alignment-nf¶

The alignment-nf pipeline performs alignment for wild isolate sequence data at the strain level, and outputs BAMs and related information. Those BAMs can be used for downstream analysis including variant calling, concordance analysis, wi-gatk-nf (variant calling) and other analyses.

This page details how to run the pipeline and how to add new wild isolate sequencing data.

Pipeline overview¶

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    parameters              description                                 Set/Default
    ==========              ===========                                 ========================
    --debug                 Use --debug to indicate debug mode          null
    --sample_sheet          See test_data/sample_sheet for example      null
    --species               Species to map: 'ce', 'cb' or 'ct'          null
    --fq_prefix             Path to fastq if not in sample_sheet        /projects/b1059/data/{species}/WI/fastq/dna/
    --kmers                 Whether to count kmers                      false
    --reference             genome.fasta.gz to use in place of default  defaults for c.e, c.b, and c.t
    --output                Output folder name.                         alignment-{date}

    HELP: http://andersenlab.org/dry-guide/pipeline-alignment/

Software requirements¶

• Nextflow v20.01+ (see the dry guide on Nextflow here or the Nextflow documentation here). On QUEST, you can access this version by loading the nf20 conda environment prior to running the pipeline command:

module load python/anaconda3.6
source activate /projects/b1059/software/conda_envs/nf20_env

• Currently only runs on Quest with conda environments installed at /projects/b1059/software/conda_envs/

Usage¶

Testing on Quest¶

This command uses a test dataset

nextflow run andersenlab/alignment-nf --debug -profile quest

Running on Quest¶

You should run this in a screen session.

Note: if you are having issues running Nextflow or need reminders, check out the Nextflow page.

nextflow run andersenlab/alignment-nf --sample_sheet <path_to_sample_sheet> --species c_elegans -profile quest -resume

Parameters¶

-profile¶

There are three configuration profiles for this pipeline.

• local - Used for local development.
• quest - Used for running on Quest.
• gcp - For running on Google Cloud (not currently active?).

--sample_sheet¶

The sample sheet for alignment is the output from the trim-fq-nf pipeline. The sample sheet must be tsv formatted, is the full path to the sample sheet (even if it is in your current directory), and has the following columns:

• strain - the name of the strain. Multiple sequencing runs of the same strain are merged together.
• id - A unique ID for each sequencing run. This must be unique for every single pair of FASTQs.
• lb - A library ID. This should uniquely identify a DNA sequencing library.
• fq1 - The path to FASTQ1
• fq2 - The path to FASTQ2

The library column is a useful tool for identifying errors by variant callers. For example, if the same library is sequenced twice, and a variant is only observed in one sequencing run then that variant may be excluded as a technical / PCR artifact depending on the variant caller being used.

--debug (optional)¶

You should use --debug true for testing/debugging purposes. This will run the debug test set (located in the test_data folder) using your specified configuration profile (e.g. local / quest / gcp).

For example:

nextflow run andersenlab/alignment-nf -profile quest --debug -resume

Using --debug will automatically set the sample sheet to test_data/sample_sheet.tsv

--species (optional)¶

Defaults to "c_elegans", change to "c_briggsae" or "c_tropicalis" to select correct reference file.

--fq_prefix (optional)¶

Within a sample sheet you may specify the locations of FASTQs using an absolute directory or a relative directory. If you want to use a relative directory, you should use the --fq_prefix to set the path that should be prefixed to each FASTQ.

--kmers (optional)¶

default = false

Toggles kmer-analysis

--reference (optional)¶

A fasta reference indexed with BWA. WS245 is packaged with the pipeline for convenience when testing or running locally.

On Quest, the default references are here:

c_elegans: /projects/b1059/data/c_elegans/genomes/PRJNA13758/WS276/c_elegans.PRJNA13758.WS276.genome.fa.gz
c_briggsae: /projects/b1059/data/c_briggsae/genomes/QX1410_nanopore/Feb2020/c_briggsae.QX1410_nanopore.Feb2020.genome.fa.gz
c_tropicalis: /projects/b1059/data/c_tropicalis/genomes/NIC58_nanopore/June2021/c_tropicalis.NIC58_nanopore.June2021.genome.fa.gz

The current C. briggsae genome does not contain the Mitochondria DNA. This needs to be addressed later.

