Adapter trimming

“plus ça change, plus c’est la même chose “

Jean-Baptiste Alphonse Karr

So its 2023 and the new [sequencing] kid on the block, MGI, hasn’t figured out adapter trimming.

Here is a quick primer [pun intended] on the easiest way to remove primers and filter reads.

Use fastp and give it this file of Illumina adapters to trim against.

I use this command to filter and remove adapters from our sequences

mkdir output
fastp -n 1 -l 100 -i fastq/$R1 -I fastq/$R2 -o output/$R1 -O output/$R2 --adapter_fasta IlluminaAdapters.fa

If you have a lot of files, you can easily wrap this in a for loop to process all the files in a directory

mkdir fastq_fastp
for R1 in $(find fastq -name \*R1\* -printf "%f\n"); do
    fastp -n 1 -l 100 -i fastq/$R1 -I fastq/$R2 -o fastq_fastp/$R1 -O fastq_fastp/$R2 --adapter_fasta IlluminaAdapters.fa 

In addition to trimming all adapter sequences, this will remove any sequence with an N (yes, one or more), and remove any sequence shorter than 100 bp. fastp will also make sure that your reads are paired as well!