phred score mis-detection by dada2

[amakunin]

Description of the bug

I’ve been testing ampliseq on some Element Biosciences data using standard Illumina settings and discovered an interesting issue where DADA2_ERR fails with error that looks like this

  Error in dada(drps, err = NULL, errorEstimationFunction = errorEstimationFunction,  : 
    Invalid derep$quals matrix. Quality values must be positive integers.
  Calls: learnErrors -> dada
	  Execution halted

Turns out, the readFastq function that is being called under the hood mis-interprets ElemBio Phred scores as Phred+64, not Phred+33 - probably because maximium score in ElemBio is 55, while for Illumina it is 41.

I was able to work around this by changing qualityType = "Auto" to qualityType = "FastqQuality" in DADA2_ERR module config.

In addition, I needed to adjust DADA2_DENOISING code to use appropriate qualityType in reading fastq files:

        filtFs <- sort(list.files("./filtered/", pattern = "_1.filt.fastq.gz", full.names = TRUE), method = "radix")
        filtRs <- sort(list.files("./filtered/", pattern = "_2.filt.fastq.gz", full.names = TRUE), method = "radix")

        #denoising
        sink(file = "${prefix}.dada.log")
        derepFs <- derepFastq(filtFs, qualityType="FastqQuality")
        dadaFs <- dada(derepFs, err = errF, $args, multithread = $task.cpus)
        saveRDS(dadaFs, "${prefix}_1.dada.rds")
        derepRs <- derepFastq(filtRs, qualityType="FastqQuality")
        dadaRs <- dada(derepRs, err = errR, $args, multithread = $task.cpus)
        saveRDS(dadaRs, "${prefix}_2.dada.rds")
        sink(file = NULL)

I would appreciate if it would be possible to explicitly set qualityType in DADA2_ERR and DADA2_DENOISING modules - though I’m not sure if this should be:

• a sequencing platform parameter - this might be useful if any other tweaks would be need. Problem is I’m only testing steps up to DADA2 ASV generation, so won’t be able to advice on any downstream implications
• or a separate pipeline parameter setting phred to default (“Auto” ), 33 (“FastqQuality”) or 64 (“SFastqQuality”) - this might be useful for other platforms with non-standard Phred encoding - @erikrikarddaniel suggested this would be a better solution

Command used and terminal output

nextflow run nf-core/ampliseq -r 2.11.0 --input $manifest \
        -c $config -profile singularity --outdir ampliseq --skip_fastqc \
        --FW_primer AGATCGGAAGAGCACACGTCTGAACTCCAGTCAC --RV_primer AGATCGGAAGAGCGTCGTGTAGGGAAAGAGTGTA \
        --retain_untrimmed --illumina_pe_its --skip_qiime --skip_taxonomy \
        --ignore_empty_input_files --ignore_failed_trimming --ignore_failed_filtering --skip_barrnap -resume

Relevant files

No response

System information

linux x86_64
nextflow =24.04.02
ampliseq =2.11.0
singularity =3.10.0
local executor

[amakunin]

UPD - another place where qualities should be specified is DADA2_FILTNTRIM - qualityType = "Auto" is specified in ext.args section of module config as of ampliseq 2.11.0.

I am also unsure if qualityType should be specified in DADA2_QUALITY - the main functionplotQualityProfile does not seem to accept qualityType as an argument thogh

[erikrikarddaniel]

Thanks for reporting, and thanks for the update. The main developer is on vacation, so nothing will happen for a few weeks. We might come back to you with further questions when he's back.

[amakunin]

And another small update - unfortunately I won't be able to finish the test run. The reverse reads in my dataset had very low qualities, so did not pass DADA2_DENOISING (1_2.dada.rds file was not generated). As a result DADA2_STATS module failed with dadaRs = readRDS("${denoised[1]}") being transformed into dadaRs = readRDS("null") - maybe it would be possible to add a check if two files are being generated by denoising for paired-end data here?

[d4straub]

Hi there,

a separate pipeline parameter setting phred to default (“Auto” ), 33 (“FastqQuality”) or 64 (“SFastqQuality”)

that sounds like a good addition to me. However, test data (that could also be uploaded to our test data repo https://github.com/nf-core/test-datasets/tree/ampliseq) would be extremely important.

DADA2_QUALITY is primarily for visualization and also aids to find length cutoffs for DADA2_FILTNTRIM, so it seems not surprising that DADA2_QUALITY has no quality type setting.

[amakunin]

Hi @d4straub - it seems that DADA2_QUALITY also auto-infers Phred score, but in a different way - plotErrors function calls qa function from ShortRead library - I could not figure out if it supports different Phred scores, or just auto-infers Phred+33. In my case it correctly interpreted the data as Phred+33.

I'd be happy to provide some test data, but for now I only have forward reads of decent quality, while all reverse reads have low quality - I'll need to either generate fake Phred scores or wait for better data to appear. Please also keep in mind that my data is not compatible with downstream applications like QIIME, so it might be easier to replace a few Phred chars corresponding to base qualities between 41-55 in your existing test datasets

[d4straub]

If only forward reads are ok, isnt that also sufficient for implementing phred score flexibility? I would think so.

[amakunin]

Hi @d4straub, apologies for the delay. It seems that we might get proper paired end data soon. I'd prefer to provide that rather than just single end data - our usage of ampliseq relies heavily on the paired reads. Would this extra delay work for you? If not, I could introduce some higher base quality values, e.g. into https://github.com/nf-core/test-datasets/blob/ampliseq/testdata/1_S103_L001_R1_001.fastq.gz and its reverse counterpart - this should be accurate enough representation of ElemBio data...