A different --project and --wsbuild can be used with the --species parameter to generate the path to other reference genomes such as:

nextflow run andersenlab/alignment-nf --species c_elegans --project PRJNA13758 --wsbuild WS280

--ncbi (optional)¶

Default - /projects/b1059/data/other/ncbi_blast_db/

Path to the NCBI blast database used for blobtool analysis. Should not need to change.

--output (optional)¶

Default - alignment-YYYYMMDD

A directory in which to output results. If you have set --debug true, the default output directory will be alignment-YYYYMMDD-debug.

Output¶

├── _aggregate
│   ├── kmers.tsv
│   └── multiqc
│       ├── strain_data/
│       │   ├── mqc_mosdepth-coverage-dist-id_1.txt
│       │   ├── mqc_mosdepth-coverage-per-contig_1.txt
│       │   ├── mqc_mosdepth-coverage-plot-id_1.txt
│       │   ├── mqc_picard_deduplication_1.txt
│       │   ├── mqc_samtools-idxstats-mapped-reads-plot_Counts.txt
│       │   ├── mqc_samtools-idxstats-mapped-reads-plot_Normalised_Counts.txt
│       │   ├── mqc_samtools_alignment_plot_1.txt
│       │   ├── multiqc.log
│       │   ├── multiqc_data.json
│       │   ├── multiqc_general_stats.txt
│       │   ├── multiqc_picard_dups.txt
│       │   ├── multiqc_qualimap_bamqc_genome_results.txt
│       │   ├── multiqc_samtools_flagstat.txt
│       │   ├── multiqc_samtools_idxstats.txt
│       │   ├── multiqc_samtools_stats.txt
│       │   └── multiqc_sources.txt
│       ├── strain_multiqc_report.html
│       ├── id_data/
│       │   └──... (same as strain_data/)
│       └── id_multiqc_report.html
├── bam
│   ├── [strain].bam
│   └── [strain].bam.bai
├── blobtools
│   ├── {strain}.*.blobplot.bam0.png
│   ├── {strain}.*.blobplot.read_cov.bam0.png
│   └── {strain}.*.blobplot.stats.txt
├── software_versions.txt
├── sample_sheet.tsv
├── strain_summary.tsv
├── stats_strain_all.tsv
├── stats_strains_with_low_values.tsv
├── sample_sheet_for_seq_sheet.tsv
├── sample_sheet_for_seq_sheet_ALL.tsv
├── low_map_cov_for_seq_sheet.Rmd
├── low_map_cov_for_seq_sheet.html
└── summary.txt

Most files should be obvious. A few are detailed below.

• software_versions.txt - Outputs the software versions used for every process (step) of the pipeline.
• summary.txt - Outputs a summary of the parameters used.
• sample_sheet.tsv - The sample sheet (input file) that was used to produce the alignment directory.
• strain_summary.tsv - A summary of all strains and bams in the alignment directory.
• aggregate - Stores data that has been aggregated across all strains or sequencing IDs.
• coverage - Contains coverage data at the strain or id level, presented in a variety of ways.
• low_map_cov_for_seq_sheet.(Rmd/html) - Report showing low coverage or problematic strains to remove.
• stats_strain_all.tsv - contains stats for all strains, with all replicates combined
• stats_strains_with_low_values.tsv - contains stats for strains with either (1) low number of reads, (2) low mapping rate, and/or (3) low coverage
• sample_sheet_for_seq_sheet.tsv - sample sheet to be added to google sheet, filtered to remove low coverage strains
• sample_sheet_for_seq_sheet_ALL.tsv - sample sheet to be added to google sheet, contains all strains (use this one)
• blobplot/ - contains plots for low coverage strains to see if they show contamination issues and if they should be resequenced.

Data storage¶

Cleanup¶

Once the alignment-nf pipeline has completed successfully and you have removed low coverage strains (see pipeline overview), all BAM files can be moved to /projects/b1059/data/{species}/WI/alignments/ prior to variant calling.

Low coverage or otherwise problematic BAM files can be moved to /projects/b1059/data/{species}/WI/alignments/_bam_not_for_cendr/. Make sure to update the _README.md file in this folder with the reason each BAM was moved here. This will help remind people which files might be used again in the future.

---

Archive¶

The following sections have been integrated into other code that no longer needs to be run manually, but I am keeping the documentation here in case we need to go back to it. It is important to always check that the sample sheet is generated appropriately. If there are errors in teh sample sheet, one can be constructed manually using the following code:

construct_sample_sheet.sh¶

The scripts/construct_sample_sheet.sh script generates the WI_sample_sheet.tsv file.

