After using files that I downloaded from the SRA with fasterq-dump, I realize I am not 100% sure that I have all the data.

I noticed in my downstream analysis that I seem to be missing the .1 and .2 numbers in the code associated with individual reads with the same spot ID. Then I noticed that my initial fastq file doesn't seem to have those either.

So I'm wondering if the -split-spot option that I used is doing what I want it do do, which is 'get all the sequence data', and how does it differ from --concatenate-reads. I am afraid I did not understand this from the program description. Please help?

  • $\begingroup$ Hi Laura, do you have paired-end reads? Can you provide the SRA accession too? $\endgroup$ – StupidWolf Oct 24 '19 at 11:35
  • $\begingroup$ @StupidWolf It looks like they're paired. Eg. SRR6462984.63.1 and SRR6462984.63.2 . $\endgroup$ – Laura Oct 24 '19 at 13:05

Fasterq comes from the latest version of sratools. So if you check the manual , it says the equivalence is:

fastq-dump SRRXXXXXX --split-3 --skip-technical

fasterq-dump SRRXXXXXX

In older versions of sratoolkit, if you use fastq-dump without specifying --split-3 for paired-end reads, you get the format mentioned, spotID.1 for forward, spotID.2 for reverse:

fastq-dump SRR6462984.sra
more SRR6462984.fastq

You get:

@SRR6462984.1 1 length=301
+SRR6462984.1 1 length=301
@SRR6462984.2 2 length=302
+SRR6462984.2 2 length=302

With the latest version, it by defaults splits the paired end sra file into forward and reverse fastq (i.e --split-3):

fasterq-dump SRR6462984.sra 
head -2 SRR6462984_*.fastq

And you get

==> SRR6462984_1.fastq <==
@SRR6462984.1 1 length=150

==> SRR6462984_2.fastq <==
@SRR6462984.1 1 length=151

So if you need to get back the .1 and .2 format, use fastq-dump from the latest installation.

I would suggest sticking to the newest format, with 2 files, because fastq-dump will be deprecated in the future.


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