Read Mapping¶
In this part of the tutorial we will look at the assemblies by mapping the reads to the assembled contigs. Different tools exists for mapping reads to genomic sequences such as bowtie or bwa. Today, we will use the tool BBMap.
BBMap: Short read aligner for DNA and RNA-seq data. Capable of handling arbitrarily large genomes with millions of scaffolds. Handles Illumina, PacBio, 454, and other reads; very high sensitivity and tolerant of errors and numerous large indels. Very fast. See the BBTools home page for more info.
bbmap
needs to build an index for the contigs sequences before it
can map the reads onto them. Here is an example command line for
mapping the reads back to the MEGAHIT assembly:
cd /mnt/WGS-data/megahit_out
bbmap.sh ref=final.contigs.fa
Now that we have an index, we can map the reads:
bbmap.sh in=../read1.fq in2=../read2.fq out=megahit.bam threads=14
bbmap
produces output in BAM format (the binary version of the SAM format). BAM files can be viewed and manipulated with SAMtools. Let’s first build an index for the FASTA file:
samtools faidx final.contigs.fa
We have to sort the BAM file by starting position of the alignments. This can be done using samtools again:
samtools sort -o megahit_sorted.bam -@ 14 megahit.bam
Now we have to index the sorted BAM file:
samtools index megahit_sorted.bam
To look at the BAM file use:
samtools view megahit_sorted.bam | less
We will use the IGV genome browser to look at the mappings:
igv
Now let’s look at the mapped reads:
- Load the contig sequences into IGV. Use the menu
Genomes->Load Genome from File...
- Load the BAM file into IGV. Use menu
File->Load from File...
- Load the predicted genes as another track. Use menu
File->Load from File...
to load the GFF file.