Heterogeneous Evolution Among SARS-CoV-2 Genes and Variants of Concern.

Challenges persist regarding the influence of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) on public health, with growing interest in future viral molecular variants. In this context, accurate predictions demand a thorough understanding of the virus's molecular evolution, especially proteins targeted by therapies, where certain discrepancies among studies exist. We analyzed thousands of SARS-CoV-2 genomes to assess the rate of evolution and molecular adaptation in the various SARS-CoV-2 coding regions. We found an overall low genetic diversity along the genome, with fluctuations over time and among genomic regions, and a notable increase in the Omicron variant, especially in the S and ORF6 genes. We also estimated an overall rate of molecular evolution of approximately 10 substitutions per site per year, though it varied among genomic regions and over time. Actually, most genomic regions did not follow the strict molecular clock. Regarding selective pressures, the protein-coding regions of SARS-CoV-2 generally exhibited evidence of purifying selection, with local diversifying selection associated with virus transmission and replication. Overall, the molecular evolution of SARS-CoV-2 displays heterogeneity among genomic regions and over time. These findings suggest that forecasting SARS-CoV-2 molecular evolution is not straightforward and remark the importance of continuing to monitor SARS-CoV-2 evolution.