Methodology to Define the Origin of SARS-CoV-2

The lineage-defining sequences of SARS-CoV-2 Lineage A and Lineage B act as the most ancestral roots. Lineage A (EPI_ISL_406801) is from GISAID, and Lineage B (MN908947) is from GenBank.

Register for a free GISAID account (https://gisaid.org/register/) to obtain EPI_ISL_406801

GenBank MN908947 (https://www.ncbi.nlm.nih.gov/nuccore/MN908947.3)

Identify lineages of interest in an area:

filter GISAID by location (e.g.: North America/USA/Hawai’i) and download all sequences. For VOC with >10,000 sequences, GISAID sequences were downloaded in batches due to GISAID maximum download size. Similarly, all geographically similar sequences reported in GenBank were downloaded using the search term SARS-CoV-2 and state abbreviation (e.g., “SARS-CoV-2 HI”) and the sequence length filter (20,000 - 40,000).

Combine the GISAID and GenBank sequences into one .fasta file using AliView, Geneious Prime, or a text editor, and assign lineages using Pangolin Lineage Assigner (pangolin.cog-uk.io).

Geneious Prime (http://www.geneious.com)

AliView (https://ormbunkar.se/aliview/)

Determine prevalence of Each Lineage:

Download the results to Microsoft Excel, use advanced filter to copy unique records of lineages to a new column (ex: column M), then use COUNTIF (e.g., =COUNTIF(2:1432,M2)) to determine prevalence of each lineage. Alternatively, upload the results to Google Sheets and use the =UNIQUE command (e.g., =UNIQUE(B2:B1432) followed by the above COUNTIF command.

Filter GISAID and GenBank by the lineage of interest (e.g., B.1.429) and download all sequences.

Combine lineage of interest (B.1.429) GenBank sequences, GISAID (B.1.429) sequences, and EPI_ISL_406801 into one fasta file.

Align sequences using multiple alignment using fast Fourier transform (MAFFT) program or server with MN908947 as a reference and do not remove any uninformative sequences and all parameters set as “same as input.

MAFFT (https://mafft.cbrc.jp/alignment/server/add_fragments.html?frommanualnov6)

Remove the newly added MN908947 sequence that MAFFT places at the beginning of the alignment using AliView, Geneious Prime, or a text editor. If not, the sRNA toolbox will remove the MN908947 sequence during the duplicate removal step, and Lineage B will not serve as an ancestral root in the phylogenetic tree.

Import Multiple Sequence Alignment (MSA) file into Geneious Prime or AliView, search for the orf1a 5’ start of the entire alignment (5’- atg gagagccttgtccctggtttca-3’) and remove the 5’ untranslated region (UTR) by deleting the upstream region (~265 bp) from the MSA. Next, search for ORF10 3’ end (5’-tgtagttaactttaatctcaca tag -3’) and remove the entire 3’ UTR by deleting the downstream region (~229 bp) from the MSA.

Create a duplicate file for the MN908947 sequence and remove the 5’ UTR and 3’ UTR from MN908947 as described above.

Using MAFFT, align the trimmed MSA with the trimmed MN908947 as a reference and delete sequences with uncalled nucleotides ‘n’. Set the “remove uninformative sequences” parameter in the MAFFT at >0%.

Using sRNAtoolbox program or server, load the updated alignment to remove duplicate sequences and merge identifications (also referred to as sequence accession numbers) of duplicates. This merger will create “appendages” in the phylogenetic tree where the sRNA toolbox will line up identical sequences together with equal signs (=).

sRNAtoolbox ((https://arn.ugr.es/srnatoolbox/helper/removedup/)

Import the final alignment into Geneious Prime and create an approximately maximum-likelihood phylogenetic tree using the FastTree program. Alternatively, FastTree can run as standalone software, and FastTreeMP is appropriate when multiple CPU cores/threads are available.

Root the tree with Lineage A (EPI_ISL_406801), which should then be the most recent common ancestor (MRCA) to Lineage B (MN908947) if performing phylogenetics on a Lineage B subgroup. Identify the MRCA of each sequence of interest.