A global phylogeny of SARS-CoV-2

This repository provides a regularly-updated global phylogeny of SARS-CoV-2 data from GISAID, as well as reproducile methods with which to estimate it.

We gratefully acknowledge the authors, originating and submitting laboratories of the sequences from GISAID’s EpiFlu™ Database on which this research is based. The list is detailed in this file.

Can I use it and cite it

Yes. Every new release from 22-July-20 to 13-November-20 is available at this DOI:

Releases subsequent to 13-November-20 are available on the GISAID website. To obtain the trees and alignments from GISAID you'll need a membership, but this is free and new requests are processed quickly. Once you have a GISAID membership, follow these steps to get the tree and the corresponding alignment:

1. Log in to GISAID
2. In the top menu bar, click the "EpiCoV" tab
3. Click the "Audacity" tile (with a big rainbow tree on it, near the bottom) to get the tree
4. Click the "Downloads" link just under the "EpiCoV" tab, then choose the alignment you want to download

The alignment that is used to make the tree is the one labelled "MSA masked" followed by the date. Older alignments are in the 'Archive' box.

Key files

The latest tree and alignment, with the associated code to reproduce that tree can always be obtained from GISAID (see above)

How do I look at these trees?

The trees contain more than 30,000 sequences. If you would like to visualise them, I recommend using Dendroscope. This is very fast, has a very quick search that allows you to find sequences of interest, and allows you to switch easily between cladograms, phylogenies, etc. The image above is a screenshot from Dendroscope.

Why isn't the sequence I'm intersted in in the tree?

Sequences are filtered out for a few reasons:

1. Sequence is not marked as both 'complete' and 'high coverage' by GISAID
2. Shorter than 28,000 bp after alignment
3. Contains more than 1000 ambiguities after alignment
4. Identified as being on a long branch by TreeShrink
5. Identified as questionable in the excluded_sequences.tsv file

If your sequence is in GISAID, and was submitted before the date noted in the latest release of the repository, but it is not in the tree, then it was filtered for one of the above reasons.

Why are there three trees in some releases, and what are all the numbers?

Up to and including 31-7-20, each release contains three trees. This section describes the differences between those trees. After 31-7-20 there is only one tree in each release.

The topology and branch lengths of the three trees are identical. In all cases, the topology is the best topology estimated by fasttree with options tuned specifically for this dataset, see here and here. The branch lengths represent substitutions per site. You will see that if you multiply the branchlenghts by about 30,000 (which is roughly the length of the alignments) many of the branchlengths are close to integers. That's because there's very little variation in these sequences, meaning that many branches have some integer number of changes inferred on them.

The only difference between the three trees are the support values represented for each branch. One tree (ft_FBP.tree) contains standard bootstrap values (also known as Felsenstein Bootstrap Proportions, or FBP). The next has support values calculated using the Transfer Bootstrap Expectation (or TBE). You can read more about these two values and how they differe here and here. The third tree has support values calculated by the SH test in fasttree.

The difference between the TBE and FBP is fairly simple to describe. The FBP value represents the percentage of bootstrap trees in which a split appeared. The TBE value represents the proportion of taxa in a clade that are, on average, stable across bootstrap replicates. The TBE is always at least as high as the FBP. In trees of thousands of taxa (this tree has 10s of 1000s), FBP values can often be very close to zero simply because branches are very short and some taxa can move around a lot. The TBE is more informative in this case, because it measures how many taxa have to be moved to recover a branch in a tree, rather than just the presence/absence of that branch.

The measures are all designed to estimate a similar quantity - something related to the confidence one might have in a branch. But they take very different approaches to this. Here's a plot which shows what they look like on a tree of >30K SARS-CoV-2 sequences from 24/6/20. The plot shows clearly that FBPs tend to be the lowest, TBEs are higher, and SH values are even higher. FBPs and TBEs are well correlated, but neither correlates that well to the SH values from fasttree (there is a very significant correlation though, as you'd expect!).

Why bother?

For most people this repository won't contain much of use. Instead, most people should use:

• Nextstrain
• PANGOLIN

This repository exists for two reasons:

1. To provide a global phylogeny of SARS-CoV-2 sequences that others might find useful
2. To develop and provide reproducible best-practice analyses for estimating that tree

Figuring out the best approach for estimating a very large phylogeny like this is an open question. I have kept notes of methods I have tried and compared here, as well as sometimes in the issues of this repository. In short, all choices and decisions made at all stages of the analysis are evidnece based either from my own analyses or from published analyses of SARS-CoV-2 sequences. A big part of the reason for working on this is that my research group develops phylogenetic methods like IQ-TREE, PartitionFinder, and RWTY. We are always working on improving these methods (indeed, some of these methods are used internally by Nextstrain and PANGOLIN), and as such we are working hard to make sure we understand these data as much as possible.

If you want to dig deeper into how we estimate the trees, and/or contribute by trying other methods or making suggestions, please read on. Feel free to use these trees for your work, but please cite this repository, and more importantly all the relevant tools that are used to make the trees.

