Next-generation sequencing (NGS) has transformed the field of genetics into genomics by providing DNA sequence data at an ever increasing rate and reduced cost (Mardis, 2008). The nascent field of population genomics relies on NGS coupled with laboratory methods to reproducibly reduce genome complexity to a few thousand loci. The most common approach, restriction-site associated DNA sequencing (RADseq), uses restriction endonucleases to randomly sample the genome at locations adjacent to restriction-enzyme recognition sites that, when coupled with Illumina sequencing, produces high coverage of homologous SNP (Single Nucleotide Polymorphism) loci. As such, RADseq provides a powerful method for population level genomic studies (Ellegren, 2014; Narum et al., 2013; Rowe, Renaut & Guggisberg, 2011).
The original RADseq approach (Baird et al., 2008; Miller et al., 2007), and initial population genomic studies employing it (Hohenlohe et al., 2010), focused on SNP discovery and genotyping on the first (forward) read only. This is because the original RADseq method (Baird et al., 2008; Miller et al., 2007) utilized random shearing to produce RAD loci; paired-end reads were not of uniform length or coverage, making it problematic to find SNPs at high and uniform levels of coverage across a large proportion of individuals. As a result, the most comprehensive and widely used software package for analysis of RADseq data, Stacks (Catchen et al., 2013; Catchen et al., 2011), provides SNP genotypes based only on first-read data. In contrast, RADseq approaches such as ddRAD (Peterson et al., 2012), 2bRAD (Wang et al., 2012), and ezRAD (Toonen et al., 2013) rely on restriction enzymes to define both ends of a RAD locus, largely producing RAD loci of fixed length (flRAD). Paired-end Illumina sequencing of flRAD fragments provides an opportunity to significantly expand the number of SNPs that can be genotyped from a single RADseq library.
Here, the variant-calling pipeline dDocent is introduced as a tool for generating population genomic data; a brief methodological outline of the analysis pipeline also is presented. dDocent is a wrapper script designed to take raw flRAD data and produce population informative SNP calls (SNPs that are shared across the majority of individuals and populations), taking full advantage of both paired-end reads. dDocent is configured for organisms with high levels of nucleotide and Indel polymorphisms, such as are found in many marine organisms (Guo, Zou & Wagner, 2012; Keever et al., 2009; Sodergren et al., 2006; Waples, 1998; Ward, Woodwark & Skibinski, 1994); however, the pipeline also can be adjusted for low polymorphism species. As input, dDocent takes paired FASTQ files for individuals and outputs raw SNP and Indel calls as well as filtered SNP calls in VCF format. The pipeline and a comprehensive online manual can be found at (http://dDocent.wordpress.com). Finally, results of pipeline analyses, using both dDocent and Stacks, of populations of three species of marine fishes are provided to demonstrate the utility of dDocent compared to Stacks, the first and most comprehensive, existing software package for RAD population genomics.
Implementation and basic usage
The dDocent pipeline is written in BASH and will run using most Unix-like operating systems. dDocent is largely dependent on other bioinformatics software packages, taking advantage of programs designed specifically for each task of the analysis and ensuring that each modular component can be updated separately. Proper implementation depends on the correct installation of each third-party packages/tools. A full list of dependencies can be found in the user manual at (http://ddocent.wordpress.com/ddocent-pipeline-user-guide/) and a sample script to automatically download and install the packages in a Linux environment can be found at the dDocent repository (https://github.com/jpuritz/dDocent).
dDocent is run by simply switching to a directory containing input data and starting the program. There is no configuration file, and dDocent will proceed through a short series of command-line prompts, allowing the user to establish analysis parameters. After all required variables are configured, including an e-mail address for a completion notification, dDocent provides instructions on how to move the program to the background and run, undisturbed, until completion. The pipeline is designed to take advantage of multiple processing-core machines and, whenever possible, processes are invoked with multiple threads or occurrences. For most Linux distributions, the number of processing cores should be automatically detected. If dDocent cannot determine the number of processors, it will ask the user to input the value.
