Table of Contents

Freebayes

Description

FreeBayes is a genetic variant detector designed to find small polymorphisms (SNPs, indels, MNPs and complex events).

FreeBayes is haplotype-based, in the sense that it calls variants based on the literal sequences of reads aligned to a particular target, not their precise alignment. This model is a straightforward generalization of previous ones (e.g. PolyBayes, samtools, GATK) which detect or report variants based on alignments. This method avoids one of the core problems with alignment-based variant detection, that identical sequences may have multiple possible alignments.

FreeBayes uses short-read alignments (BAM files) for any number of individuals from a population and a reference genome to determine the most-likely combination of genotypes for the population at each position in the reference. It reports positions which it finds putatively polymorphic in variant call file (VCF) format. It can also use an input set of variants (VCF) as a source of prior information, and a copy number variant map (BED) to define non-uniform ploidy variation across the samples under analysis.

Available

  • FreeBayes version 1.3.1 is available in Puhti

Usage

First load the bioconda module and activate FreeBayes environment

module load bioconda
source activate freebayes

After this you can launch Freebayes. For example:

freebayes -f reference.fa input.bam > results.vcf

Note that freebayes requires that bam file is indexed. Un-indexed bam file can be indexed with command:

samtools index input.bam

FreeBayes analysis jobs can be computationally heavy and should be run as batch jobs in Puhti.

In Puhti, you can use freebayes-puhti to automatically submit a Freebayes job to the batch job system. This tool also speeds up the analysis by running the ananlysis as several simultaneous tasks in parallel. To be able to use freebayes-puhti you first need to define a regions file for you reference fasta file. This can be done with command:

fasta_generate_regions.py reference.fa.fai 100000 > regions.txt

For small datasets you may decrease the region size in the command above so that you will get more than 100 regions to the regions file.

Once you have the regions file created you can launch you analysis task with command like:

freebayes-puhti -regions regions.txt -f reference.fa input.bam -out results.vcf

freebayes-puhti will execute your FreeBayes analysis as an automatically generated array batch job. The results will also be automatically merged and sorted once the batch jobs have finished. By default freebayes-puhti allows each subjob to use 16 GB of memory and to run for 24 hours. For massive FreeBayes jobs this may not be sufficient. In that case you can try to use options -mem and -time to extend the limits. -mem option defines the memory reservation in Gigabytes -time the time reservation in hours. For example, extending the task to 64 GB of memory and 48 hours of running time could be done with command:

freebayes-puhti -mem 64 -time 48 -regions regions.txt -f reference.fa input.bam -out results.vcf

Once launched, FreeBayes starts monitoring the progress of the job. As the job may take several days, the connection may break or you may need to close the connection. This does not harm the actual computing task. Once all subjobs have completed, you can use command freebayes-puhti-recover to collect the results. For example:

freebayes-puhti-recover freebayes_jobnum_tmp 

Where the freebayes_jobnum_tmp is the temporary FreeBayes directory that was created by the freebayes-puhti command to the same directory where the command was launched.

Manual

Detailed information on FreeBayes functionality: