BWA-MEM2 — DNA Short-Read Aligner

Overview

BWA-MEM2 aligns short DNA reads (Illumina, 50–250 bp) to a reference genome using the BWT-FM index. It is the standard aligner for whole-genome sequencing (WGS), whole-exome sequencing (WES), ChIP-seq, and ATAC-seq DNA alignment. BWA-MEM2 is 2× faster than the original BWA-MEM while producing identical results. It outputs SAM format with proper read group (

@RG

When to Use

• Aligning WGS or WES Illumina reads to a reference genome for variant calling (SNP, indel, SV)
• ChIP-seq or ATAC-seq DNA alignment to produce BAM files for peak calling with MACS3
• Producing GATK-compatible BAM files with

  @RG

  read group tags
• Aligning reads ≥ 50 bp; for shorter reads (< 50 bp), BWA-backtrack may be more appropriate
• Re-aligning legacy FASTQ files to an updated reference genome assembly
• Use STAR instead for RNA-seq reads that span splice junctions
• Use Bowtie2 as an alternative for local alignment or when index size must be minimized

Prerequisites

• Software: bwa-mem2 (conda or pre-compiled binary), samtools
• Reference: genome FASTA (e.g., GRCh38, hg19, mm10)
• RAM: ~28 GB for human genome index; 6–8 GB for mouse

Check before installing: The tool may already be available in the current environment (e.g., inside a

pixi

/

conda

env). Run

command -v bwa-mem2

first and skip the install commands below if it returns a path. When running inside a pixi project, invoke the tool via

pixi run bwa-mem2

rather than bare

bwa-mem2

.

# Install with conda (recommended)
conda install -c bioconda bwa-mem2 samtools

# Or download pre-compiled binary
wget https://github.com/bwa-mem2/bwa-mem2/releases/download/v2.2.1/bwa-mem2-2.2.1_x64-linux.tar.bz2
tar -jxf bwa-mem2-2.2.1_x64-linux.tar.bz2
export PATH="$PWD/bwa-mem2-2.2.1_x64-linux:$PATH"

# Verify
bwa-mem2 version
# 2.2.1

Quick Start

# 1. Build genome index (~30 min, run once)
bwa-mem2 index GRCh38.fa

# 2. Align paired-end reads and sort
bwa-mem2 mem -t 16 -R "@RG\tID:sample1\tSM:sample1\tPL:ILLUMINA" \
    GRCh38.fa sample1_R1.fastq.gz sample1_R2.fastq.gz \
    | samtools sort -@ 8 -o sample1.sorted.bam

# 3. Index the BAM
samtools index sample1.sorted.bam
echo "Aligned reads: $(samtools view -c -F 4 sample1.sorted.bam)"

Workflow

Step 1: Download Reference Genome

Obtain the reference genome FASTA file matching the target assembly.

# Download GRCh38 primary assembly (human)
wget https://ftp.ncbi.nlm.nih.gov/genomes/all/GCA/000/001/405/GCA_000001405.15_GRCh38/seqs_for_alignment_pipelines.ucsc_ids/GCA_000001405.15_GRCh38_no_alt_analysis_set.fna.gz
gunzip GCA_000001405.15_GRCh38_no_alt_analysis_set.fna.gz
mv GCA_000001405.15_GRCh38_no_alt_analysis_set.fna GRCh38.fa

# Or use ENSEMBL/GENCODE
wget https://ftp.ebi.ac.uk/pub/databases/gencode/Gencode_human/release_47/GRCh38.primary_assembly.genome.fa.gz
gunzip GRCh38.primary_assembly.genome.fa.gz

echo "Reference size: $(du -sh GRCh38.fa)"

Step 2: Build BWA-MEM2 Index

Index the reference genome — required once per genome, takes ~25-35 min for human.

