# An introduction to Rbwa

Jean-Philippe Fortin1*

1Department of Data Science and Statistical Computing, gRED, Genentech

*fortin946@gmail.com

#### 2023-04-25

# 1 Introduction

The Rbwa package provides an R wrapper around the two popular BWA aligners `BWA-backtrack` (Li and Durbin 2009) and `BWA-MEM` (Li 2013).

As mentioned in the BWA manual (see http://bio-bwa.sourceforge.net/bwa.shtml), BWA-backtrack is designed for short Illumina reads (up to 100bp), while BWA-MEM is more suitable for longer sequences (70bp to 1Mbp) and supports split alignment.

# 2 Installation

`Rbwa` can be installed from Bioconductor using the following commands in a fresh R session:

    if (!requireNamespace("BiocManager", quietly = TRUE))
        install.packages("BiocManager")

    BiocManager::install("Rbwa")

# 3 Overview

The two main alignment functions are:

- BWA-backtrack: `bwa_aln`
- BWA-MEM: `bwa_mem`

The package also includes the following convenience functions:

- Genome indexing: `bwa_build_index`
- Conversion of `bwa_aln` output into SAM output: `bwa_sam`
- Generation of secondary alignments: `xa2multi`

# 4 Build a reference index with `bwa_build_index`

Both `bwa_aln` and `bwa_mem` require first to create a genome index from a FASTA file. This is done only once for a given genome. This can be done using the function `bwa_build_index`.

First, we load `Rbwa`:

    library(Rbwa)

In the following example code, we build a BWA index for a small portion of human chr12 that we stored in a FASTA file located within the `Rbwa` package. We store the index files in a temporary directory.

    dir <- tempdir()
    fasta <- system.file(package="Rbwa",
                         "fasta/chr12.fa")
    index_prefix <- file.path(dir, "chr12")
    bwa_build_index(fasta,
                    index_prefix=index_prefix)
    list.files(dir)

    ## [1] "BiocStyle" "chr12.amb" "chr12.ann" "chr12.bwt" "chr12.pac" "chr12.sa"

# 5 Aligment with `bwa_aln`

We now align read sequences stored in the toy example FASTQ file `fastq/sequences.fastq`, provided in the `Rbwa` package, to our indexed genome:

    fastq <- system.file(package="Rbwa",
                         "fastq/sequences.fastq")
    bwa_aln(index_prefix=index_prefix,
            fastq_files=fastq,
            sai_files=file.path(dir, "output.sai"))

Any valid BWA arguments can be passed to the `bwa_aln` function. To see the complete list of valid arguments, please visit the BWA reference manual: http://bio-bwa.sourceforge.net/bwa.shtml.

For instance, we can specify the maximal edit distance between the query sequence and the reference genome to be 3 using `n`, as well as the maximal edit distance in the seed sequence `k` to be 3, where we specify that the length of the seed sequence is 13 using the argument `l`:

    bwa_aln(index_prefix=index_prefix,
            fastq_files=fastq,
            sai_files=file.path(dir, "output.sai"),
            n=3, k=3, l=13)

## 5.1 Creating a SAM file

The output of `bwa_aln` is an intermediate `sai` file that should be converted into a `sam` file using the `bwa_sam` function as follows:

    bwa_sam(index_prefix=index_prefix,
            fastq_files=fastq,
            sai_files=file.path(dir, "output.sai"),
            sam_file=file.path(dir, "output.sam"))

Let’s read the first few lines of the SAM file:

    strtrim(readLines(file.path(dir, "output.sam")), 65)

