pywgsim

pywgsim is a modified version of the wgsim short read simulator.

• https://github.com/lh3/wgsim

The code for wgsim has been modified to allow visualizing the simulated mutations as a GFF file.

The package provides both a python wrapper and standalone compiled executables for Linux and MacOS.

Installation

Using pip

pip install pywgsim

PyPI page: https://pypi.org/project/pywgsim/

Using conda

conda install -c conda-forge -c bioconda pywgsim

Usage

$ pywgsim -h

prints:

usage: pywgsim [-h] [-e 0.02] [-D 500] [-s 50] [-N 1000] [-1 70] [-2 70]
               [-r 0.001] [-R 0.15] [-X 0.25] [-S 0] [-A 0.05] [-f]
               genome [read1] [read2]

positional arguments:
  genome                FASTA reference sequence
  read1                 FASTQ file for first in pair (read1.fq)
  read2                 FASTQ file for second in pair (read2.fq)

optional arguments:
  -h, --help            show this help message and exit
  -e 0.02, --err 0.02   the base error rate
  -D 500, --dist 500    outer distance between the two ends
  -s 50, --stdev 50     standard deviation
  -N 1000, --num 1000   number of read pairs
  -1 70, --L1 70        length of the first read
  -2 70, --L2 70        length of the second read
  -r 0.001, --mut 0.001
                        rate of mutations
  -R 0.15, --frac 0.15  fraction of indels
  -X 0.25, --ext 0.25   probability an indel is extended
  -S 0, --seed 0        seed for the random generator
  -A 0.05, --amb 0.05   disregard if the fraction of ambiguous bases higher
                        than FLOAT
  -f, --fixed           each chromosome gets N sequences

Changes compared to wgsim

The original code for wgsim has been modified as follows:

1. The output describing the mutations introduced by wgsim are generated in GFF format.
2. The separator character in the read name has been changed from _ to |.
3. There is a new flag called --fixed that generates the same N number of reads for each chromosome.

Read naming

The read naming now follows a more widely accepted convention (i.e. NCBI) and allows for contigs with underscores in them. In addition the visual inspection of the read names is easier:

@NC_002945.4|1768156|1768694|0:0:0|4:0:0|4

Fixed mode

In the default operation of wgsim the N reads are distributed such to create a uniform coverage across all chromosomes (longer chromosomes get a larger fraction of N).

When the --fixed mode is enabled N reads will be generated for each chromosome. The --fixed mode was introduced to simplify the evaluation of classifiers. Since the same number of reads is generated from each input sequence it becomes simpler the assess the quality of classifications (i.e. how many out of N were classified correctly)

Mutation output

The tool simulates mutations assuming a diploid genome. The output generated by pywgsim will look like this:

##gff-version 3
#
# N=10000 err_rate=0 mut_rate=0.001 indel_frac=0.15000001 indel_ext=0.25 size=500 std=50 len1=70 len2=70 seed=1607013056
#
NC_001416.1    wgsim    snp    89      89      .    +    .    Name=A/R;Ref=A;Alt=R;Type=het
NC_001416.1    wgsim    snp    2825    2825    .    +    .    Name=-/A;Ref=-;Alt=A;Type=het
NC_001416.1    wgsim    snp    3712    3712    .    +    .    Name=G/A;Ref=G;Alt=A;Type=hom
NC_001416.1    wgsim    snp    4622    4622    .    +    .    Name=G/-;Ref=G;Alt=-;Type=hom

Interpretation:

• A/R means heterozygous mutations with A/A and A/G alleles.
• -/A means an insertion of a A relative to the reference, the type field indicates heterozygous mutation.
• G/A means homozygous mutations with G/A alleles in both copies.
• G/- means a deletion of a G from the reference, the type field indicates homozygous mutation.

Ambiguity codes

A           A           T
C           C           G
G           G           C
T/U         T           A
M         A or C        K
R         A or G        Y
W         A or T        W
S         C or G        S
Y         C or T        R
K         G or T        M
V      A or C or G      B
H      A or C or T      D
D      A or G or T      H
B      C or G or T      V
N    G or A or T or C   N

Read name conventions

The read names are now of the form:

   @NC_002945.4|1768156|1768694|0:0:0|4:0:0|4

Where:

• NC_002945.4 is the contig name that the fragment was generated from.
• 1768156 is the left-most position of the fragment.
• 1768694 is the right-most position of the fragment.
• 0:0:0 are the number of errors, substitutions and indels in the left-most read of the pair.
• 4:0:0 are the number of errors, substitutions and indels in the right-most read of the pair.
• 4 is the read pair number, unique, per contig.

API

The C interface to wgsim is accessible as a single function call

from pywgsim import wgsim

wgsim.core(r1="read1.fq", r2="read2.fq", ref="genome.fa", err_rate=0.02, mut_rate=0.001, indel_frac=0.15, indel_ext=0.25, max_n=0.05, is_hap=0, N=100000,  dist=500, stdev=50, size_l=100, size_r=100, is_fixed=0, seed=0)

The function creates the files r1 and r2.