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first adapter in linked pair is incorrectly trimmed from 5' end of read #394

dmelu opened this issue Aug 10, 2019 · 2 comments


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commented Aug 10, 2019

Using cutadapt 2.1 compiled from source code, the below command trims the first adapter of the specified liked pair from the read in file temp1.fq. This behavior seems incorrect because many bases are mismatched near the 3' end of the trimmed adapter. The correct behavior should be that the read is left unchanged. Also, the last two bases of the adapter are not trimmed.
The observed behavior would be correct if option --no-indels was not specified. So the issue seems to be that cutadapt ignores option --no-indels when searching for linked adapters.
Another weird behavior is that the numbers of allowed errors are calculate using a 10% error rate, instead of the 20% rate specified by option -e.

cat ~/tmp/temp1.fq
This is cutadapt 2.1 with Python 3.7.2
Command line parameters: --no-indels -e 0.2 -y  {name} -a 3=^CGGGCAGGCTATGTTTTCTTTCTACTGGATC...AGATCGGAAGAGCGTCGTGTAGGGAAAGAGTGT temp1.fq
Processing reads on 1 core in single-end mode ...
[8<----------] 00:00:00             1 reads  @   1153.2 µs/read;   0.05 M reads/minute
@bug 3
Finished in 0.00 s (3591 us/read; 0.02 M reads/minute).

=== Summary ===

Total reads processed:                       1
Reads with adapters:                         1 (100.0%)
Reads written (passing filters):             1 (100.0%)

Total basepairs processed:           101 bp
Total written (filtered):             72 bp (71.3%)

=== Adapter 1 ===

Sequence: CGGGCAGGCTATGTTTTCTTTCTACTGGATC...AGATCGGAAGAGCGTCGTGTAGGGAAAGAGTGT; Type: linked; Length: 31+33; 5' trimmed: 1 times; 3' trimmed: 0 times

No. of allowed errors:
0-9 bp: 0; 10-19 bp: 1; 20-29 bp: 2; 30-31 bp: 3

No. of allowed errors:
0-9 bp: 0; 10-19 bp: 1; 20-29 bp: 2; 30-33 bp: 3

Overview of removed sequences at 5' end
length	count	expect	max.err	error counts
29	1	0.0	2	0 0 0 1

Overview of removed sequences at 3' end
length	count	expect	max.err	error counts


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commented Aug 10, 2019

Thanks for catching this! The example made it easy to reproduce the problem.

I’ve fixed this now. Indeed, the command-line options -e, -O and --no-indels were accidentally being ignored for linked adapters.

@marcelm marcelm closed this in eb5e089 Aug 10, 2019


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commented Aug 11, 2019

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