Title: ABySS README Author: Shaun Jackman, Anthony Raymond Affiliation: Canada's Michael Smith Genome Science Centre CSS: README.css

ABySS

ABySS is a de novo sequence assembler intended for short paired-end reads and large genomes.

Contents

• [Quick Start]
  • [Install ABySS on Debian or Ubuntu]
  • [Install ABySS on Mac OS X]
• [Dependencies]
• [Compiling ABySS from source]
• [Assembling a paired-end library]
• [Assembling multiple libraries]
• [Scaffolding]
• [Optimizing the parameter k]
• [Parallel processing]
• [Running ABySS on a cluster]
• [Assembly Parameters]
• [ABySS programs]
• [Mailing List]
• [Authors]

Quick Start

Install ABySS on Debian or Ubuntu

Run the command

sudo apt-get install abyss

or download and install the Debian package.

Install ABySS on Mac OS X

Download and install the Mac OS X app.

Assemble a small synthetic data set

wget http://www.bcgsc.ca/platform/bioinfo/software/abyss/releases/1.3.4/test-data.tar.gz
tar xzvf test-data.tar.gz
abyss-pe k=25 name=test \
	in='test-data/reads1.fastq test-data/reads2.fastq'

Calculate assembly contiguity statistics

abyss-fac test-unitigs.fa

Dependencies

ABySS requires the following libraries:

• Boost
• sparsehash
• Open MPI

ABySS requires a C++ compiler that supports OpenMP such as GCC.

ABySS will receive an error when compiling with Boost 1.51.0 or 1.52.0 since they contain a bug. Download an earlier version such as Boost 1.50.0.

Compiling ABySS from source

To compile and install ABySS in /usr/local:

./configure
make
sudo make install

To install ABySS in a specified directory:

./configure --prefix=/opt/abyss
make
sudo make install

ABySS uses OpenMP for parallelization, which requires a modern compiler such as GCC 4.2 or greater. If you have an older compiler, it is best to upgrade your compiler if possible. If you have multiple versions of GCC installed, you can specify a different compiler:

./configure CC=gcc-4.6 CXX=g++-4.6

ABySS requires the Boost C++ libraries. Many systems come with Boost installed. If yours does not, you can download Boost. It is not necessary to compile Boost before installing it. The Boost header file directory should be found at /usr/include/boost, in the ABySS source directory, or its location specified to configure:

./configure --with-boost=/usr/local/include

If you wish to build the parallel assembler with MPI support, MPI should be found in /usr/include and /usr/lib or its location specified to configure:

./configure --with-mpi=/usr/lib/openmpi

ABySS should be built using the sparsehash library to reduce memory usage, although it will build without. sparsehash should be found in /usr/include or its location specified to configure:

./configure CPPFLAGS=-I/usr/local/include

The default maximum k-mer size is 64 and may be decreased to reduce memory usage or increased at compile time:

./configure --enable-maxk=96

If you encounter compiler warnings, you may ignore them like so:

make AM_CXXFLAGS=-Wall

To run ABySS, its executables should be found in your PATH. If you installed ABySS in /opt/abyss, add /opt/abyss/bin to your PATH:

PATH=/opt/abyss/bin:$PATH

Assembling a paired-end library

To assemble paired reads in two files named reads1.fa and reads2.fa into contigs in a file named ecoli-contigs.fa, run the command:

abyss-pe name=ecoli k=64 in='reads1.fa reads2.fa'

The parameter in specifies the input files to read, which may be in FASTA, FASTQ, qseq, export, SRA, SAM or BAM format and compressed with gz, bz2 or xz and may be tarred. The assembled contigs will be stored in ${name}-contigs.fa.

A pair of reads must be named with the suffixes /1 and /2 to identify the first and second read, or the reads may be named identically. The paired reads may be in separate files or interleaved in a single file.

Reads without mates should be placed in a file specified by the parameter se (single-end). Reads without mates in the paired-end files will slow down the paired-end assembler considerably during the abyss-fixmate stage.

Assembling multiple libraries

The distribution of fragment sizes of each library is calculated empirically by aligning paired reads to the contigs produced by the single-end assembler, and the distribution is stored in a file with the extension .hist, such as ecoli-3.hist. The N50 of the single-end assembly must be well over the fragment-size to obtain an accurate empirical distribution.

Here's an example scenario of assembling a data set with two different fragment libraries and single-end reads:

• Library pe200 has reads in two files, pe200_1.fa and pe200_2.fa.
• Library pe500 has reads in two files, pe500_1.fa and pe500_2.fa.
• Single-end reads are stored in two files, se1.fa and se2.fa.

The command line to assemble this example data set is:

abyss-pe k=64 name=ecoli lib='pe200 pe500' \
	pe200='pe200_1.fa pe200_2.fa' pe500='pe500_1.fa pe500_2.fa' \
	se='se1.fa se2.fa'

The empirical distribution of fragment sizes will be stored in two files named pe200-3.hist and pe500-3.hist. These files may be plotted to check that the empirical distribution agrees with the expected distribution. The assembled contigs will be stored in ${name}-contigs.fa.