    ## [1] "@SQ\tSN:chr12\tLN:171330"
    ## [2] "@PG\tID:bwa\tPN:bwa\tVN:0.7.17-r1198-dirty\tCL:/tmp/RtmpkvPHJW/Rinst1ca83"
    ## [3] "ACATCAGAAAGAGCGGCAG\t0\tchr12\t170881\t37\t19M\t*\t0\t0\tACATCAGAAAGAGCGGCAG\t~~~~~~~"
    ## [4] "CAACCCAGCCCCCCTCCAA\t0\tchr12\t170801\t37\t19M\t*\t0\t0\tCAACCCAGCCCCCCTCCAA\t~~~~~~~"
    ## [5] "CCTGTGATCCACGGAGGCT\t0\tchr12\t170765\t37\t19M\t*\t0\t0\tCCTGTGATCCACGGAGGCT\t~~~~~~~"
    ## [6] "GCACTGCGGTGAGTGCTGT\t0\tchr12\t170665\t37\t19M\t*\t0\t0\tGCACTGCGGTGAGTGCTGT\t~~~~~~~"
    ## [7] "GCCTTTTACAGTTCGTACT\t0\tchr12\t170820\t37\t19M\t*\t0\t0\tGCCTTTTACAGTTCGTACT\t~~~~~~~"
    ## [8] "GTCATGCCCCCTCAGCCAG\t0\tchr12\t170703\t37\t19M\t*\t0\t0\tGTCATGCCCCCTCAGCCAG\t~~~~~~~"
    ## [9] "TCGGCTCTCACCGTGTCCG\t0\tchr12\t170646\t37\t19M\t*\t0\t0\tTCGGCTCTCACCGTGTCCG\t~~~~~~~"

## 5.2 Creating a SAM file with secondary alignments

By default, each row of the SAM output corresponds to the best alignment hit for a given input query sequence. Other alignments (secondary alignments, or other loci in case of multiple alignments) are stored in the XA tag.

The function `xa2multi` conveniently extracts the alignments from the XA tags and represent them as additional rows in the SAM format. This can be executed as follows:

    xa2multi(file.path(dir, "output.sam"),
             file.path(dir, "output.multi.sam"))
    strtrim(readLines(file.path(dir, "output.multi.sam")), 65)

    ## [1] "@SQ\tSN:chr12\tLN:171330"
    ## [2] "@PG\tID:bwa\tPN:bwa\tVN:0.7.17-r1198-dirty\tCL:/tmp/RtmpkvPHJW/Rinst1ca83"
    ## [3] "ACATCAGAAAGAGCGGCAG\t0\tchr12\t170881\t37\t19M\t*\t0\t0\tACATCAGAAAGAGCGGCAG\t~~~~~~~"
    ## [4] "CAACCCAGCCCCCCTCCAA\t0\tchr12\t170801\t37\t19M\t*\t0\t0\tCAACCCAGCCCCCCTCCAA\t~~~~~~~"
    ## [5] "CCTGTGATCCACGGAGGCT\t0\tchr12\t170765\t37\t19M\t*\t0\t0\tCCTGTGATCCACGGAGGCT\t~~~~~~~"
    ## [6] "GCACTGCGGTGAGTGCTGT\t0\tchr12\t170665\t37\t19M\t*\t0\t0\tGCACTGCGGTGAGTGCTGT\t~~~~~~~"
    ## [7] "GCCTTTTACAGTTCGTACT\t0\tchr12\t170820\t37\t19M\t*\t0\t0\tGCCTTTTACAGTTCGTACT\t~~~~~~~"
    ## [8] "GTCATGCCCCCTCAGCCAG\t0\tchr12\t170703\t37\t19M\t*\t0\t0\tGTCATGCCCCCTCAGCCAG\t~~~~~~~"
    ## [9] "TCGGCTCTCACCGTGTCCG\t0\tchr12\t170646\t37\t19M\t*\t0\t0\tTCGGCTCTCACCGTGTCCG\t~~~~~~~"

# 6 Aligment with `bwa_mem`

The `bwa_mem` function works similar to the `bwa_aln` function, except that it does not produce intermediate `.sai` files; it outputs a SAM file directly:

    fastq <- system.file(package="Rbwa",
                         "fastq/sequences.fastq")
    bwa_mem(index_prefix=index_prefix,
            fastq_files=fastq,
            sam_file=file.path(dir, "output.sam"))

    strtrim(readLines(file.path(dir, "output.sam")), 65)

