feat(ancestral): Add optional Nextclade GFF3 compatibility mode

[corneliusroemer]

Description of proposed changes

Until this PR, it was hard to use augur ancestral with a genome annotation gff3 from a Nextclade dataset for multiple reasons:

• The GFF reader in augur looks for "gene" features only, while Nextclade 3 uses primarily CDS (gene only for backwards compatibility)
• Augur's GFF reader doesn't support compound features
• It takes the "gene"/"feature" name in the order "gene" > "gene_name" > "locus_tag", which differs from Nextclade's order

This meant that users had to create a Genbank file, just for the purpose of running augur ancestral.
That was doable but tedious.

This PR solves the problem by adding a new GFF reader mode that is designed to be as compatible with Nextclade as possible. That means:

• It looks for CDS features (for simplicity it ignores Nextclade's fallback support of gene features)
• It supports compound features
• It takes the feature name in the exact same order as Nextclade

The new mode is activated by passing the --use-nextclade-gff-parsing flag to augur ancestral.

It does no harm to anyone who does not want to use it.

It results in some code duplication, but I think it's totally worth it for the sake of usability.
I have already started using it in some of my private builds and it works like a charm.

Feedback requested

I'm not sure about the best name for the flag. Even I can't remember whether I chose --nextclade-compatible or --nextclade-compatibility. Maybe I should shorten to --nextclade-compat, or use --nextclade-gff-mode instead?

For now I've chosen --use-nextclade-gff-parsing but there's scope for bike shedding.

Alternatives considered

Work around with other tooling to generate gb file from gff

I've tried various things to work around the issue with Nextclade GFF not working with augur ancestral. Around a year ago I made a gff-to-genbank CLI pip installable (https://github.com/corneliusroemer/gff-to-genbank) - I considered reusing it for my current project but ended up aborting that approach as it ended up being complicated: while I could write compound locations correctly, I still needed to do sed replacements of the qualifiers, so that spurious locus_tags in the GFF wouldn't end up getting priority over the feature names that Nextclade would use. I had to do yet another sed replacement on the produced genbank file to replace the "region" feature with "source".

The solution proposed here is better in that it doesn't require users to install another tool, doesn't require any extra sed replacements.

Yes it does cause a bit of code duplication, and users need to know about the special --nextclade-compatible flag, but I just can't think of a better solution - this problem has existed for more than a year and 3 weeks we didn't seem any closer to a solution.

Related issue(s)

addresses the point raised in #1655

Future work

There's a small follow-up PR that further increases usability of ancestral for translations: #1668 - it drops the requirement to pass --genes and by default uses all genes.

Checklist

• Automated checks pass
• Check if you need to add a changelog message
• Check if you need to add tests
• Check if you need to update docs

[codecov]

Codecov Report

Attention: Patch coverage is 74.71264% with 22 lines in your changes missing coverage. Please review.

Project coverage is 72.25%. Comparing base (2e993df) to head (566150e).

Files with missing lines	Patch %	Lines
augur/utils.py	73.17%	9 Missing and 13 partials ⚠️

Additional details and impacted files

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  Coverage   72.25%   72.25%           
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  Files          79       79           
  Lines        8276     8320   +44     
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+ Hits         5980     6012   +32     
- Misses       2011     2014    +3     
- Partials      285      294    +9     

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[corneliusroemer]

Sorry for the noise closing, I typed the wrong gh cli command, which closed instead of checked out the PR 🤦

[rneher]

I think this would be useful to have. reduces friction using nextclade datasets as annotation.

alternatively, we could use the nextclade conversion of the gff to json and parse the json, but this here should cover the majority of use cases.

[genehack]

One nitpick, just based on the description of the work: if the flag is going to be --use-nextclade-gff-parsing, it strikes me as potentially deeply confusing and hard to untangle that the fallback of looking for gene features is present in Nextclade, but not here.

[corneliusroemer]

One nitpick, just based on the description of the work: if the flag is going to be --use-nextclade-gff-parsing, it strikes me as potentially deeply confusing and hard to untangle that the fallback of looking for gene features is present in Nextclade, but not here.

Good point, this is not 100% compatible, so maybe best to not mention Nextclade so explicitly in the arg name, i.e. use a different name. Proposals welcome!

Nextclade only looks at the genes if there are no cdses present at all.

If someone tries to use a gff like that, we could error if the flag is present.

https://github.com/nextstrain/nextclade/blob/c95471b4faa9121feea6102a333672ffcca74a06/packages/nextclade/src/gene/gene.rs#L72-L75