Read Mapping

There are two subcommands used when mapping reads to a reference genome, index and align. index is used to index the specified reference genome to allow for quicker mapping during the alignment process. align performs the actual alignment of reads to the indexed reference genome.

Creating the Reference Index

Before performing the read alignment, an index of the reference genome (referred to as my_reference.fa below). must be created. This step only needs to be run once for each reference.

To create the index, run

biscuit index my_reference.fa

The index of BISCUIT is composed of the 2-bit packed reference (*.bis.pac, *.bis.amb, *.bis.ann). The FM-index and suffix array of the parent strand (*.par.bwt and * and the daughter strand (*.dau.bwt and * Here, * refers to the input reference file, my_reference.fa.

For more help with index, run biscuit index in the terminal or check out the index help page.

Aligning Reads to the Reference

After creating an index of the reference genome, the sequenced reads can be mapped to the reference genome. In conjunction with samtools and dupsifter, BISCUIT can be used to map, duplicate mark, sort, and index the provided reads.

The suggested one-line command to generate an aligned, duplicate marked, and sorted BAM is referred to as the biscuitSifter pipeline:

# Uncompressed single end FASTQ example
biscuit align -@ NTHREADS -R "my_rg" /path/to/my_reference.fa read1.fq | \
dupsifter /path/to/my_reference.fa | \
samtools sort -@ NTHREADS -o my_output.bam -O BAM -

# Uncompressed paired end FASTQ example
biscuit align -@ NTHREADS -R "my_rg" /path/to/my_reference.fa read1.fq read2.fq | \
dupsifter /path/to/my_reference.fa | \
samtools sort -@ NTHREADS -o my_output.bam -O BAM -

# Gzipped paired end FASTQ example
biscuit align -@ NTHREADS -R "my_rg" /path/to/my_reference.fa read1.fq.gz read2.fq.gz | \
dupsifter /path/to/my_reference.fa | \
samtools sort -@ NTHREADS -o my_output.bam -O BAM -

Indexing the aligned BAM is done with samtools:

samtools index my_output.bam

In its suggested form, the biscuitSifter pipeline creates a BAM file that retains the duplicate marked reads. If desired, the --remove-dups flag in dupsifter will remove duplicates during processing, but BISCUIT will skip marked duplicates when running biscuit pileup by default (see Read Pileup for more details). If duplicates want to be retained when running biscuit pileup, including the -u flag will retain duplicates when generating the pileup VCF file.

For more help with align, run biscuit align in the terminal or check out the align help page.

Mapping a Single Read

While the above example highlights using FASTQ files for read mapping, BISCUIT can also map single reads on the fly using the -1 option for a single end read or -1 and -2 for a pair of reads from paired end sequencing.

# Single end example
biscuit align /path/to/my_reference.fa -1 AATTGGCC

# Paired end example
biscuit align /path/to/my_reference.fa -1 AATTGGCC -2 AGCTAGCT

Which Strand to Map?

Depending on the library construction strategy, bisulfite-converted (or bisulfite-like) reads can come from one of four types of strands. These types are determined by whether the target is the top or bottom strand and whether the DNA is the original strand that underwent conversion (parent strand, original top (OT) or original bottom (OB)), or the complement strand created during PCR amplification (daughter strand, complement to original top (CTOT) or complement to original bottom (CTOB)). In order to properly use a read aligner, it is critical to understand the strands from which the sequenced reads can be generated. In BISCUIT, the strands the reads can be mapped to is controlled by the -b option.

Single-End Library

By default, BISCUIT maps reads to all four strands (OT/CTOT/OB/CTOB, -b 0). This works for most cases, including coventional libraries (such as TCGA WGBS and Roadmap Epigenome Project), the PBAT library, and single cell libraries (where tagging happens after amplification). When -b 1 is specified, reads are forced to map only to the parent strands (OT/OB). This is more efficient and less error-prone for conventional libraries, such as TCGA and the Roadmap Project, as their library preparations only generate reads from the parent strands. If, for some reason, reads must be mapped to only the daughter strands (CTOT/CTOB), then include -b 3 in the options for biscuit align.

Paired-End Library

When -b 0 is used (the default behavior), BISCUIT maps read 1 (from read1.fq.gz) to one strand (parent or daughter) and read 2 (from read2.fq.gz) to the other strand. Alternatively, when using -b 1, read 1 is forced to map to the parent strand and read 2 is mapped to the daughter strand. In order to map read 1 to the daughter and read 2 to the parent, simply swap read1.fq.gz and read2.fq.gz in the command line prompt.

An Example

An example of using the default functionality in BISCUIT is:

# Note, this is a reduced representation of the output
inputread    0    chr10    3386516    3    35M    *    0    0
    ZR:i:0    AS:i:35    XS:i:34    XL:i:35    XA:Z:chr10,+3386516,35M,0
    XB:Z:1,0    MC:Z:*    MQ:i:0    YD:A:f

In this example, where the strand is not specified, BISCUIT maps the read to the original top (YD:A:f) strand automatically, without any mismatches (NM:i:0).

Conversely, when using -b 3:

inputread    0    chr10    3386516    60    35M    *    0    0
    ZR:i:17    AS:i:34    XS:i:0    XL:i:35    MC:Z:*    MQ:i:0    YD:A:r

In this case, BISCUIT maps the read to the original bottom (YD:A:r) strand with one mismatch (NM:i:1).

Distinguishing Decoy Chromosomes in Human and Mouse Genomes

As is done in BWA, BISCUIT makes a distinction of primary chromosomes from alternative/decoy chromosomes. For human and mouse genomes, this inference is turned on by default, which results in a preferential alignment of reads to the primary chromosomes. This can be turned off through the -i option in biscuit align. The inference of alternative/decoy chromosomes is (generically) done in the following manner: If the chromosome names are listed as chr1, chr2, and so on, then chromosomes with the name pattern chrUn, _random, _hap, or _alt are set as ALT chromosomes.

Other Alignment Features

  • Asymmetric scoring for C to T and G to A is used in local mapping
  • Generates a consistent mapping quality calculation with destination strand specification
  • Produces consistent NM and MD tags under asymmetric scoring
  • Produces ZR and ZC tags for retention count and conversion count
  • Separate seeding for parent and daughter strands for mapping efficiency
  • Indices make efficient use of disk-space and there is no need to store a bisulfite-converted reference
  • BWA-mem-like parameters that are visible to the users
  • Few dependencies (zlib and ncurses)
  • Robust to OS build and stable multi-threading
  • Optional parent and daughter strand restriction for both single- and paired-end reads
  • Optional OT/OB strand restriction, tightly integrated in mapping
  • Native inclusion of mate auxiliary tags (MC/MQ) in alignments

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