Installation

You should be able to re-run the scripts I provide here, provided you have access to a reasonably large server (e.g. ~500GB RAM and about 50 cores). With those resources I can run the scripts with ~35 threads (as of early June 2020, each fasttree run takes about 10GB), and it will complete in around 24 hours.

To start, you will need some version of conda: https://docs.conda.io/projects/conda/en/latest/user-guide/install/

Once you have that, getting all the pacakges you need is simple:

# 1. Clone the repo
git clone https://github.com/roblanf/sarscov2phylo

# 2. change to the dir
cd sarscov2phylo/

# 3. download and install all the packages you need 
conda env create -f environment.yml

# 4. acviate the conda environment with all the packages
conda activate sarscov2phylo

Finally you will need to compile and install the MP version of FastTree from here: http://www.microbesonline.org/fasttree/#Install

Quickstart

1. Get the latest GISAID data in a single fasta file, e.g. gisaid.fa, you can do this via the batch download feature on GISAID, though you will have to become a member to do this.

2. Run this: bash global_tree_gisaid.sh -i gisaid.fasta -o global.fa -t 20

You can get more information on what is happening from the scripts themselves, and below.

What does it do, exactly?

Here's what the code does:

1. Fixes known issues with GISAID sequences. This includes: (i) ensuring all sequences start on a new line; (ii) excluding problematic sequences identified in a raft of different ways, listed and explained in this file; (iii) replaces spaces in sequence names with a "_"; (iv) replaces brackets in a species name with "-"; (v) repalces spaces in sequences themselves (!!) with "N". Note that the EPI_ID in each sequence name still forms a unique identifier for each sequence, which can always be reconciled with data downloaded from GISAID. All of this is accomplished with the clean_gisaid.sh script, and uses the following tools: sed and faSomeRecords. This creates the file ending with _cleaned.fa, where the start of the filename matches the name of the raw sequence data.

2. Creates a global sequence alignment. This is done by aligning every sequence to NC_045512.2 from NCBI. This is achieved quickly by doing it one sequence at a time in parallel in the global_profile_alignment.sh script, then joining the individually-aligned sequences into a global alignment at the end. It uses the following tools: mafft, faSplit, faSomeRecords, grep, GNU parallel, and find. This creates the file aln_global_unmasked.fa.

3. Masks sites that are likely dominated by errors of various kinds from the global alignment. Specifically, all sites suggested in this post are masked by replacing them with gaps, by downloading the latest (when the script is run) version of this file. Masking is performed with the mask_alignment.sh script, which uses the following tools: wget, faSplit, seqmagick, grep, GNU parallel, find. This creates the file aln_global_masked.fa. Note that this also masks the first and last 30 informative base pairs of every sequence, as well as any 7bp window which contains at least 2 uninformative characters (gaps or N's).

4. Filters seuqences that are shorter than 28000 bp and/or have >1000 ambiguities. This is because both of these filters tend to be associated with lower quality sequences, and or those that tend to be missing sufficient data that it is hard to place them meaningfully in a phylogeny. This uses esl-alimanip, and creates the file aln_global_filtered.fa.

5. Removes sites that are >50% gaps, after converting N's to gaps. I convert N's to gaps because maximum likelihood phylogenetic analyses treat them as the same thing. I then remove all sites with >50% gaps (at the time of writing this has only ever been the sites masked in step 3) using esl-alimask. This creates the file global.fa (or whatever you specified as your output file name when running the script).

6. Calculates and prints to alignments.log the stats of all alignments, for simple sanity checking.

7. Estimates a global ML tree from global.fa. [NB: the methods here changed substantially subsequent to 31-7-20; please see previous releases for the previous methods]. This is done by using fasttree with settings determined empirically to be the best, which are constantly updated see here and here and also here and here. The global tree is estimated in two steps. First, we start with the best tree from the previous release and add any new sequences to that tree using Maximum Parsimony in IQ-TREE (see here and here for a demonstration that MP works extremely well with these data). Second, we further optimise that tree with a series of minimum evolution SPR moves and Maximum Likelihood NNI moves in fasttree. Details of the benchmarking behind these choices are here and here. We use fasttree to calculate SH supports on the branches of that tree. The resulting tree is rooted with the seuqence 'hCoV-19/Wuhan/WH04/2020|EPI_ISL_406801|2020-01-05' as suggested in this preprint, using nw_reroot. This creates the file global.fa_ft_SH.tree.

8. Removes sequences on very long branches from the tree using TreeShrink. These sequences are likely to be either of poor quality and/or poorly aligned, so rather unreliable to interpret in a phylogeny with such limited variation. They are subsequently added to the list of excluded seuqences so they are not included in future iterations of the pipeline. This creates the file ft_SH.tree.

9. Roots the final two trees with nw_reroot as in step 7.

One last thing

The trees and alignments produced by the scripts here are checked regularly by eye. No matter all the clever software tools, there is no replacement for the human eye when it comes to spotting anomalies. If I find an anomaly, I go back and change the scripts to deal with it automatically (e.g. by removing a seuqence or masking a site, changing alignment or tree building parameters, etc). Rest assured that each release contains the exact code that built the trees included in that release, and also that I would not publish a release with a known anomaly. Obviously I can't guarantee to find all of the problematic sequences or sites, but I try.