Evaluation of global evolutionary variations in the early stage of SARS-CoV-2 pandemic.

To understand the origin of variants and their evolutionary history in the early stage of the COVID-19 pandemic, time-scaled phylogenetic and gene variation analyses were performed. The mutation patterns and evolution characteristics were examined using the Bayesian Evolutionary Analysis Sampling Trees (BEAST) with 349 whole-genome sequences available by March 2020. The results revealed five phylogenetic clusters (Groups A-E), with 408 nucleotide variants. The mutations including the deletion of three nucleotides underwent various and complicated changes in the whole genome over time, while some frequency or transient mutations were also observed. Phylogenetic analysis demonstrated that SARS-CoV-2 originated from China and was transmitted to other Asian countries, followed by North America and Europe. This study could help to comprehensively understand the evolutionary characteristics of SARS-CoV-2 with a special emphasis on its global variation patterns.