The authors were interested in the genetic changes that occur when organisms become able to infect novel hosts. To investigate this they experimentally evolved replicate lineages of an RNA virus (Drosophila C Virus) in 19 different species of Drosophilidae and then sequenced the viral genomes. They observed a strong pattern of parallel evolution, with viral lineages from the same host becoming more genetically similar to each other than to lineages from other host species. When the authors compared viruses that had evolved in different host species, they found that these parallel genetic changes were more likely to occur if the two host species were closely related. This suggests that by becoming adapted to one host, a virus also becomes better adapted to closely related host species. Therefore this may explain why host shifts tend to occur between related species and therefore when a pathogen emerges in one species, any closely related species may become vulnerable to the new disease.
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