The construct_sample_sheet.sh script does a few things.

(1) Parses FASTQ Filenames

Unfortunately, no two sequencing centers are alike and they use different formats for naming sequencing files. For example:

ECA768_RET-S11_S79_L001_2P.fq.gz      [strain]_[lib_lib#]_[sample_#]_[lane]_[read].fq.gz
XZ1734_S573_L007_2P.fq.gz             [strain]_[sample_#]_[lane]_[read].fq.gz

In some cases they even changed formats over time!

The script parses the FASTQ filenames from different sequencing centers, extracting the strain name, and a unique ID. Note that the library and unique sequencing run ID (id) are named somewhat arbitrarily. The most imporant aspect of these columns is that any DNA library that has been sequenced multiple times possess the same library, and that every pair of FASTQs possess a unique sequencing ID.

Consider the following (fake) example:

strain	isotype	reference_strain	id	library
AB1	AB1	TRUE	BGI2-RET2-AB1	RET2
AB1	AB1	TRUE	BGI2-RET3-AB1	RET3
AB4	CB4858	FALSE	BGI1-RET2-AB4	RET2
AB4	CB4858	FALSE	BGI2-RET2-AB4	RET2

AB1 was sequenced twice, however two different DNA libraries were produced for each sequencing run (RET2 and RET3). AB4 was also sequenced twice, but both sequencing runs were of the same DNA library (called RET2). Note that the id column is always unique across all sequencing runs.

If you look at the WI_sample_sheet.tsv in more detail you will observe that the id and library columns are not consistantly named. This is not ideal, but it works. The inconsistancy does not affect analysis, and exists because the filenames are not consistant, but unique library and sequencing run IDs must be derived from them.

(2) Clean up strain names

The second thing the construct_sample_sheet.sh script does is that it replaces shorthand strain names or innapropriately named strains with the 3-letter system. For example, N2Baer is renamed to ECA254.

(3) Integrate metadata

The C. elegans WI Strain Info google spreadsheet is a master spreadlist containing every strain, reference_strain, and isotype for C. elegans wild isolates. The script downloads this dataset and uses it to integrate the isotype and reference strain into the sample sheet.

Adding new sequencing datasets¶

Sequencing data should be added to QUEST and processed through the trimming pipeline before being added to WI_sample_sheet.tsv. Before proceeding, be sure to read pipeline-trimming

To add new sequencing datasets you will need to devise a strategy for extracting the strain name, a unique ID, and sequencing library from the FASTQ filenames. This may be the same as a past dataset, in which case you can append the sequencing run folder name to the list with that format. Alternatively, you may need to create a custom set of bash commands for generating the rows corresponding to each FASTQ pair.

Here is an example from the construct_sample_sheet.sh script.

#===================================#
# BGI-20161012-ECA23                #
#===================================#

out=`mktemp`
seq_folder=BGI-20161012-ECA23
>&2 echo ${seq_folder}
prefix=${fastq_dir}/WI/dna/processed/$seq_folder
for i in `ls -1 $prefix/*1P.fq.gz`; do
    bname=`basename ${i}`;
    barcode=`zcat ${i} | grep '@' | cut -f 10 -d ':' | sed 's/_//g' | head -n 100 | uniq -c | sort -k 1,1n | cut -c 9-100 | tail -n 1`
    echo -e "${bname}\t${i}\t${barcode}" >> ${out}
done;

cat ${out} |\
awk -v prefix=${prefix} -v seq_folder=${seq_folder} '{
    fq1 = $1;
    fq2 = $1;
    LB = $3;
    gsub("N", "", LB);
    gsub("1P.fq.gz", "2P.fq.gz", fq2);
    ID = $1;
    gsub("_1P.fq.gz", "", ID);
    split(ID, a, "[-_]")
    SM=a[2];
    print SM "\t" ID "\t" LB "\t" prefix "/" fq1 "\t" prefix "/" fq2 "\t" seq_folder;
}' >> ${fq_sheet}

Notes on this snippet:

• SM=strain, LB=library, and ID=id in the final output file.
• The sequencing run is listed in the comment box at the top.
• Barcodes are extracted from each FASTQ in the first forloop. These are used to define the library.
• The id is defined using the basename of the file.
• A final column corresponding to the seq_folder is always added.