In vitro evolution of Cyprinid herpesvirus 3

Viruses are able to evolve in vitro by mutation after serial passages on cell cultures, which can lead to either a loss or an increase of virulence. Cyprinid herpesvirus 3 (CyHV-3), a 295-kb double stranded DNA virus, is the etiological agent of the koi herpesvirus disease. To assess the influence of serial passages on its genome, an isolate of CyHV-3 (KHV-T) was passaged 99 times onto common carp brain cells and passages 78 (P78) and 99 (P99) were fully sequenced and compared with the wild-type form (P0). Genome comparisons revealed a limited number (<50) of small variations spread all over the genomes. The largest one was a deletion of 1 363 bp in the predicted ORF150, which was detected in P78 but not in P99 or P0. This unexpected finding, which was confirmed by PCR, suggests that CyHV-3 evolves, at least in vitro, through an assemblage of haplotypes that alternatively become dominant or under-represented.