Engineering iridoviruses: Development of reverse genetics and virus rescue systems

Iridoviruses are a family of large DNA viruses that infect insects as well as poikilotherm vertebrates including amphibians, reptiles, and fish. Particularly members of the genus Ranavirus are known for causing mass mortality in fish and amphibians, posing a threat to aquaculture and contributing to global amphibian decline. Despite their omnipresence and negative impact, many important aspects of iridovirus biology remain unknown, largely due to the absence of a reverse genetics system.In this study, we developed, characterized, and utilized a reverse genetics system for Frog Virus 3 (FV3), a prototypic ranavirus and one of the most widely studied members of the iridovirus family. The virus recovered from this system exhibits growth and phenotypic properties identical to those of the parental virus isolate. Furthermore, we established an alternative approach of virus reconstitution from genomic DNA, utilizing a heterologous iridovirus as a helper virus. We show that the novel approach enables rapid and effortless recovery of modified viruses from naked DNA.The reverse genetics and rescue systems described in this study will greatly advance iridovirus research by facilitating efficient genetic modification of virus genome in yeast or bacteria. This could clear the path to elucidating functions of virus genes, and allow a much more detailed understanding of iridovirus biology. Moreover, owing to the promiscuous nature of FV3 with its ability to infect hosts from different classes of animals, the FV3 system has the potential to serve as a platform for the development of modified live vaccines for a variety of fish and amphibian species.