    ## [1] "@SQ\tSN:chr12\tLN:171330"
    ## [2] "@PG\tID:bwa\tPN:bwa\tVN:0.7.17-r1198-dirty\tCL:/tmp/RtmpkvPHJW/Rinst1ca83"
    ## [3] "ACATCAGAAAGAGCGGCAG\t4\t*\t0\t0\t*\t*\t0\t0\tACATCAGAAAGAGCGGCAG\t~~~~~~~~~~~~~~~~~~~\t"
    ## [4] "CAACCCAGCCCCCCTCCAA\t4\t*\t0\t0\t*\t*\t0\t0\tCAACCCAGCCCCCCTCCAA\t~~~~~~~~~~~~~~~~~~~\t"
    ## [5] "CCTGTGATCCACGGAGGCT\t4\t*\t0\t0\t*\t*\t0\t0\tCCTGTGATCCACGGAGGCT\t~~~~~~~~~~~~~~~~~~~\t"
    ## [6] "GCACTGCGGTGAGTGCTGT\t4\t*\t0\t0\t*\t*\t0\t0\tGCACTGCGGTGAGTGCTGT\t~~~~~~~~~~~~~~~~~~~\t"
    ## [7] "GCCTTTTACAGTTCGTACT\t4\t*\t0\t0\t*\t*\t0\t0\tGCCTTTTACAGTTCGTACT\t~~~~~~~~~~~~~~~~~~~\t"
    ## [8] "GTCATGCCCCCTCAGCCAG\t4\t*\t0\t0\t*\t*\t0\t0\tGTCATGCCCCCTCAGCCAG\t~~~~~~~~~~~~~~~~~~~\t"
    ## [9] "TCGGCTCTCACCGTGTCCG\t4\t*\t0\t0\t*\t*\t0\t0\tTCGGCTCTCACCGTGTCCG\t~~~~~~~~~~~~~~~~~~~\t"

# 7 Session info

    sessionInfo()

    ## R version 4.3.0 RC (2023-04-13 r84269)
    ## Platform: x86_64-pc-linux-gnu (64-bit)
    ## Running under: Ubuntu 22.04.2 LTS
    ##
    ## Matrix products: default
    ## BLAS:   /home/biocbuild/bbs-3.17-bioc/R/lib/libRblas.so
    ## LAPACK: /usr/lib/x86_64-linux-gnu/lapack/liblapack.so.3.10.0
    ##
    ## locale:
    ##  [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C
    ##  [3] LC_TIME=en_GB              LC_COLLATE=C
    ##  [5] LC_MONETARY=en_US.UTF-8    LC_MESSAGES=en_US.UTF-8
    ##  [7] LC_PAPER=en_US.UTF-8       LC_NAME=C
    ##  [9] LC_ADDRESS=C               LC_TELEPHONE=C
    ## [11] LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C
    ##
    ## time zone: America/New_York
    ## tzcode source: system (glibc)
    ##
    ## attached base packages:
    ## [1] stats     graphics  grDevices utils     datasets  methods   base
    ##
    ## other attached packages:
    ## [1] Rbwa_1.4.0       BiocStyle_2.28.0
    ##
    ## loaded via a namespace (and not attached):
    ##  [1] digest_0.6.31       R6_2.5.1            bookdown_0.33
    ##  [4] fastmap_1.1.1       xfun_0.39           cachem_1.0.7
    ##  [7] knitr_1.42          htmltools_0.5.5     rmarkdown_2.21
    ## [10] cli_3.6.1           sass_0.4.5          jquerylib_0.1.4
    ## [13] compiler_4.3.0      tools_4.3.0         evaluate_0.20
    ## [16] bslib_0.4.2         yaml_2.3.7          BiocManager_1.30.20
    ## [19] jsonlite_1.8.4      rlang_1.1.0

# References

Li, Heng. 2013. “Aligning Sequence Reads, Clone Sequences and Assembly Contigs with Bwa-Mem.” arXiv Preprint arXiv:1303.3997.

Li, Heng, and Richard Durbin. 2009. “Fast and Accurate Short Read Alignment with Burrows–Wheeler Transform.” Bioinformatics 25 (14): 1754–60.