There are two distinct modules of dDocent: dDocent.FB and dDocent.GATK. dDocent.FB uses minimal, BAM-file preparation steps before calling SNPs and Indel s, simultaneously using FreeBayes (Garrison & Marth, 2012). dDocent.GATK uses GATK (McKenna et al., 2010) for Indel realignment, SNP and Indel genotyping (using HaplotypeCaller), and variant quality-score recalibration, largely following GATK Best Practices recommendations (Van der Auwera et al., 2013; DePristo et al., 2011). The modules represent two different strategies for SNP/Indel calling that are completely independent of one another. Currently, dDocent.FB is easier to implement, substantially faster to execute, and depends on software that is commercially unrestricted; consequently, the remainder of this paper focuses on dDocent.FB. Additional information on dDocent.GATK may be found in the user guide.
Data input requirements
dDocent requires demultiplexed forward and paired-end FASTQ files for every individual in the analysis (flRAD data only). A simple naming convention (a single-word locality code/name and a single-word sample identifier separated by an underscore) must be followed for every sample; examples are LOCA_IND01.F.fq and LOCA_IND01.R.fq. A sample script for using a text file containing barcodes and sample names and process_radtags from Stacks (Catchen et al., 2013) to properly demultiplex samples and put them in the proper dDocent naming convention, can be found at the dDocent repository (https://github.com/jpuritz/dDocent).
After dDocent checks that it is recognizing the proper number of samples in the current directory, it asks the user if s/he wishes to proceed with quality trimming of sequence data. If directed, dDocent can use the program Trim Galore! (http://www.bioinformatics.babraham.ac.uk/projects/trim_galore/) to simultaneously remove Illumina adapter sequences and trim ends of reads of low quality. By default, Trim Galore! looks for double-digest RAD adapters (Peterson et al., 2012) and trims bases with quality scores less than PHRED 10 (corresponding to a 10% chance of error in the base call). The read mapping and variant calling steps of dDocent account for base quality, so minimal trimming of the data is needed. Typically, quality trimming only needs to be performed once, so the option exists to skip this step in subsequent dDocent analyses.
De novo assembly
Without reference material, population genomic analyses from RADseq depend on de novo assembly of a set of reference contigs. Intrinsically, not all RAD loci appear in all individuals due to stochastic processes inherent in library preparation and sequencing and to polymorphism in restriction-enzyme restriction sites (Catchen et al., 2011). Moreover, populations can contain large levels of within-locus polymorphism, making generation of a reference sequence computationally difficult. dDocent minimizes the amount of data used for assembly by taking advantage of the fact that flRAD loci present in multiple individuals should have higher levels of exactly matching reads (forward and reverse) than loci that are only present in a few individuals. Caution is advised for unique reads with low levels of coverage throughout the data set as they likely represent sequencing errors or polymorphisms that are shared only by a few individuals.
In the first step of the assembly process, untrimmed, paired-end reads are reverse complemented and concatenated to forward reads. Unique paired reads are identified and their occurrences are counted in the entire data set. These data are tabulated into the number of unique reads per levels of 1X to 50X coverage; a graph is then generated and printed to the terminal. The distribution usually follows an asymptotic relationship (Fig. 1), with a large proportion of reads only having one or two occurrences, meaning they likely will not be informative on a population scale. Highly polymorphic RAD loci still should have at least one allele present at the level of expected sequence coverage, so this can be used as a guide for informative data. The user chooses a cut-off level of coverage for reads to be used for assembly—note that all reads are still used for subsequent steps of the pipeline.
After a cut-off level is chosen, remaining concatenated reads are divided back into forward- and reverse-read files and then input directly into the RADseq assembly program Rainbow (Chong, Ruan & Wu, 2012). The default parameters of Rainbow are used except that the maximum number of mismatches used in initial clustering is changed from four to six to help account for highly polymorphic species with large effective population sizes. In short, Rainbow clusters forward reads based on similarity; clusters are then recursively divided, based on reverse reads, into groups representing single alleles. Reads in merged clusters are then assembled using a greedy algorithm (Pop & Salzberg, 2008). dDocent then selects the longest contig for each cluster as the representative reference sequence for that RAD locus. If the forward read does not overlap with the reverse read (almost always the case with flRAD), the forward read is concatenated to the reverse read with ten ‘N’ characters as padding to represent the unknown insert. If forward and reverse reads do overlap, then a full contig is created without N padding. Finally, reference sequences are clustered based on overall sequence similarity (chosen by user, 90% by default), using the program CD-HIT (Fu et al., 2012; Li & Godzik, 2006). This final cluster step reduces the data set further, based on overall sequence identity after assembly. Alternatively, de novo assembly can be skipped and the user can provide a FASTA file with reference sequences.
dDocent uses the MEM algorithm (Li, 2013) of BWA (Li & Durbin, 2009; Li & Durbin, 2010) to map quality-trimmed reads to the reference contigs. Users can deploy the default values of BWA or set an alternative value for each mapping parameter (match score, mismatch score, and gap-opening penalty). The default settings are meant for mapping reads to the human genome, so users are encouraged to experiment with mapping parameters. BWA output is ported to SAMtools (Li et al., 2009), saving disk space, and alignments are saved to the disk as binary alignment/Map (BAM). BAM files are then sorted and indexed.
SNP and Indel discovery and genotyping
dDocent uses a two-step process to optimize the computationally intensive task of SNP/Indel calling. First, quality-trimmed forward and reverse reads are reduced to unique reads. This data set is then mapped to all reference sequences, using the previously entered mapping settings (see Read Mapping above). From this alignment, a set of intervals is created using BEDtools (Quinlan & Hall, 2010). The interval set saves computational time by directing the SNP-/Indel-calling software to examine only reference sequences along contigs that have high quality mappings. Second, the interval list is then split into multiple files, one for each processing core, allowing SNP/Indel calling to be optimized with a scatter-gather technique. The program FreeBayes (Garrison & Marth, 2012) is then executed multiple times simultaneously (one execution per processor and genomic interval). FreeBayes is a Bayesian-based, variant-detection software that uses assembled haplotype sequences to simultaneously call SNPs, Indels, multi-nucleotide polymorphisms (MNPs), and complex events (e.g., composite insertion and substitution events) from alignment files; FreeBayes has the added benefit for population genomics of using reads across multiple individuals to improve genotyping (Garrison & Marth, 2012). FreeBayes is run with minimal changes to the default parameter minimum mapping quality score and base quality score are set to PHRED 10. After all executions of FreeBayes are completed, raw SNP/Indel calls are concatenated into a single variant call file (VCF), using VCFtools (Danecek et al., 2011).
Final SNP data-set requirements are likely to be highly dependent on specific goals and aims of individual projects. To that end, dDocent uses VCFtools (Danecek et al., 2011) to provide only basic level filtering, mostly for run diagnostic purposes. dDocent produces a final VCF file that contains all SNPs, Indels, MNPs, and complex events that are called in 90% of all individuals, with a minimum quality score of 30. Users are encouraged to use VCFtools and vcflib (part of the FreeBayes package; https://github.com/ekg/vcflib) to fully explore and filter data appropriately.
Comparison between dDocent and Stacks
Two sample localities, each comprising 20 individuals, were chosen randomly from unpublished RADseq data sets of three different, marine fish species: red snapper (Lutjanus campechanus), red drum (Sciaenops ocellatus), and silk snapper (Lutjanus vivanus). These three species are part of ongoing RADseq projects in our laboratory, and preliminary analyses indicated high levels of nucleotide polymorphisms across all populations. Double-digest RAD libraries were prepared, generally following Peterson et al. (2012). Individual DNA extractions were digested with EcoRI and MspI. A barcoded adapter was ligated to the EcoRI site of each fragment and a generic adapter was ligated to the MspI site. Samples were then equimollarly pooled and size-selected between 350 and 400 bp, using a Qiagen Gel Extraction Kit. Final library enhancement was completed using 12 cycles of PCR, simultaneously enhancing properly ligated fragments and adding an Illumina Index for additional barcoding. Libraries were sequenced on three separate lanes of an Illumina HiSeq 2000 at the University of Texas Genomic Sequencing and Analysis Facility. Raw sequence data were archived at NCBI’s Short Read Archive (SRA) under Accession SRP041032.
Demultiplexed individual reads were analyzed with dDocent (version 1.0), using three different levels of final reference contig clustering (90%, 96%, and 99% similarity) in an attempt to alter the most comparable analysis variable in dDocent to match the maximum distance between stacks parameter and the maximum distance between stacks from different individuals parameter of Stacks. The coverage cut-off for assembly was 12 for red snapper, 13 for red drum, and nine for silk snapper. All dDocent runs used mapping variables of one, three, and five for match-score value, mismatch score, and gap-opening penalty, respectively. For comparisons, complex variants were decomposed into canonical SNP and Indel representation from the raw VCF files, using vcfallelicprimitives from vcflib (https://github.com/ekg/vcflib).
For analysis with Stacks (version 1.08), reads were demultiplexed and cleaned using process_radtags, removing reads with ‘N’ calls and low-quality base scores. Because dDocent inherently uses both reads for SNP/Indel genotyping, forward reads and reverse reads were processed separately with denovo_map.pl, using three different sets of parameters. The first set had a minimum depth of coverage of two to create a stack, a maximum distance of two between stacks, and a maximum distance of four between stacks from different individuals, with both the deleveraging algorithm and removal algorithms enabled. The second set had a minimum depth of coverage of three to create a stack, a maximum distance of four between stacks, and a maximum distance of eight between stacks from different individuals, with both the deleveraging algorithm and removal algorithms enabled. The third set had a minimum depth of coverage of three to create a stack, a maximum distance of four between stacks, and a maximum distance of 10 between stacks from different individuals, with both the deleveraging algorithm and removal algorithms enabled. SNP calls were output in VCF format.
For both dDocent and Stacks runs, VCFtools was used to filter out all Indel s and SNPs that had a minor allele count of less than five. SNP calls were then evaluated at different individual-coverage levels: the total number of SNPs; the number of SNPs called in 75%, 90%, and 99% of individuals at 3X coverage; the number of SNPs called in 75% and 90% of individuals at 5X coverage; the number of SNPs called in 75% and 90% of individuals at 10X coverage; and the number of SNPS called in 75% and 90% of individuals at 20X coverage. Overall coverage levels for red snapper were lower and likely impacted by a few low-quality individuals; consequently, the number of 5X and 10X SNPs shared among 90% of individuals (after removing the bottom 10% of individuals in terms of coverage) were compared instead of SNP loci shared at 20X coverage. Results from two runs of Stacks (one using forward and one using reverse reads) were combined for comparison with dDocent, which inherently calls SNPs on both reads. All analyses and computations were performed on a 32-core Linux workstation with 128 GB of RAM.
Results and Discussion
Results of SNP calling, including run times (in minutes) for each analysis (not including quality trimming), are presented in Table 1. Data from high coverage SNP calls, averaged over all runs for each pipeline, are presented in Fig. 2. While Stacks called a larger number of low coverage SNPs, limiting results to higher individual coverage and to higher individual call rates revealed that dDocent consistently called more high-quality SNPs. Run times were equivalent for both pipelines.
|dDocent A||dDocent B||dDocent C||Stacks A||Stacks B||Stacks C|
|Total 3X SNPS||53,298||53,316||53,361||28,817||33,479||34,459|
|75% 3X SNPs||21,195||20,990||20,724||4,150||5,735||5,728|
|90% 3X SNPs||9,102||8,850||8,639||675||987||983|
|99% 3X SNPs||78||47||15||–||–||–|
|75% 5X SNPs||14,881||14,594||14,339||2,632||4,351||4,324|
|90% 5X SNPs||5,021||4,925||4,785||179||579||574|
|75% 10X SNPs||7,556||7,318||7,154||783||1,618||1,579|
|90% 10X SNPS||1,414||1,340||1,286||7||48||47|
|90% IND 90% 5X||10,267||10,026||9,798||806||1,807||1,079|
|90% IND 90% 10x||4,242||4,093||3,974||129||441||434|
|Total 3X SNPS||46,378||46,688||46,832||45,792||50,821||52,366|
|75% 3X SNPs||36,745||36,905||36,900||24,134||28,991||28,981|
|90% 3X SNPs||32,356||32,424||32,330||13,439||17,946||17,874|
|99% 3X SNPs||11,906||11,910||11,774||828||1,264||1,259|
|75% 5X SNPs||34,279||34,393||34,336||21,021||26,526||26,464|
|90% 5X SNPs||28,532||28,566||28,431||10,494||15,282||15,207|
