Methods for making multiple alignment of genomic sequences for severe acute respiratory syndrome coronavirus 2

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) emerged in December 2019 and caused a pandemic. To monitor the global transmission pattern of SARS-CoV-2, it is required to constantly update the phylogenetic tree of genomic sequences with 29.9 kb, which may be time consuming. Phylogenetic analysis of SARS-CoV-2 may be accelerated by making a multiple alignment of nucleotide sequences using the CPA (combining pairwise alignments) method, in which a pairwise alignment is made for a reference and each of other sequences, and the pairwise alignments are combined into a multiple alignment. Here it is shown from the analysis of 3729 genomic sequences for SARS-CoV-2 and outgroup strains that the CPA method can produce a multiple alignment with an elevated or a reduced number of variable sites depending on the reference compared to the OMA (ordinary multiple alignment) method, which was considered to be the most reliable. In particular, the topology of the phylogenetic tree constructed from the multiple alignment made using the CPA method adopting the outgroup sequence as the reference was considerably different from that using the OMA method, suggesting that the outgroup sequence may not be suitable as the reference in the CPA method.