Insertion and deletion mutations preserved in SARS-CoV-2 variants

The insertion/deletion (indel) mutation profiles of SARS-CoV-2 variants, including Omicron, remain unclear. We compared whole-genome sequences from various lineages and used preserved indels to infer the ancestral relationships between different lineages. Thirteen indel patterns from twelve sites were seen in >= 2 sequences; six of these sites were located in the N-terminal domain of the viral spike gene. Preserved indels in the coding regions were also identified in the non-structural protein 3 (Nsp3), Nsp6, and nucleocapsid genes. Seven of the thirteen indel patterns were specific to the Omicron variants, four of which were observed in BA.1, making it the most mutated variant. Other preserved indels observed in the Omicron variants were also seen in Alpha and/or Gamma, but not Delta, suggesting that Omicron is phylogenetically more proximal to Alpha. We demonstrated distinct profiles of preserved indels among SARS-CoV-2 variants and sublineages, suggesting the importance of indels in viral evolution.