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QTL-seq User Guide

version 2.2.4

Table of contents

What is QTL-seq?

Bulked segregant analysis, as implemented in QTL-seq (Takagi et al., 2013), is a powerful and efficient method to identify agronomically important loci in crop plants. QTL-seq was adapted from MutMap to identify quantitative trait loci. It utilizes sequences pooled from two segregating progeny populations with extreme opposite traits (e.g. resistant vs susceptible) and a single whole-genome resequencing of either of the parental cultivars.

Citation

Installation

Dependencies

Softwares

Python (>=3.5) libraries

  • matplotlib
  • numpy
  • pandas
  • seaborn (optional)

Installation using bioconda

You can install QTL-seq using bioconda.

conda install -c bioconda qtlseq

Alternatively, if you want to create QTL-seq specific environment with Python3.

conda create -n qtlseq python=3 qtlseq

Manual installation

If you got a error during installation, you can install QTL-seq, manually.

git clone https://github.com/YuSugihara/QTL-seq.git
cd QTL-seq
pip install -e .

Then you have to install other dependencies by yourself. We highly recommend you to install SnpEff and Trimmomatic using bioconda.

conda install -c bioconda snpeff
conda install -c bioconda trimmomatic

After installation, please check whether SnpEff and Trimmomatic work through the commands like below.

snpEff --help
trimmomatic --help

Usage

If your reference genome has more than 50 contigs (or chromosomes), only significant contigs will be plotted.

qtlseq -h

usage: qtlseq -r <FASTA> -p <BAM|FASTQ> -b1 <BAM|FASTQ>
              -b2 <BAM|FASTQ> -n1 <INT> -n2 <INT> -o <OUT_DIR>
              [-F <INT>] [-T] [-e <DATABASE>] [--species <NAME>]

QTL-seq version 2.2.4

optional arguments:
  -h, --help         show this help message and exit
  -r , --ref         Reference fasta.
  -p , --parent      fastq or bam of parent. If you specify
                     fastq, please separate pairs by comma,
                     e.g. -p fastq1,fastq2. You can use this
                     optiion multiple times
  -b1 , --bulk1      fastq or bam of bulk 1. If you specify
                     fastq, please separate pairs by comma,
                     e.g. -b1 fastq1,fastq2. You can use this
                     optiion multiple times
  -b2 , --bulk2      fastq or bam of bulk 2. If you specify
                     fastq, please separate pairs by comma,
                     e.g. -b2 fastq1,fastq2. You can use this
                     optiion multiple times
  -n1 , --N-bulk1    Number of individuals in bulk 1.
  -n2 , --N-bulk2    Number of individuals in bulk 2.
  -o , --out         Output directory. Specified name must not
                     exist.
  -F , --filial      Filial generation. This parameter must be
                     more than 1. [2]
  -t , --threads     Number of threads. If you specify the number
                     below one, then QTL-seq will use the threads
                     as many as possible. [2]
  -w , --window      Window size (kb). [2000]
  -s , --step        Step size (kb). [100]
  -D , --max-depth   Maximum depth of variants which will be used. [250]
  -d , --min-depth   Minimum depth of variants which will be used. [8]
  -N , --N-rep       Number of replicates for simulation to make 
                     null distribution. [5000]
  -T, --trim         Trim fastq using trimmomatic.
  -a , --adapter     FASTA of adapter sequences. This will be used
                     when you specify "-T" for trimming.
  --trim-params      Parameters for trimmomatic. Input parameters
                     must be separated by comma with following
                     order: phred, ILLUMINACLIP, LEADING, TRAILING,
                     SLIDINGWINDOW, MINLEN. If you want to remove
                     adapters of illumina, please specify FASTA of
                     the adapter sequences with "--adapter". Specified
                     name will be inserted into <ADAPTER_FASTA>. If you
                     don't specify it, adapter trimming will be skipped.
                     [33,<ADAPTER_FASTA>:2:30:10,20,20,4:15,75]
  -e , --snpEff      Predict causal variant using SnpEff. Please
                     check available databases in SnpEff.
  --mem              Maximum memory per thread when bam sorted;
                     suffix K/M/G recognized. [1G]
  -q , --min-MQ      Minimum mapping quality in mpileup. [40]
  -Q , --min-BQ      Minimum base quality in mpileup. [18]
  -C , --adjust-MQ   "adjust-MQ" in mpileup. Default parameter
                     is suited for BWA. [50]
  --species          Consider multiple test correction derived by
                     Huang et al. (2019). Please spesify a species name.
                     With this option. QTL-seq produces a theoretical threshold.
                     Currently, Arabidopsis, Cucumber, Maize, Rapeseed,
                     Rice, Tobacco, Tomato, Wheat, and Yeast are supported.
  -v, --version      show program's version number and exit

QTL-seq can run from FASTQ (without or with trimming) and BAM. If you want to run QTL-seq from VCF, please use QTL-plot (example 5). Once you run QTL-seq, QTL-seq automatically complete the subprocesses.

Example 1 : run QTL-seq from FASTQ without trimming

qtlseq -r reference.fasta \
       -p parent.1.fastq,parent.2.fastq \
       -b1 bulk_1.1.fastq,bulk_1.2.fastq \
       -b2 bulk_2.1.fastq,bulk_2.2.fastq \
       -n1 20 \
       -n2 20 \
       -o example_dir

-r : reference fasta

-p : FASTQs of parent. Please input pair-end reads separated by comma. FASTQs can be gzipped.

-b1 : FASTQs of bulk 1. Please input pair-end reads separated by comma. FASTQs can be gzipped.

-b2 : FASTQs of bulk 2. Please input pair-end reads separated by comma. FASTQs can be gzipped.

-n1 : number of individuals in bulk 1.

-n2 : number of individuals in bulk 2.

-o : name of output directory. Specified name should not exist.

Example 2 : run QTL-seq from FASTQ with trimming

qtlseq -r reference.fasta \
       -p parent.1.fastq,parent.2.fastq \
       -b1 bulk_1.1.fastq,bulk_1.2.fastq \
       -b2 bulk_2.1.fastq,bulk_2.2.fastq \
       -n1 20 \
       -n2 20 \
       -o example_dir \
       -T

-r : reference fasta

-p : FASTQs of parent. Please input pair-end reads separated by comma. FASTQs can be gzipped.

-b1 : FASTQs of bulk 1. Please input pair-end reads separated by comma. FASTQs can be gzipped.

-b2 : FASTQs of bulk 1. Please input pair-end reads separated by comma. FASTQs can be gzipped.

-n1 : number of individuals in bulk 1.

-n2 : number of individuals in bulk 2.

-o : name of output directory. Specified name should not exist.

-T : trim your reads using trimmomatic.

Example 3 : run QTL-seq from BAM

qtlseq -r reference.fasta \
       -p parent.bam \
       -b1 bulk_1.bam \
       -b2 bulk_2.bam \
       -n1 20 \
       -n2 20 \
       -o example_dir

-r : reference fasta

-p : BAM of parent.

-b1 : BAM of bulk 1.

-b2 : BAM of bulk 2.

-n1 : number of individuals in bulk 1.

-n2 : number of individuals in bulk 2.

-o : name of output directory. Specified name should not exist.

Example 4 : run QTL-seq from multiple FASTQs and BAMs

qtlseq -r reference.fasta \
       -p parent_1.1.fastq,parent_1.2.fastq \
       -p parent_1.bam \
       -b1 bulk_11.1.fastq,bulk_11.2.fastq \
       -b1 bulk_12.bam \
       -b1 bulk_13.bam \
       -b2 bulk_21.1.fastq,bulk_21.2.fastq \
       -b2 bulk_22.bam \
       -b2 bulk_23.bam \
       -n1 20 \
       -n2 20 \
       -o example_dir

QTL-seq can automatically merge multiple FASTQs and BAMs. Of course, you can merge FASTQs or BAMs using cat or samtools merge before input them to QTL-seq. If you specify -p multiple times, please make sure that those files include only 1 individual. On the other hand, -b1 and -b2 can include more than 1 individuals because those are bulked samples. QTL-seq can automatically classify FASTQs and BAMs from whether comma exits or not.

