Prowler: A novel trimming algorithm for Oxford Nanopore sequence data.

Trimming and filtering tools are useful in DNA sequencing analysis because they increase the accuracy of sequence alignments and thus the reliability of results. Oxford Nanopore Technologies (ONT) trimming and filtering tools are currently rudimentary, generally only filtering reads based on whole read average quality. This results in discarding reads that contain regions of high quality sequence. Here we propose Prowler, a a trimmer that uses a window-based approach inspired by algorithms used to trim short read data. Importantly, we retain the phase and read length information by optionally replacing trimmed sections with Ns. Prowler was applied to mammalian and bacterial datasets, to assess its effect on alignment and assembly respectively. Compared to data filtered with Nanofilt, alignments of data trimmed with Prowler had lower error rates and more mapped reads. Assemblies of Prowler trimmed data had a lower error rate than those filtered with Nanofilt, however this came at some cost to assembly contiguity. Prowler is implemented in Python and is available at: https://github.com/ProwlerForNanopore/ProwlerTrimmer. Supplementary data are available at Bioinformatics online.