Scaffolding

Long-distance mate-pair libraries may be used to scaffold an assembly. Specify the names of the mate-pair libraries using the parameter mp. The scaffolds will be stored in the file ${name}-scaffolds.fa. Here's an example of assembling a data set with two paired-end libraries and two mate-pair libraries:

abyss-pe k=64 name=ecoli lib='pe1 pe2' mp='mp1 mp2' \
	pe1='pe1_1.fa pe1_2.fa' pe2='pe2_1.fa pe2_2.fa' \
	mp1='mp1_1.fa mp1_2.fa' mp2='mp2_1.fa mp2_2.fa'

The mate-pair libraries are used only for scaffolding and do not contribute towards the consensus sequence.

Optimizing the parameter k

To find the optimal value of k, run multiple assemblies and inspect the assembly contiguity statistics. The following shell snippet will assemble for every value of k from 20 to 40.

export k
for k in {20..40}; do
	mkdir k$k
	abyss-pe -C k$k name=ecoli in=../reads.fa
done
abyss-fac k*/ecoli-contigs.fa

The default maximum value for k is 64. This limit may be changed at compile time using the --enable-maxk option of configure. It may be decreased to 32 to decrease memory usage or increased to 96.

Parallel processing

The np option of abyss-pe specifies the number of processes to use for the parallel MPI job. Without any MPI configuration, this will allow you to use multiple cores on a single machine. To use multiple machines for assembly, you must create a hostfile for mpirun, which is described in the mpirun man page.

Do not run mpirun -np 8 abyss-pe. To run ABySS with 8 threads, use abyss-pe np=8. The abyss-pe driver script will start the MPI process, like so: mpirun -np 8 ABYSS-P.

The paired-end assembly stage is multithreaded, but must run on a single machine. The number of threads to use may be specified with the parameter j. The default value for j is the value of np.

Running ABySS on a cluster

ABySS integrates well with cluster job schedulers, such as:

• SGE (Sun Grid Engine)
• Portable Batch System (PBS)
• Load Sharing Facility (LSF)
• IBM LoadLeveler

For example, to submit an array of jobs to assemble every odd value of k between 51 and 63 using 64 processes for each job:

mkdir k{51..63}
qsub -N ecoli -pe openmpi 64 -t 51-63:2 \
	<<<'abyss-pe -C k$SGE_TASK_ID in=/data/reads.fa'

Assembly Parameters

Parameters of the driver script, abyss-pe

• a: maximum number of branches of a bubble [2]
• b: maximum length of a bubble (bp) [10000]
• c: minimum mean k-mer coverage of a unitig [sqrt(median)]
• d: allowable error of a distance estimate (bp) [6]
• e: minimum erosion k-mer coverage [sqrt(median)]
• E: minimum erosion k-mer coverage per strand [1]
• j: number of threads [2]
• k: size of k-mer (bp)
• l: minimum alignment length of a read (bp) [k]
• m: minimum overlap of two unitigs (bp) [30]
• n: minimum number of pairs required for building contigs [10]
• N: minimum number of pairs required for building scaffolds [n]
• p: minimum sequence identity of a bubble [0.9]
• q: minimum base quality [3]
• s: minimum unitig size required for building contigs (bp) [200]
• S: minimum contig size required for building scaffolds (bp) [s]
• t: minimum tip size (bp) [2k]
• v: use v=-v to enable verbose logging [disabled]

Please see the abyss-pe manual page for more information on assembly parameters.

ABySS programs

abyss-pe is a driver script implemented as a Makefile. Any option of make may be used with abyss-pe. Particularly useful options are:

• -C dir, --directory=dir
  Change to the directory dir and store the results there.
• -n, --dry-run
  Print the commands that would be executed, but do not execute them.

abyss-pe uses the following programs, which must be found in your PATH:

• ABYSS: de Bruijn graph assembler
• ABYSS-P: parallel (MPI) de Bruijn graph assembler
• AdjList: find overlapping sequences
• DistanceEst: estimate the distance between sequences
• MergeContigs: merge sequences
• MergePaths: merge overlapping paths
• Overlap: find overlapping sequences using paired-end reads
• PathConsensus: find a consensus sequence of ambiguous paths
• PathOverlap: find overlapping paths
• PopBubbles: remove bubbles from the sequence overlap graph
• SimpleGraph: find paths through the overlap graph
• abyss-fac: calculate assembly contiguity statistics
• abyss-filtergraph: remove shim contigs from the overlap graph
• abyss-fixmate: fill the paired-end fields of SAM alignments
• abyss-map: map reads to a reference sequence
• abyss-scaffold: scaffold contigs using distance estimates
• abyss-todot: convert graph formats and merge graphs

For a flowchart showing the relationship between these programs, see doc/flowchart.pdf.

Mailing List

Subscribe to the [ABySS mailing list] (http://groups.google.com/group/abyss-users), abyss-users@googlegroups.com.

For questions related to transcriptome assembly, contact the [Trans-ABySS mailing list] (http://groups.google.com/group/trans-abyss), trans-abyss@googlegroups.com.

Authors

This document is written by Shaun Jackman.

ABySS is written by Shaun Jackman, Tony Raymond and Jared Simpson.

Copyright 2012 Canada's Michael Smith Genome Science Centre