Mutation rate of SARS-CoV-2 and emergence of mutators during experimental evolution

Background and objectivesTo understand how organisms evolve, it is fundamental to study how mutations emerge and establish. Here, we estimated the rate of mutation accumulation of SARS-CoV-2 in vitro and investigated the repeatability of its evolution when facing a new cell type but no immune or drug pressures. MethodologyWe performed experimental evolution with two strains of SARS-CoV-2, one carrying the originally described spike protein (CoV-2-D) and another carrying the D614G mutation that has spread worldwide (CoV-2-G). After 15 passages in Vero cells and whole genome sequencing, we characterized the spectrum and rate of the emerging mutations and looked for evidences of selection across the genomes of both strains. ResultsFrom the mutations accumulated, and excluding the genes with signals of selection, we estimate a spontaneous mutation rate of 1.25x10-6 nt-1 per infection cycle for both lineages of SARS-CoV-2. We further show that mutation accumulation is heterogeneous along the genome, with the spike gene accumulating mutations at rate five-fold higher than the genomic average. We also observe the emergence of mutators in the CoV-2-G background, likely linked to mutations in the RNA-dependent RNA polymerase and/or in the error-correcting exonuclease protein. Conclusions and implicationsThese results provide valuable information on how spontaneous mutations emerge in SARS-CoV-2 and on how selection can shape its genome towards adaptation to new environments. Lay summaryMutation is the ultimate source of variation. We estimated how the SARS-COV-2 virus--cause of the COVID-19 pandemic--mutates. Upon infecting cells, its genome can change at a rate of 0.04 per replication. We also find that this rate can change and that its spike protein can adapt, even within few replications.