Rsamtools

Dependencies & Environment

Package-intrinsic requirements from the Bioconductor landing page — reproduce in any R environment.

• Version: 2.28.0 · Bioconductor: 3.23 · R: ≥ 4.6
• Depends: Seqinfo, GenomicRanges, Biostrings
• Imports: BiocGenerics, S4Vectors, IRanges, XVector, bitops, BiocParallel
• System requirements: GNU make
• Install: BiocManager::install("Rsamtools")

When to Use

• Low-level Alignment Import: Importing and parsing BAM files directly into R lists or DataFrame objects using scanBam.
• Targeted Read Extraction: Extracting specific genomic coordinates and alignment fields (e.g., read sequence, strand, position) using ScanBamParam.
• BAM File Management: Managing collections of BAM files and their metadata programmatically using BamViews.
• Coverage Calculation: Calculating read coverage over specific genomic ranges using coverage on imported BAM data.

When NOT to Use

• For high-level representation of gapped alignments, use GenomicAlignments (specifically readGAlignments) because Rsamtools provides lower-level list-based outputs.
• For iterating through BAM files in parallel, use GenomicFiles because it implements higher-level strategies for file iteration.
• For whole-genome manipulation and DNA sequence operations, use Biostrings or BSgenome instead.

Data Requirements

• Input Format: BAM files (e.g., .bam) and their corresponding index files (.bai).
• Genomic Coordinates: Specified as GRanges objects for targeted queries.

Key Parameters

• file: Path to the BAM file to be parsed by scanBam.
• param: A ScanBamParam object determining which genomic coordinates and components to input.
• which: A GRanges object specifying the genomic ranges to extract.
• what: A character vector (e.g., c("rname", "pos", "qwidth")) specifying which BAM record fields to retrieve.
• flag: A filter created by scanBamFlag (e.g., isUnmappedQuery=FALSE) to restrict reads.
• bamRanges: A GRanges object defining the genomic rows for a BamViews instance.

Best Practices

• Always ensure a .bai index file is available locally when querying specific ranges with the which argument.
• Restrict imported fields using the what argument in ScanBamParam to minimize memory consumption.
• Process large BAM files in chunks (e.g., by chromosome) using summary functions to avoid memory exhaustion.
• Use BamViews to marshal references to multiple BAM files without loading them immediately into memory.

Common Pitfalls

• Querying by range without an index: Fails because providing a which argument to ScanBamParam requires a BAM index. Fix: Ensure the .bai file exists or create one using indexBam.
• "EOF marker is absent" warning: Occurs with some BAM files. Fix: This message can safely be ignored as noted in the vignette.
• Chromosome naming mismatches: Ensembl vs UCSC naming (e.g., "chr1" vs "1") causes empty queries. Fix: Use seqlevels<- to map between naming schemes before querying.

Alternatives

• GenomicAlignments: Provides a more useful, higher-level representation of BAM files in R (e.g., GAlignments objects).
• GenomicFiles: Useful for iterating through BAM and other files, including in parallel.
• ShortRead: Better suited for I/O and quality assessment of ungapped short read alignments.

Citations

• Morgan M, Pagès H, Obenchain V, Hayden N. Rsamtools: Binary alignment (BAM), FASTA, variant call (BCF), and tabix file import. Bioconductor.

References

• Homepage: https://bioconductor.org/packages/Rsamtools
• Vignette: https://bioconductor.org/packages/release/bioc/vignettes/Rsamtools/inst/doc/Rsamtools-Overview.pdf

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