Mapping of reads to assembly¶
TODO: anpassen
In order to correct a given assembly, nanopolish needs a mapping of the original reads to this assembly. We are using the software package BWA to do this. BWA is a software package for mapping low-divergent sequences against a large reference genome. It consists of three algorithms: BWA-backtrack, BWA-SW and BWA-MEM. The first algorithm is designed for Illumina sequence reads up to 100bp, while the rest two for longer sequences ranged from 70bp to 1Mbp. BWA-MEM and BWA-SW share similar features such as long-read support and split alignment, but BWA-MEM, which is the latest, is generally recommended for high-quality queries as it is faster and more accurate.
First we need to create an index on our assembly, this has already been done for the pilon polishing:
#already done for pilon
bwa index ~/workdir/assembly/assembly.contigs.fasta
Then we will run the mapping. Check the usage of bwa mem:
Usage: bwa mem [options] <idxbase> <in1.fq> [in2.fq]
Algorithm options:
-t INT number of threads [1]
-k INT minimum seed length [19]
-w INT band width for banded alignment [100]
-d INT off-diagonal X-dropoff [100]
-r FLOAT look for internal seeds inside a seed longer than {-k} * FLOAT [1.5]
-y INT seed occurrence for the 3rd round seeding [20]
-c INT skip seeds with more than INT occurrences [500]
-D FLOAT drop chains shorter than FLOAT fraction of the longest overlapping chain [0.50]
-W INT discard a chain if seeded bases shorter than INT [0]
-m INT perform at most INT rounds of mate rescues for each read [50]
-S skip mate rescue
-P skip pairing; mate rescue performed unless -S also in use
-e discard full-length exact matches
Scoring options:
-A INT score for a sequence match, which scales options -TdBOELU unless overridden [1]
-B INT penalty for a mismatch [4]
-O INT[,INT] gap open penalties for deletions and insertions [6,6]
-E INT[,INT] gap extension penalty; a gap of size k cost '{-O} + {-E}*k' [1,1]
-L INT[,INT] penalty for 5'- and 3'-end clipping [5,5]
-U INT penalty for an unpaired read pair [17]
-x STR read type. Setting -x changes multiple parameters unless overridden [null]
pacbio: -k17 -W40 -r10 -A1 -B1 -O1 -E1 -L0 (PacBio reads to ref)
ont2d: -k14 -W20 -r10 -A1 -B1 -O1 -E1 -L0 (Oxford Nanopore 2D-reads to ref)
intractg: -B9 -O16 -L5 (intra-species contigs to ref)
Input/output options:
-p smart pairing (ignoring in2.fq)
-R STR read group header line such as '@RG\tID:foo\tSM:bar' [null]
-H STR/FILE insert STR to header if it starts with @; or insert lines in FILE [null]
-j treat ALT contigs as part of the primary assembly (i.e. ignore <idxbase>.alt file)
-v INT verbose level: 1=error, 2=warning, 3=message, 4+=debugging [3]
-T INT minimum score to output [30]
-h INT[,INT] if there are <INT hits with score >80% of the max score, output all in XA [5,200]
-a output all alignments for SE or unpaired PE
-C append FASTA/FASTQ comment to SAM output
-V output the reference FASTA header in the XR tag
-Y use soft clipping for supplementary alignments
-M mark shorter split hits as secondary
-I FLOAT[,FLOAT[,INT[,INT]]]
specify the mean, standard deviation (10% of the mean if absent), max
(4 sigma from the mean if absent) and min of the insert size distribution.
FR orientation only. [inferred]
Note, that there is an option for Oxford Nanopore 2D-reads:
-x STR read type. Setting -x changes multiple parameters unless overridden [null]
pacbio: -k17 -W40 -r10 -A1 -B1 -O1 -E1 -L0 (PacBio reads to ref)
ont2d: -k14 -W20 -r10 -A1 -B1 -O1 -E1 -L0 (Oxford Nanopore 2D-reads to ref)
intractg: -B9 -O16 -L5 (intra-species contigs to ref)
We use this default option for our mapping:
cd ~/workdir/
mkdir nanopore_mapping
bwa mem -t 14 -x ont2d ~/workdir/assembly/assembly.contigs.fasta ~/workdir/basecall/ONT.fastq.gz | samtools view - -Sb | samtools sort - -@14 > ~/workdir/nanopore_mapping/mapping.sorted.bam
We need to convert the resulting sam file to a sorted and indexed bam file:
samtools index ~/workdir/nanopore_mapping/mapping.sorted.bam