# Build index (~28 GB RAM required for human genome)
bwa-mem2 index GRCh38.fa

# This creates: GRCh38.fa.0123, GRCh38.fa.amb, GRCh38.fa.ann,
#               GRCh38.fa.bwt.2bit.64, GRCh38.fa.pac
echo "Index files: $(ls GRCh38.fa.* | wc -l) created"
ls -lh GRCh38.fa.*

Step 3: Align Paired-End Reads

Align FASTQ reads with a read group header required for GATK compatibility.

# Align with read group (required for GATK)
# @RG fields: ID (run ID), SM (sample name), PL (platform), LB (library), PU (flowcell)
bwa-mem2 mem \
    -t 16 \
    -R "@RG\tID:sample1_run1\tSM:sample1\tPL:ILLUMINA\tLB:lib1\tPU:flowcell1" \
    GRCh38.fa \
    sample1_R1.fastq.gz \
    sample1_R2.fastq.gz \
    | samtools sort -@ 8 -m 2G -o sample1.sorted.bam

samtools index sample1.sorted.bam
echo "Alignment complete."
echo "Total reads:   $(samtools view -c sample1.sorted.bam)"
echo "Mapped reads:  $(samtools view -c -F 4 sample1.sorted.bam)"

Step 4: Mark PCR Duplicates

Remove or mark optical and PCR duplicates before variant calling.

# Option A: samtools markdup (fast)
samtools fixmate -m sample1.sorted.bam sample1.fixmate.bam
samtools sort -@ 8 -o sample1.fixmate.sorted.bam sample1.fixmate.bam
samtools markdup -@ 8 sample1.fixmate.sorted.bam sample1.markdup.bam
samtools index sample1.markdup.bam

echo "Duplication rate:"
samtools flagstat sample1.markdup.bam | grep "duplicate"

# Option B: Picard MarkDuplicates (GATK best practices)
picard MarkDuplicates \
    INPUT=sample1.sorted.bam \
    OUTPUT=sample1.markdup.bam \
    METRICS_FILE=sample1.dupmetrics.txt \
    REMOVE_DUPLICATES=false \
    CREATE_INDEX=true
cat sample1.dupmetrics.txt | grep -A2 "ESTIMATED"

Step 5: Assess Alignment Quality

Generate alignment statistics and check key quality metrics.

# Full alignment statistics
samtools flagstat sample1.markdup.bam > sample1.flagstat.txt
cat sample1.flagstat.txt

# Coverage statistics
samtools coverage sample1.markdup.bam | head -30

# Parse key metrics with Python
python3 - << 'EOF'
from pathlib import Path

flagstat = Path("sample1.flagstat.txt").read_text()
for line in flagstat.splitlines():
    if any(kw in line for kw in ["total", "mapped", "properly paired", "duplicate"]):
        print(line)
EOF

Step 6: Complete WGS/WES Pipeline → Variant Calling

Pipe BWA-MEM2 output directly into GATK HaplotypeCaller.

#!/bin/bash
# Complete WGS alignment → variant calling pipeline
GENOME="GRCh38.fa"
SAMPLE="sample1"
R1="data/${SAMPLE}_R1.fastq.gz"
R2="data/${SAMPLE}_R2.fastq.gz"
THREADS=16
OUTDIR="results/${SAMPLE}"
mkdir -p "$OUTDIR"

# Step 1: Align + sort
bwa-mem2 mem -t $THREADS \
    -R "@RG\tID:${SAMPLE}\tSM:${SAMPLE}\tPL:ILLUMINA\tLB:lib1\tPU:run1" \
    $GENOME $R1 $R2 \
    | samtools sort -@ 8 -o $OUTDIR/${SAMPLE}.sorted.bam
samtools index $OUTDIR/${SAMPLE}.sorted.bam

# Step 2: Mark duplicates
samtools fixmate -m $OUTDIR/${SAMPLE}.sorted.bam - \
    | samtools sort -@ 8 \
    | samtools markdup -@ 8 - $OUTDIR/${SAMPLE}.markdup.bam
samtools index $OUTDIR/${SAMPLE}.markdup.bam

# Step 3: GATK variant calling
gatk HaplotypeCaller \
    -R $GENOME \
    -I $OUTDIR/${SAMPLE}.markdup.bam \
    -O $OUTDIR/${SAMPLE}.g.vcf.gz \
    -ERC GVCF \
    --native-pair-hmm-threads 4

echo "Pipeline complete: $OUTDIR/${SAMPLE}.g.vcf.gz"

Key Parameters

-t

1

-R

@RG\tID:...\tSM:...

