- Open Access
GenomeQC: a quality assessment tool for genome assemblies and gene structure annotations
BMC Genomics volume 21, Article number: 193 (2020)
Genome assemblies are foundational for understanding the biology of a species. They provide a physical framework for mapping additional sequences, thereby enabling characterization of, for example, genomic diversity and differences in gene expression across individuals and tissue types. Quality metrics for genome assemblies gauge both the completeness and contiguity of an assembly and help provide confidence in downstream biological insights. To compare quality across multiple assemblies, a set of common metrics are typically calculated and then compared to one or more gold standard reference genomes. While several tools exist for calculating individual metrics, applications providing comprehensive evaluations of multiple assembly features are, perhaps surprisingly, lacking. Here, we describe a new toolkit that integrates multiple metrics to characterize both assembly and gene annotation quality in a way that enables comparison across multiple assemblies and assembly types.
Our application, named GenomeQC, is an easy-to-use and interactive web framework that integrates various quantitative measures to characterize genome assemblies and annotations. GenomeQC provides researchers with a comprehensive summary of these statistics and allows for benchmarking against gold standard reference assemblies.
The GenomeQC web application is implemented in R/Shiny version 1.5.9 and Python 3.6 and is freely available at https://genomeqc.maizegdb.org/ under the GPL license. All source code and a containerized version of the GenomeQC pipeline is available in the GitHub repository https://github.com/HuffordLab/GenomeQC.
Over the past few decades, numerous plant genome assemblies have been generated, ranging in size from 63 Mb in Genlisea aurea  to 22 Gb in Pinus taeda . The genomic resources generated from such projects have contributed to the development of improved crop varieties, enhanced our understanding of genome size, architecture, and complexity, and uncovered mechanisms underlying plant growth and development [3, 4]. With the declining cost of sequence, the number of genome assemblies has increased exponentially (Additional file 1: Figure S1). The NCBI assembly database  currently hosts more than 800 plant genome assemblies with varying degrees of contiguity and increasingly includes multiple genome assemblies per species (Additional file 1: Figure S2).
The growing number of assemblies and gene annotations has necessitated the development of metrics that can be used to compare their quality. Such metrics also allow evaluation of the performance of various assembly and annotation methods using the same data. Length metrics (N50/NG50 and L50/LG50 values) provide a standard measure of assembly contiguity . The most commonly reported N50/NG50 values are calculated for the 50% threshold, but NG(X) plots across all thresholds (1–100%) provide a more complete picture . