NanoSim

NanoSim is a fast and scalable read simulator that captures the technology-specific features of ONT data, and allows for adjustments upon improvement of nanopore sequencing technology.

Dependencies

LAST (Tested with version 581)
R (Tested with version 3.2.3)
Python (2.6 or above)
Numpy (Tested with version 1.10.1 or above)

Usage

NanoSim is implemented using R for error model fitting and Python for read length analysis and simulation. The first step of NanoSim is read characterization, which provides a comprehensive alignment-based analysis, and generates a set of read profiles serving as the input to the next step, the simulation stage. The simulation tool uses the model built in the previous step to produce in silico reads for a given reference genome. It also outputs a list of introduced errors, consisting of the position on each read, error type and reference bases.

1. Characterization stage

Characterization stage takes a reference and a training read set in FASTA format as input. User can also provide their own alignment file in MAF format.

Usage:

./read_analysis.py <options>  
    [options]:  
    -h : print usage message  
    -i : training ONT real reads, must be fasta files  
    -r : reference genome of the training reads  
    -m : User can provide their own alignment file, with maf extension, can be omitted  
    -o : The prefix of output file, default = 'training'  

Some ONT read profiles are ready to use for users. With the profiles, users can run simulation tool directly. Please go to [ftp://ftp.bcgsc.ca/supplementary/NanoSim/] to download E. coli or S. cerevisiae datasets and profiles.

2. Simulation stage

Simulation stage takes reference genome and read profiles as input and outputs simulated reads in FASTA fomat.

Usage:

./simulator.py [command] <options>  
   [command]:  
    circular | linear  
    # Do not choose 'circular' when there is more than one sequence in the reference  
    <options>:  
    -h : print usage message
    -r : reference genome in fasta file, specify path and file name  
    -c : the prefix of training set profiles, same as the output prefix in read_analysis.py, default = training  
    -o : The prefix of output file, default = 'simulated'  
    -n : Number of generated reads, default = 20,000 reads  
    --perfect: Output perfect reads, no mutations, default = False  
    --KmerBias: prohibits homopolymers with length >= 6 bases in output reads, can be omitted  

For example:
1 If you want to simulate E. coli genome, then circular command must be chosen because it's a circular genome
./simulator.py circular -r Ecoli_ref.fasta -c ecoli

2 If you want to simulate only perfect reads, i.e. no snps, or indels, just simulate the read length distribution
./simulator.py circular -r Ecoli_ref.fasta -c ecoli --perfect

3 If you want to simulate S. cerevisiae genome with kmer bias, then linear command must be chosen because it's a linear genome
./simulator.py linear -r yeast_ref.fasta -c yeast --KmerBias

See more detailed example in example.sh

Explaination of output files

1. Characterization stage

1. training_aligned_length_ecdf Length distribution of aligned regions on aligned reads
2. training_aligned_reads_ecdf Length distribution of aligned reads
3. training_align_ratio Empirical distribution of align ratio of each read
4. training_besthit.maf The best alignment of each read based on length
5. training_match.hist/training_mis.hist/training_del.hist/training_ins.hist Histogram of match, mismatch, and indels
6. training_first_match.hist Histogram of the first match length of each alignment
7. training_error_markov_model Markov model of error types
8. training_ht_ratio Empirical distribution of the head region vs total unaligned region
9. training.maf The output of LAST, alignment file in MAF format
10. training_match_markov_model Markov model of the length of matches (stretches of correct base calls)
11. training_model_profile Fitted model for errors
12. training_processed.maf A re-formatted MAF file for user-provided alignment file
13. training_unaligned_length_ecdf Length distribution of unaligned reads

2. Simulation stage

1. simulated.log
   Log file for simulation process

2. simulated_reads.fasta
   FASTA file of simulated reads. Each reads has "unaligned", "aligned", or "perfect" in the header determining their error rate. "unaligned" means that the reads have an error rate over 90% and cannot be aligned. "aligned" reads have the same error rate as training reads. "perfect" reads have no errors.

To explain the information in the header, we have two examples:

• >ref|NC-001137|-[chromosome=V]_468529_unaligned_0_F_0_3236_0
  All information before the first _ are chromosome information. 468529 is the start position and unaligned suggesting it should be unaligned to the reference. The first 0 is the sequence index. F represents a forward strand. 0_3236_0 means that sequence length extracted from the reference is 3236 bases.
• >ref|NC-001143|-[chromosome=XI]_115406_aligned_16565_R_92_12710_2
  This is an aligned read coming from chromosome XI at position 115406. 16565 is the sequence index. R represents a reverse complement strand. 92_12710_2 means that this read has 92-base head region (cannot be aligned), followed by 12710 bases of middle region, and then 2-base tail region.

The information in the header can help users to locate the read easily.

3. simulated_error_profile
   Contains all the information of errors introduced into each reads, including error type, position, original bases and current bases.