iVar
Manual

Table of Contents

Available Commands

Command Description
trim Trim reads in aligned BAM
variants Call variants from aligned BAM file
filtervariants Filter variants across replicates or multiple samples aligned using the same reference
consensus Call consensus from aligned BAM file
getmasked Detect primer mismatches and get primer indices for the amplicon to be masked
removereads Remove reads from trimmed BAM file
version Show version information
trimadapter (EXPERIMENTAL) Trim adapter sequences from reads

To view detailed usage for each command type ivar <command> Note : Commands maked (EXPERIMENTAL) are still under active development.

Description of commands

Trim primer sequences with iVar

iVar uses primer positions supplied in a BED file to soft clip primer sequences from an aligned and sorted BAM file. Following this, the reads are trimmed based on a quality threshold(Default: 20). To do the quality trimming, iVar uses a sliding window approach(Default: 4). The windows slides from the 5' end to the 3' end and if at any point the average base quality in the window falls below the threshold, the remaining read is soft clipped. If after trimming, the length of the read is greater than the minimum length specified(Default: 30), the read is written to the new trimmed BAM file.

Please note that the strand is taken into account while doing the trimming so forward primers are trimmed only from forward strand and reverse primers are trimmed from reverse strand.

To sort and index an aligned BAM file (OPTIONAL, if index is not present iVar will create one), the following command can be used,

# Input file - test.bam
samtools sort -o test.sorted.bam test.bam && samtools index test.sorted.bam.

Note: All the trimming in iVar is done by soft-clipping reads in an aligned BAM file. This information is lost if reads are extracted in fastq or fasta format from the trimmed BAM file.

Command:

ivar trim
Usage: ivar trim -i <input.bam> -b <primers.bed> -p <prefix> [-m <min-length>] [-q <min-quality>] [-s <sliding-window-width>]
Input Options Description
-i (Required) Sorted bam file, with aligned reads, to trim primers and quality
-b (Required) BED file with primer sequences and positions
-m Minimum length of read to retain after trimming (Default: 30)
-q Minimum quality threshold for sliding window to pass (Default: 20)
-s Width of sliding window (Default: 4)
-e Include reads with no primers. By default, reads with no primers are excluded
Output Options Description
-p (Required) Prefix for the output BAM file

Example Usage:

ivar trim -b test_primers.bed -p test.trimmed -i test.bam -q 15 -m 50 -s 4

The command above will produce a trimmed BAM file test.trimmed.bam after trimming the aligned reads in test.bam using the primer positions specified in test_primers.bed and a minimum quality threshold of 15, minimum read length of 50 and a sliding window of 4.

Example BED file

Puerto 28 52 400_1_out_L 60 +
Puerto 482 504 400_1_out_R 60 -
Puerto 359 381 400_2_out_L 60 +
Puerto 796 818 400_2_out_R 60 -
Puerto 658 680 400_3_out_L* 60 +
Puerto 1054 1076 400_3_out_R* 60 -
.
.
.
.

Call variants with iVar

iVar uses the output of the samtools mpileup command to call variants - single nucleotide variants(SNVs) and indels. In order to call variants correctly, the reference file used for alignment must be passed to iVar using the -r flag. The output of samtools pileup is piped into ivar variants to generate a .tsv file with the variants. There are two parameters that can be set for variant calling using iVar - minimum quality(Default: 20) and minimum frequency(Default: 0.03). Minimum quality is the minimum quality for a base to be counted towards the ungapped depth to canculate iSNV frequency at a given position. For insertions, the quality metric is discarded and the mpileup depth is used directly. Minimum frequency is the minimum frequency required for a SNV or indel to be reported.

Amino acid translation of iSNVs

iVar can identify codons and translate variants into amino acids using a GFF file in the https://github.com/The-Sequence-Ontology/Specifications/blob/master/gff3.md "GFF3" format containing the required coding regions (CDS). In absence of a GFF file, iVar will not perform the translation and "NA" will be added to the output file in place of the reference and alternate codons and amino acids. The GFF file in the GFF3 format can be downloaded via ftp from NCBI RefSeq/Genbank. They are usually the files with the extension ".gff.gz". For example, the GFF file for Zaire Ebolavirus can be found here. More details on GFF3 files hosted by NCBI can be found in their ftp FAQs.

Account for RNA editing through polymerase slippage

Some RNA viruses such as Ebola virus, might have polymerase slippage causing the insertion of a couple of nucleotides. More details can be found here. iVar can account for this editing and identify the correct open reading frames. The user will have to specify two additional parameters, EditPosition: Position at which edit occurs and EditSequence: The sequence tht is inserted at the given positon, in the "attributes" column of the GFF file to account for this. A test example is given below,

test Genbank CDS 2 292 . + . ID=id-testedit1;Note=PinkFloyd;EditPosition=100;EditSequence=A
test Genbank CDS 2 292 . + . ID=id-testedit2;Note=AnotherBrickInTheWall;EditPosition=102;EditSequence=AA

If a certain base is present in multiple CDSs, iVar will add a new row for each CDS frame and distinguish the rows by adding the ID (specified in attributes of GFF) of the GFF feature used for the translation. This is shown for position 42 in the example output below. There are two rows with two different GFF features: id-test3 and id-test4.

Command:

Usage: samtools mpileup -aa -A -d 0 -B -Q 0 --reference [<reference-fasta] <input.bam> | ivar variants -p <prefix> [-q <min-quality>] [-t <min-frequency-threshold>] [-m <minimum depth>] [-r <reference-fasta>] [-g GFF file]
Note : samtools mpileup output must be piped into ivar variants
Input Options Description
-q Minimum quality score threshold to count base (Default: 20)
-t Minimum frequency threshold(0 - 1) to call variants (Default: 0.03)
-m Minimum read depth to call variants (Default: 0)
-r Reference file used for alignment. This is used to translate the nucleotide sequences and identify intra host single nucleotide variants
-g A GFF file in the GFF3 format can be supplied to specify coordinates of open reading frames (ORFs). In absence of GFF file, amino acid translation will not be done.
Output Options Description
-p (Required) Prefix for the output tsv variant file

Example Usage:

samtools mpileup -aa -A -d 600000 -B -Q 0 test.trimmed.bam | ivar variants -p test -q 20 -t 0.03 -r test_reference.fa -g test.gff

The command above will generate a test.tsv file.

Example of output .tsv file.

REGION POS REF ALT REF_DP REF_RV REF_QUAL ALT_DP ALT_RV ALT_QUAL ALT_FREQ TOTAL_DP PVAL PASS GFF_FEATURE REF_CODON REF_AA ALT_CODON ALT_AA
test 42 G T 0 0 0 1 0 49 1 1 1 FALSE id-test3 AGG R ATG M
test 42 G T 0 0 0 1 0 49 1 1 1 FALSE id-test4 CAG Q CAT H
test 320 A T 1 1 35 1 1 46 0.5 2 0.666667 FALSE NA NA NA NA NA
test 365 A T 0 0 0 1 1 27 1 1 1 FALSE NA NA NA NA NA

Description

Field Description
REGION Region from BAM file
POS Position on reference sequence
REF Reference base
ALT Alternate Base
REF_DP Ungapped depth of reference base
REF_RV Ungapped depth of reference base on reverse reads
REF_QUAL Mean quality of reference base
ALT_DP Ungapped depth of alternate base.
ALT_RV Ungapped deapth of alternate base on reverse reads
ALT_QUAL Mean quality of alternate base
ALT_FREQ Frequency of alternate base
TOTAL_DP Total depth at position
PVAL p-value of fisher's exact test
PASS Result of p-value <= 0.05
GFF_FEATURE ID of the GFF feature used for the translation
REF_CODON Codong using the reference base
REF_AA Amino acid translated from reference codon
ALT_CODON Codon using the alternate base
ALT_AA Amino acid translated from the alternate codon

Note: Please use the -B options with samtools mpileup to call variants and generate consensus. When a reference sequence is supplied, the quality of the reference base is reduced to 0 (ASCII: !) in the mpileup output. Disabling BAQ with -B seems to fix this. This was tested in samtools 1.7 and 1.8.

Filter variants across replicates with iVar

iVar can be used to get an intersection of variants(in .tsv files) called from any number of replicates or from different samples using the same reference sequence. This intersection will filter out any iSNVs that do not occur in a minimum fraction of the files supplied. This parameter can be changed using the -t flag which range from 0 to 1 (default). Fields that are different across replicates(fields apart from REGION, POS, REF, ALT, REF_CODON, REF_AA, ALT_CODON, ALT_AA) will have the filename added as a suffix. If there are a large number of files to be filtered, the -f flag can be used to supply a text file with one sample/replicate variant .tsv file per line.

Command:

Usage: ivar filtervariants -p <prefix> replicate-one.tsv replicate-two.tsv ... OR ivar filtervariants -p <prefix> -f <text file with one variant file per line>
Input: Variant tsv files for each replicate/sample
Input Options Description
-t Minimum fration of files required to contain the same variant. Specify value within [0,1]. (Default: 1)
-f A text file with one variant file per line.
Output Options Description
-p (Required) Prefix for the output filtered tsv file

Example Usage: The command below only retains those variants that are found in atleast 50% of the fiels supplied

ivar filtervariants -t 0.5 -p test.filtered test.1.tsv test.2.tsv test.3.tsv

The three replicates can also be supplied using a text file as shown below

ivar filtervariants -t 0.5 -p test.filtered -f filter_files.txt

filter_files.txt

./path/to/test.1.tsv
./path/to/test.2.tsv
./path/to/test.3.tsv

The command above will prodoce an output .tsv file test.filtered.tsv.

Example output of filtered .tsv file from three files test_rep1.tsv and test_rep2.tsv

REGION POS REF ALT GFF_FEATURE REF_CODON REF_AA ALT_CODON ALT_AA REF_DP_test.1.tsv REF_RV_test.1.tsv REF_QUAL_test.1.tsv ALT_DP_test.1.tsv ALT_RV_test.1.tsv ALT_QUAL_test.1.tsv ALT_FREQ_test.1.tsv TOTAL_DP_test.1.tsv PVAL_test.1.tsv PASS_test.1.tsv REF_DP_test.2.tsv REF_RV_test.2.tsv REF_QUAL_test.2.tsv ALT_DP_test.2.tsv ALT_RV_test.2.tsv ALT_QUAL_test.2.tsv ALT_FREQ_test.2.tsv TOTAL_DP_test.2.tsv PVAL_test.2.tsv PASS_test.2.tsv REF_DP_test.3.tsv REF_RV_test.3.tsv REF_QUAL_test.3.tsv ALT_DP_test.3.tsv ALT_RV_test.3.tsv ALT_QUAL_test.3.tsv ALT_FREQ_test.3.tsv TOTAL_DP_test.3.tsv PVAL_test.3.tsv PASS_test.3.tsv
test 139 T A id-test3 GCT A GCA A 1 0 32 1 0 55 0.5 2 0.666667 FALSE 1 0 32 1 0 55 0.5 2 0.666667 FALSE NA NA NA NA NA NA NA NA NA NA
test 320 A T NA NA NA NA NA 1 1 35 1 1 46 0.5 2 0.666667 FALSE NA NA NA NA NA NA NA NA NA NA 1 1 35 1 1 46 0.5 2 0.666667 FALSE
test 365 A T NA NA NA NA NA 0 0 0 1 1 27 1 1 1 FALSE 0 0 0 1 1 27 1 1 1 FALSE 0 0 0 1 1 27 1 1 1 FALSE
test 42 G T id-test4 CAG Q CAT H 0 0 0 1 0 49 1 1 1 FALSE 0 0 0 1 0 49 1 1 1 FALSE NA NA NA NA NA NA NA NA NA NA
test 42 G T id-testedit1 AGG R ATG M 0 0 0 1 0 49 1 1 1 FALSE 0 0 0 1 0 49 1 1 1 FALSE 0 0 0 1 0 49 1 1 1 FALSE
test 69 T G id-testedit2 TTG L TGG W 1 0 57 1 0 53 0.5 2 0.666667 FALSE 1 0 57 1 0 53 0.5 2 0.666667 FALSE 1 0 57 1 0 53 0.5 2 0.666667 FALSE

Description of fields

No Field Description
1 REGION Common region across all replicate variant tsv files
2 POS Common position across all variant tsv files
3 REF Common reference base across all variant tsv files
4 ALT Common alternate base across all variant tsv files
5 GFF_FEATURE GFF feature used for the translation
6 REF_CODON The codon using the reference base
7 REF_AA Reference codon translated into amino acid
8 ALT_CODON Codon using the alternate base
9 ALT_AA Alternate codon translated into amino acid
10 REF_DP_<rep1-tsv-file-name> Depth of reference base in replicate 1
11 REF_RV_<rep1-tsv-file-name> Depth of reference base on reverse reads in replicate 1
12 REF_QUAL_<rep1-tsv-file-name> Mean quality of reference base in replicate 1
13 ALT_DP_<rep1-tsv-file-name> Depth of alternate base in replicate 1
14 ALT_RV_<rep1-tsv-file-name> Deapth of alternate base on reverse reads in replicate 1
15 ALT_QUAL_<rep1-tsv-file-name> Mean quality of alternate base in replicate 1
16 ALT_FREQ_<rep1-tsv-file-name> Frequency of alternate base in replicate 1
17 TOTAL_DP_<rep1-tsv-file-name> Total depth at position in replicate 1
18 PVAL_<rep1-tsv-file-name> p-value of fisher's exact test in replicate 1
19 PASS_<rep1-tsv-file-name> Result of p-value <= 0.05 in replicate 1
20 Continue rows 10 - 19 for every replicate provided

Generate a consensus sequences from an aligned BAM file

To generate a consensus sequence iVar uses the output of samtools mpileup command. The mpileup output must be piped into ivar consensus. There are five parameters that can be set - minimum quality(Default: 20), minimum frequency threshold(Default: 0), minimum depth to call a consensus(Default: 10), a flag to exclude nucleotides from regions with depth less than the minimum depth and a character to call in regions with coverage lower than the speicifed minimum depth(Default: 'N'). Minimum quality is the minimum quality of a base to be considered in calculations of variant frequencies at a given position. Minimum frequency threshold is the minimum frequency that a base must match to be called as the consensus base at a position. If one base is not enough to match a given frequency, then an ambigious nucleotide is called at that position. Minimum depth is the minimum required depth to call a consensus. If '-k' flag is set then these regions are not included in the consensus sequence. If '-k' is not set then by default, a 'N' is called in these regions. You can also specfy which character you want to add to the consensus to cover regions with depth less than the minimum depth. This can be done using -n option. It takes one of two values: '-' or 'N'.

As an example, consider a position with 6As, 3Ts and 1C. The table below shows the consensus nucleotide called at different frequencies.

Minimum frequency threshold Consensus
0 A
0.5 A
0.6 A
0.7 W(A or T)
0.9 W (A or T)
1 H (A or T or C)

If there are two nucleotides at the same frequency, both nucleotides are used to call an ambigious base as the consensus. As an example, consider a position wiht 6 Ts, 2As and 2 Gs. The table below shows the consensus nucleotide called at different frequencies.

Minimum frequency threshold Consensus
0 T
0.5 T
0.6 T
0.7 D(A or T or G)
0.9 D(A or T or G)
1 D(A or T or G)

The output of the command is a fasta file with the consensus sequence and a .txt file with the average quality of every base used to generate the consensus at each position. For insertions, the quality is set to be the minimum quality threshold since mpileup doesn't give the quality of bases in insertions.