-k

19

-w

100

-M

-a

-p

-c

500

-T

30

-Y

Common Recipes

Recipe 1: Batch Align Multiple Samples

#!/bin/bash
# Align all samples in parallel using GNU parallel or sequential loop
GENOME="GRCh38.fa"
SAMPLES=(ctrl_1 ctrl_2 treat_1 treat_2)
THREADS=12

for sample in "${SAMPLES[@]}"; do
    echo "=== Aligning $sample ==="
    bwa-mem2 mem -t $THREADS \
        -R "@RG\tID:${sample}\tSM:${sample}\tPL:ILLUMINA\tLB:lib1\tPU:run1" \
        $GENOME \
        data/${sample}_R1.fastq.gz \
        data/${sample}_R2.fastq.gz \
        | samtools sort -@ 4 -m 2G -o results/${sample}.sorted.bam
    samtools index results/${sample}.sorted.bam
    
    MAPPED=$(samtools view -c -F 4 results/${sample}.sorted.bam)
    TOTAL=$(samtools view -c results/${sample}.sorted.bam)
    echo "$sample: $MAPPED / $TOTAL reads mapped"
done

Recipe 2: Parse Alignment Metrics with Python

import subprocess
import pandas as pd
from pathlib import Path

samples = ["ctrl_1", "ctrl_2", "treat_1", "treat_2"]
metrics = []

for sample in samples:
    bam = f"results/{sample}.sorted.bam"
    result = subprocess.run(
        ["samtools", "flagstat", bam], capture_output=True, text=True
    )
    stats = {}
    for line in result.stdout.splitlines():
        if "total" in line:
            stats["total"] = int(line.split()[0])
        elif "mapped" in line and "%" in line:
            stats["mapped"] = int(line.split()[0])
            stats["pct_mapped"] = float(line.split("(")[1].split("%")[0])
        elif "properly paired" in line:
            stats["properly_paired"] = int(line.split()[0])
    stats["sample"] = sample
    metrics.append(stats)

df = pd.DataFrame(metrics).set_index("sample")
print(df[["total", "mapped", "pct_mapped", "properly_paired"]])
df.to_csv("alignment_metrics.tsv", sep="\t")

Expected Outputs

*.sorted.bam

samtools index

*.sorted.bam.bai

*.flagstat.txt

*.markdup.bam

*.dupmetrics.txt

Troubleshooting

@RG

-R

-R "@RG\tID:...\tSM:...\tPL:ILLUMINA"

-p

zcat r1.fq.gz | wc -l

[E::bwa_idx_load_from_disk]

bwa-mem2 index genome.fa

.0123

.bwt.2bit.64

-t 16

samtools sort

-M

References

• BWA-MEM2 GitHub: bwa-mem2/bwa-mem2 — source code, pre-compiled binaries, and benchmarks
• Vasimuddin M et al. (2019) "Efficient Architecture-Aware Acceleration of BWA-MEM for Multicore Systems" — IPDPS 2019. DOI:10.1109/IPDPS.2019.00041
• Li H & Durbin R (2009) "Fast and accurate short read alignment with Burrows-Wheeler Aligner" — Bioinformatics 25(14):1754-1760. DOI:10.1093/bioinformatics/btp324
• GATK Best Practices for germline short variant discovery — GATK workflow recommending BWA-MEM2 alignment