Example 5 : run QTL-plot from VCF

qtlplot -h

usage: qtlplot -v <VCF> -n1 <INT> -n2 <INT> -o <OUT_DIR>
              [-F <INT>] [-t <INT>] [-w <INT>] [-s <INT>] [-D <INT>]
              [-d <INT>] [-N <INT>] [-m <FLOAT>] [-S <INT>] [-e <DATABASE>]
              [--igv] [--indel]

QTL-plot version 2.2.4

optional arguments:
  -h, --help            show this help message and exit
  -v , --vcf            VCF file which contains parent, bulk1, and bulk2
                        in this order. This VCF file must have AD field.
  -n1 , --N-bulk1       Number of individuals in bulk 1.
  -n2 , --N-bulk2       Number of individuals in bulk 2.
  -o , --out            Output directory. Specified name can exist.
  -F , --filial         Filial generation. This parameter must be
                        more than 1. [2]
  -t , --threads        Number of threads. If you specify the number
                        below one, then QTL-plot will use the threads
                        as many as possible. [2]
  -w , --window         Window size (kb). [2000]
  -s , --step           Step size (kb). [100]
  -D , --max-depth      Maximum depth of variants which will be used. [250]
  -d , --min-depth      Minimum depth of variants which will be used. [8]
  -N , --N-rep          Number of replicates for simulation to make 
                        null distribution. [5000]
  -m , --min-SNPindex   Cutoff of minimum SNP-index for clear results. [0.3]
  -S , --strand-bias    Filter spurious homo genotypes in cultivar using
                        strand bias. If ADF (or ADR) is higher than this
                        cutoff when ADR (or ADF) is 0, that SNP will be
                        filtered out. If you want to supress this filtering,
                        please set this cutoff to 0. [7]
  -e , --snpEff         Predict causal variant using SnpEff. Please
                        check available databases in SnpEff.
  --igv                 Output IGV format file to check results on IGV.
  --species             Consider multiple test correction derived by
                        Huang et al. (2019). Please spesify a species name.
                        With this option. QTL-seq produces a theoretical threshold.
                        Currently, Arabidopsis, Cucumber, Maize, Rapeseed,
                        Rice, Tobacco, Tomato, Wheat, and Yeast are supported.
  --indel               Plot SNP-index with INDEL.
  --fig-width           Width allocated in chromosome figure. [7.5]
  --fig-height          Height allocated in chromosome figure. [4.0]
  --white-space         White space between figures. (This option
                        only affect vertical direction.) [0.6]
  -f , --format         Specifiy the format of an output image.
                        eps/jpeg/jpg/pdf/pgf/png/rgba/svg/svgz/tif/tiff
  --version             show program's version number and exit

QTL-plot is included in QTL-seq. QTL-seq run QTL-plot after making VCF. Then, QTL-plot will work with default parameters. If you want to change some parameters, you can use VCF inside of (OUT_DIR/30_vcf/QTL-seq.vcf.gz) to retry plotting process like below.

qtlplot -v OUT_DIR/30_vcf/QTL-seq.vcf.gz \
        -o ANOTHER_DIR_NAME \
        -n1 20 \
        -n2 20 \
        -w 2000 \
        -s 100

Use QTL-plot for VCF which was made by yourself

In this case, please make sure that:

  1. Your VCF include AD format.
  2. Your VCF include three columns of parent, bulk1 and bulk2 in this order.

If you got a error, please try to run QTL-seq from FASTQ or BAM before asking in issues.

Outputs

Inside of OUT_DIR is like below.

