Evolution of giant pandoravirus revealed by CRISPR/Cas9

Until today, genetic tools have been lacking to enable manipulation of amoebal giant viruses (GVs) by CRISPR/Cas9 technology. Here, Bisio et al. apply S. pyogenes Cas9 together with pU6- driven guide RNAs to investigate the replication of pandoravirus, a GV replication in the nucleus. Using this tool, they provide evidence for stepwise evolution and genetic expansion of viral gigantism. Giant viruses (GVs) are a hotspot of unresolved controversies since their discovery, including the definition of Virus and their origin. While increasing knowledge of genome diversity has accumulated, GV functional genomics was largely neglected. Here, we describe an experimental framework to genetically modify nuclear GVs and their host Acanthamoeba castellanii using CRISPR/Cas9, shedding light on the evolution from small icosahedral viruses to amphora-shaped GVs. Ablation of the icosahedral major capsid protein in the phylogenetically-related mollivirus highlights a transition in virion shape and size. We additionally demonstrate the existence of a reduced core essential genome in pandoravirus, reminiscent of their proposed smaller ancestors. This proposed genetic expansion led to increased genome robustness, indicating selective pressures for adaptation to uncertain environments. Overall, we introduce new tools for manipulation of the unexplored genome of nuclear GVs and provide experimental evidence suggesting that viral gigantism has aroused as an emerging trait.

Automated summary

In this study, the researchers used CRISPR/Cas9 technology to genetically modify amoebal giant viruses, and found evidence suggesting that their giant size evolved gradually, possibly as an adaptation to uncertain environments.