Nextstrain build for novel coronavirus SARS-CoV-2
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Customizing analysis

Changing parameters

You can configure most steps of the workflow by specifying values in a .yaml configuration file. We’ve provided reasonable default values for each step in the defaults/parameters.yaml; these are the same values the Nextstrain team uses for our analyses. For more details, see the reference for all workflow configuration parameters.

We encourage you to take a few minutes to skim through the default config file. Although these default values should be fine for most users, it’s helpful to get a sense for what options are available.

If you’d like to tweak the parameterization, you can override any of these values by specifying them in the my_profiles/<name>/builds.yaml file. Any values not overridden in this way will fall back to the default values. Keeping build-specific parameters separate this way prevents mixups of settings between runs, and gives you a cleaner file to work with (rather than having to wrestle the entire default parameterization file).

Adding custom rules

Insert your own custom Snakemake rules into the default workflow without modifying the main Snakemake files, by defining a list of custom_rules in your builds.yaml file. Each entry in the custom_rules list should be a path to a valid Snakemake file (e.g., “my_rules.smk”). The main workflow will detect these custom rules and include them after all other rules have been defined.

As an example, the Nextstrain team’s workflow defines custom export rules that modify the default auspice JSONs. These rules are defined in the builds.yaml file as follows:

  - workflow/snakemake_rules/export_for_nextstrain.smk

To modify rules for the example profile, create a new file named my_profiles/example/my_rules.smk and modify the builds.yaml file for the example profile to include the following lines:

  - my_profiles/example/my_rules.smk

Adding a new place

Places are defined as one of:

  • region (e.g., North America, Asia)
  • country
  • division (i.e., state, province, or canton)
  • location (i.e., a county or city within a division)

To define a new place, you’ll need to specify its GPS coordinates and a color.

  1. Add a line to defaults/lat_longs.tsv. This file is separated into sections for each geographic resolution. This looks like:
    # resolution	place	latitude	longitude
    location	Abondant	48.790785	1.420178

Note: keep in mind that 0.0 longitude is the prime meridian; to specify something in the Western hemisphere, you’ll need to enter a negative value for longitude. Similarly, to specify something in the Southern hemisphere, you’ll need to enter a negative value for latitude

  1. Add an entry to color_ordering.tsv such that your newly-defined place is next to geographically nearby places in the list.


Basic subsampling

Reasonable defaults are pre-defined. You can find a description of them here.

Custom subsampling schemes

We implement hierarchical subsampling by producing multiple samples at different geographic scales and merge these samples into one file for further analysis. A build can specify any number of such samples which can be flexibly restricted to particular meta data fields and subsampled from groups with particular properties. When specifying subsampling in this way, we’ll first take sequences from the ‘focal’ area, and the select samples from other geographical areas. Read further for information on how we select these samples. Here, we’ll look at the advanced example (./my_profiles/example_advanced_customization) file to explain some of the options.

When specifying how many sequences you want in a subsampling level (for example, from a country or a region), you can do this using either seq_per_group or max_sequences - these work with the group_by argument. For example, switzerland subsampling rules in the advanced example looks like this:

  # Focal samples for country
    group_by: "division year month"
    max_sequences: 1500
    exclude: "--exclude-where 'country!={country}'"
  # Contextual samples from country's region
    group_by: "country year month"
    seq_per_group: 20
    exclude: "--exclude-where 'country={country}' 'region!={region}'"
      type: "proximity"
      focus: "country"
  # Contextual samples from the rest of the world,
  # excluding the current region to avoid resampling.
    group_by: "country year month"
    seq_per_group: 10
    exclude: "--exclude-where 'region={region}'"
      type: "proximity"
      focus: "country"

For country-level sampling above, we specify that we want a maximum of 1,500 sequences from the country in question (here, Switzerland). Since we set group_by to “division year month”, all the Swiss sequences will be divided into groups by their division, month, and year of sampling, and the code will try to equally sample from each group to reach 1,500 sequences total.

Alternatively, in the region-level sampling, we set seq_per_group to 20. This means that all the sequences from Europe (excluding Switzerland) will be divided into groups by their sampling country, month, and year (as defined by group_by), and then 20 sequences will taken from each group (if there are fewer than 20 in any given group, all of the samples from that group will be taken).

Now we’ll look at a subsampling scheme which defines a multi-canton build. Cantons are regional divisions in Switzerland - below ‘country,’ but above ‘location’ (often city-level). In the advanced example, we’d like to be able to specify a set of neighboring ‘cantons’ and do focal sampling there, with contextual samples from elsewhere in the country, other countries in the region, and other regions in the world.

For cantons this looks like this:

# This build will take from 3 cantons - we have a sample rule for each,
# rather than just one division that's focal build
  # focal samples
    group_by: "year month"
    seq_per_group: 300
    exclude: "--exclude-where 'division!=geneva'"
    group_by: "year month"
    seq_per_group: 300
    exclude: "--exclude-where 'division!=vaud'"
    group_by: "year month"
    seq_per_group: 300
    exclude: "--exclude-where 'division!=valais'"

  # Contextual samples from the country
    group_by: "division year month"
    seq_per_group: 20
    exclude: "--exclude-where 'country!=switzerland'"

  # Contextual samples from division's region
    group_by: "country year month"
    seq_per_group: 10
    exclude: "--exclude-where 'region!=europe'"
      type: "proximity"
      focus: "country"
  # Contextual samples from the rest of the world, excluding the current
  # division to avoid resampling.
    group_by: "country year month"
    seq_per_group: 5
    exclude: "--exclude-where 'region=europe'"
      type: "proximity"
      focus: "country"

All entries above canton level (the ‘contextual’ samples) specify priorities. Currently, we have only implemented one type of priority called proximity. It attempts to selected sequences as close as possible to the focal samples specified as focus: division. The argument of the latter has to match the name of one of the other subsamples.

In addition to the exclude filter, you can also specify strains to keep by providing a query. The query field uses augur filter’s --query argument (introduced in version 8.0.0) and supports pandas-style logical operators. For example, the following exclusionary filter,

exclude: "--exclude-where 'region!={region}' 'country!={country}' 'division!={division}'"

can also be written as an inclusionary filter like so:

query: --query "(region == {region}) & (country == {country}) & (division == '{division}')"

If you need parameters in a way that isn’t represented by the configuration file, create a new issue in the ncov repository to let us know.

Ancestral trait reconstruction

Trait reconstruction is the process by which augur infers the most likely metadata value of an internal node. For example, if an internal node (which always represents a hypothesized, ancestral virus / case) has 3 descendants, all of which were isolated in Washington State, we might infer that the ancestor was most likely also circulating in Washington State (see “Interpretation” for more).

For each build, you can specify which categorical metadata fields to use for trait reconstruction.

To specify this on a per-build basis, add a block like the following to your my_profiles/<name>/builds.yaml file:

  my_north_america_build: ### build name
    sampling_bias_correction: 2.5
    columns: ["country", "division"] ### traits to reconstruct; must match column names in metadata.tsv

Labeling clades

We assign clade labels according to this schema.

Because the exact topology of the tree will vary across runs, clades are defined based on their unique mutations. These are specified in defaults/clades.tsv like so:

# clade	gene	site	alt

A1a	ORF3a	251	V
A1a	ORF1a	3606	F