├── 10_ref
│  ├── reference.fasta
│  ├── reference.fasta.amb
│  ├── reference.fasta.ann
│  ├── reference.fasta.bwt
│  ├── reference.fasta.fai
│  ├── reference.fasta.pac
│  └── reference.fasta.sa
├── 20_bam
│  ├── bulk1.filt.bam
│  ├── bulk1.filt.bam.bai
│  ├── bulk2.filt.bam
│  ├── bulk2.filt.bam.bai
│  ├── parent.filt.bam
│  └── parent.filt.bam.bai
├── 30_vcf
│  ├── qtlseq.vcf.gz
│  └── qtlseq.vcf.gz.tbi
├── 40_qtlseq
│  ├── bulk1_SNPindex.png
│  ├── bulk2_SNPindex.png
│  ├── delta_SNPindex.png
│  ├── sliding_window.tsv
│  ├── sliding_window.p95.tsv
│  ├── sliding_window.p99.tsv
│  ├── np_index.tsv
│  ├── snp_index.p95.tsv
│  └── snp_index.p99.tsv
└── log
   ├── bcftools.log
   ├── bgzip.log
   ├── bwa.log
   ├── samtools.log
   └── tabix.log
  • If you run QTL-seq with trimming, you will get the directory of 00_fastq which includes FASTQs after trimming.
  • You can check the results in 40_QTL-seq.
    • snp_index.tsv : columns in this order.
      • CHROM : chromosome name
      • POSI : position in chromosome
      • VARIANT : SNP or INDEL
      • DEPTH 1 : depth of bulk 1
      • DEPTH 2 : depth of bulk 2
      • p99 : 99% confidence interval of simulated delta SNP-index (absolute value)
      • p95 : 95% confidence interval of simulated delta SNP-index (absolute value)
      • SNP-index 1 : real SNP-index of bulk 1
      • SNP-index 2 : real SNP-index of bulk 2
      • DELTA SNP-index : real delta SNP-index (bulk2 - bulk1)
    • sliding_window.tsv : columns in this order.
      • CHROM : chromosome name
      • POSI : central position of window
      • MEAN p99 : mean of p99 (absolute value)
      • MEAN p95 : mean of p95 (absolute value)
      • MEAN SNP-index 1 : mean SNP-index of bulk 1
      • MEAN SNP-index 2 : mean SNP-index of bulk 2
      • MEAN DELTA SNP-index : mean delta SNP-index
    • QTL-seq_plot.png : resulting plot (like below)
      • BLUE dot : variant
      • RED line : mean SNP-index
      • ORANGE line : mean p99
      • GREEN line : mean p95

About multiple testing correction

We implemented multiple testing correction in QTL-seq v2. However, since multiple testing correction changes the threshold from the original QTL-seq threshold, we highly recommend users, who expect original QTL-seq algorism identifying a lot of causal mutations in many researches, to try QTL-seq v2 without multiple testing correction at first. You can use multiple testing correction with the option --species like below:

qtlseq -r reference.fasta \
       -p parent.1.fastq,parent.2.fastq \
       -b1 bulk_1.1.fastq,bulk_1.2.fastq \
       -b2 bulk_2.1.fastq,bulk_2.2.fastq \
       -n1 20 \
       -n2 20 \
       -o example_dir \
       --species Rice

Currently, only nine species (Arabidopsis, Cucumber, Maize, Rapeseed, Rice, Tobacco, Tomato, Wheat, and Yeast) are supported, following the parameters defined in Huang et al. (2019).

Built and use your own database for snpEff

If you want to use your own database for snpEff, you need additional steps. Here we assume that you installed QTL-seq via anaconda distribution, creating new environment with conda create.

  1. Find the directory of snpEff that includes snpEff script, configuration file and database. You can find it in /home/anaconda3/envs/{your_env_name_installed_qtlseq}/share/snpeff-5.0-0/. anaconda3 may be miniconda3. Also, the version of snpeff may be different.

  2. Go to this directory and follow the snpEff manual to build the database. Don't forget to add your database info to the snpEff configuration file. https://pcingola.github.io/SnpEff/se_buildingdb/#add-a-genome-to-the-configuration-file

  3. Run QTL-seq with option -e {your_database_name}