|75% 10X SNPs||27,523||27,605||27,488||12,928||17,018||16,983|
|90% 10X SNPS||19,434||19,442||19,283||4,159||6,734||6,705|
|75% 20X SNPs||15,080||15,111||14,981||2,276||3,538||3,516|
|90% 20X SNPs||7,365||7,409||7,304||243||1,974||1,961|
|Total 3X SNPS||68,668||68,825||68,861||48,742||55,505||58,352|
|75% 3X SNPs||30,771||30,391||30,051||7,596||9,705||9,696|
|90% 3X SNPs||14,952||14,673||14,415||2,007||3,439||3,433|
|99% 3X SNPs||4,294||4,060||3,952||132||527||523|
|75% 5X SNPs||20,534||20,188||19,968||4,789||7,290||7,274|
|90% 5X SNPs||9,103||8,750||8,533||1,225||2,573||2,570|
|75% 10X SNPs||9,765||9,400||9,159||2,094||3,547||3,546|
|90% 10X SNPS||3,923||3,691||3,490||489||1,224||1,223|
|75% 20X SNPs||4,069||3,832||3,624||703||1,415||1,411|
|90% 20X SNPs||1,431||1,313||1,228||136||417||418|
At almost all levels of coverage in three different data sets, dDocent called more SNPs across more individuals than Stacks. Two key differences between dDocent and Stacks likely contribute these discrepancies: (i) quality trimming instead of quality filtering, and (ii) simultaneous use of forward and reverse reads by dDocent in assembly, mapping, and genotyping, instead of clustering as employed by Stacks. As with any data analysis, quality of data output is directly linked to the quality of data input. Both dDocent and Stacks use procedures to ensure that only high-quality sequence data are retained; however, Stacks removes an entire read when a sliding window of bases drops below a preset quality score (PHRED 10, by default), while dDocent via Trim Galore! trims off low-quality bases, preserving high-quality bases of each read. Filtering instead of trimming results in fewer reads entering the Stacks analysis (between 65% and 95% of the data compared to dDocent; data not shown), generating lower levels of coverage and fewer SNP calls than dDocent.
dDocent offers two advantages over Stacks: (i) it is specifically designed for paired-end data and utilizes both forward and reverse reads for de novo RAD loci assembly, read mapping, variant discovery, and genotyping; and (ii) it aligns reads to reference sequence instead of clustering by identity. Using both reads to cluster and assemble RAD loci helps to ensure that portions of the genome with complex mutational events, including Indel s or small repetitive regions, are properly assembled and clustered as homologous loci. Additionally, using BWA to map reads to reference loci enables dDocent to properly align reads with Indel polymorphisms, increasing coverage and subsequent variant discovery and genotyping. Clustering methods employed by Stacks, whether clustering alleles within an individual or clustering loci between individuals, effectively remove reads, alleles, and loci with Indel polymorphisms because the associated frame shift effectively inflates the observed number of base-pair differences. For organisms with large effective population sizes and high levels of genetic diversity, such as many marine organisms (Waples, 1998; Ward, Woodwark & Skibinski, 1994), removing reads and loci with Indel polymorphisms will result in a loss of shared loci and coverage.
dDocent is specifically designed to efficiently generate SNP and Indel polymorphisms that are shared across multiple individuals. To that end, the output reference contigs and variant calls represent a subset of the total, genomic information content of the raw input data; RAD loci and variants present in single individuals are largely ignored. Other analysis software, such as the scripts published by Peterson et al. (2012), represent a more comprehensive alternative for generating for a full de novo assembly of RAD loci and would increase the chance of discovering individual level polymorphisms. For population genomics, loci that are not shared by at least 50% of all individuals and/or have minor allele frequencies of less than 5% are often filtered out. dDocent saves computational time by ignoring these loci from the outset of assembly; however, users can pass in a more comprehensive reference (including an entire genome) in order to include all possible variant calls from the data.
dDocent is an open-source, freely available population genomics pipeline configured for species with high levels of nucleotide and Indel polymorphisms, such as many marine organisms. The dDocent pipeline reports more SNPs shared across greater numbers of individuals and with higher levels of coverage than current alternatives. The pipeline and a comprehensive online manual can be found at (http://dDocent.wordpress.com) and (https://github.com/jpuritz/dDocent).