Command:

ivar consensus
Usage: samtools mpileup -aa -A -d 0 -Q 0 <input.bam> | ivar consensus -p <prefix>
Note : samtools mpileup output must be piped into ivar consensus
Input Options Description
-q Minimum quality score threshold to count base (Default: 20)
-t Minimum frequency threshold(0 - 1) to call consensus. (Default: 0)
Frequently used thresholds | Description
---------------------------|------------
0 | Majority or most common base
0.2 | Bases that make up atleast 20% of the depth at a position
0.5 | Strict or bases that make up atleast 50% of the depth at a position
0.9 | Strict or bases that make up atleast 90% of the depth at a position
1 | Identical or bases that make up 100% of the depth at a position. Will have highest ambiguities
-m Minimum depth to call consensus(Default: 10)
-k If '-k' flag is added, regions with depth less than minimum depth will not be added to the consensus sequence. Using '-k' will override any option specified using -n
-n (N/-) Character to print in regions with less than minimum coverage(Default: N)
Output Options Description
-p (Required) Prefix for the output fasta file and quality file

Example Usage:

samtools mpileup -d 1000 -A -Q 0 test.bam | ivar consensus -p test -q 20 -t 0

The command above will produce a test.fa fasta file with the consensus sequence and a test.qual.txt with the average quality of each base in the consensus sequence.

Get primers with mismatches to the reference sequence

iVar uses a .tsv file with variants to get the zero based indices(based on the BED file) of mismatched primers. This command requires another .tsv file with each line containing the left and right primer names separated by a tab. This is used to get both the primers for an amplicon with a single mismatched primer. The output is a text file with the zero based primer indices delimited by a space. The output is written to a a text file using the prefix provided.

Command:

ivar getmasked
Usage: ivar getmasked -i <input-filtered.tsv> -b <primers.bed> -f <primer_pairs.tsv> -p <prefix>
Note: This step is used only for amplicon-based sequencing.
Input Options Description
-i (Required) Input filtered variants tsv generated from 'ivar filtervariants'
-b (Required) BED file with primer sequences and positions
-f (Required) Primer pair information file containing left and right primer names for the same amplicon separated by a tab
Output Options Description
-p (Required) Prefix for the output text file

Example BED file

Puerto 28 52 400_1_out_L 60 +
Puerto 482 504 400_1_out_R 60 -
Puerto 359 381 400_2_out_L 60 +
Puerto 796 818 400_2_out_R 60 -
Puerto 658 680 400_3_out_L* 60 +
Puerto 1054 1076 400_3_out_R* 60 -
.
.
.
.

Example primer pair information file

400_1_out_L 400_1_out_R
400_2_out_L 400_2_out_R
400_3_out_L 400_3_out_R
.
.
.
.

Example Usage:

ivar getmasked -i test.filtered.tsv -b primers.bed -f pair_information.tsv -p test.masked.txt

The command above produces an output file - test.masked.txt.

Example Output:

1 2 7 8

Remove reads associated with mismatched primer indices

This command accepts an aligned and sorted BAM file trimmed using ivar trim and removes the reads corresponding to the supplied primer indices, which is the output of ivar getmasked command. Under the hood, ivar trim adds the zero based primer index(based on the BED file) to the BAM auxillary data for every read. Hence, ivar removereads will only work on BAM files that have been trimmed using ivar trim.

Command:

ivar removereads
Usage: ivar removereads -i <input.trimmed.bam> -p <prefix> -t <text-file-with-primer-indices>
Note: This step is used only for amplicon-based sequencing.
Input Options Description
-i (Required) Input BAM file trimmed with ivar trim. Must be sorted and indexed, which can be done using sort_index_bam.sh
-t (Required) Text file with primer indices separated by spaces. This is the output of getmasked command.
Output Options Description
-p (Required) Prefix for the output filtered BAM file

Example Usage:

ivar trim -i test.bam -p test.trimmed
ivar removereads -i test.trimmed.bam -p test.trimmed.masked.bam -t test.masked.txt

The ivar trim command above trims test.bam and produced test.trimmed.bam with the primer indice data added. The ivar removereads command produces an output file - test.trimmed.masked.bam after removing all the reads corresponding to primer indices - 1,2,7 and 8.

(Experimental) trimadapter

Note: This feature is under active development and not completely validated yet.

trimadapter in iVar can be used to trim adapter sequences from fastq files using a supplied fasta file.