruses usually co-diverge with their hosts, forming new viral species as their hosts evolve into new species. It was additionally assumed that cross-species jumps — in which a virus jumps from one host species to another and adapts to the new host — were relatively rare and contributed less to virus evolution.

To better understand how viruses evolve, Jemma Geoghegan and her colleagues compared the evolutionary histories of viruses and host species. While previous studies had focused on narrow groups of viruses, Geoghegan’s team studied 19 virus families that infect a variety of hosts, including mammals, birds, reptiles, amphibians, fish, plants and insects.

The researchers began with branching ‘tree’ diagrams that illustrated the evolutionary history of each virus family and its host species. Like family trees, these evolutionary trees trace the lineage of species back through common ancestors that later evolved into new species.

The scientists compared the evolutionary trees of viruses and hosts, using a previously developed method that measures similarity between trees. Co-divergence resulted in host and virus trees with similar branching patterns, as the virus evolves alongside the host, while cross-species jumps resulted in dissimilar host and virus trees, as new viruses evolve and jump from host to host.

Writing in the journal PLOS Pathogens, the scientists found that cross-species transmission played a central role in evolution for all 19 virus families, while co-divergence was relatively rare. Cross-species jumps were especially frequent in virus families whose genetic material was encoded in RNA rather than DNA, particularly the virus families Rhabdoviridae and Picornaviridae.

“Jumping hosts is the way many RNA viruses live their life,” said project leader Professor Edward Holmes.

“The more new viruses we discover, then the more examples of species jumping we are likely to see.”

The study authors noted that the research was performed at the level of virus families, and not for individual viral species. Further studies with larger datasets could help confirm the findings and provide further insight into virus evolution.

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