SARS-CoV-2 variant evolution in the United States: High accumulation of viral mutations over time likely through serial Founder Events and mutational bursts
Fig 4
SARS-CoV-2 viral isolate signature frequencies change over time, but with different patterns across states, showing dynamic evolution by mutation, drift, selection and migration.
State-specific ridgeline plots indicate the density of each signature (y axis) over collection date (x axis). In each plot, peak colors gradually change to highlight transition in time (x axis), with the pink-shaded areas corresponding to periods of time where data was not available. States shown were selected only on abundance of sequence data throughout the year (n, number of viral genomes per state). Of note, the reference strain s0 is virtually absent by June 2020, while signature s48 is common from July 2020 through the end of the first quartile of 2021. Several new signatures found predominantly in 2021 (in more than 0.1% of the genomes) reveal a new and complex mutational profile, with a number of them being related to B.1.2 (blue labels), or B.1.1.7 (red labels) lineages (which were introduced by migration), as well as of other VOC lineages (S3A, S4 and S5 Figs).