Comparative analysis estimates the relative frequencies of co-divergence and cross-species transmission within viral families

Geoghegan, Jemma L.; Duchêne, Sebastián; Holmes, Edward C.

Given the evolutionary and ecological barriers a virus must overcome to cross the species barrier and successfully establish itself in a new host, it might seem reasonable to assume that successful cross-species transmission is a relatively rare occurrence [9] . Indeed, many emerging diseases are in reality 'spill-over' infections, in which onward transmission between members of a new host species is limited such that extinction of the novel virus occurs rapidly [5] . Nevertheless, it is possible that an increased sampling of hosts and their viruses will reveal more instances of host jumping, in turn implying that cross-species transmission is a fundamental aspect of virus evolution [8] . As a case in point, although there is strong evidence that hepadnaviruses have co-diverged with their vertebrate hosts over hundreds of millions of years [10] , the recent identification of hepadnaviruses in fish and amphibians has revealed more instances of cross-species transmission, potentially including that from aquatic to terrestrial vertebrates [11] .