A phylogenetic method identifies candidate drivers of the evolution of the SARS-CoV-2 mutation spectrum.

The molecular processes that generate new mutations evolve, but the causal mechanisms are largely unknown. In particular, the relative rates of mutation types (e.g., C>T), the mutation spectrum, sometimes vary among closely related species and populations. I present an algorithm for subdividing a phylogeny into distinct mutation spectra. By applying this approach to a SARS-CoV-2 phylogeny comprising approximately eight million genome sequences, I identify 10 shifts in the mutation spectrum. I find strong enrichment consistent with candidate causal amino-acid substitutions in the SARS-CoV-2 polymerase, and strikingly three appearances of the same homoplasious substitution are each associated with decreased C>T relative mutation rates. With rapidly growing genomic datasets, this approach and future extensions promises new insights into the mechanisms of evolution of mutational processes.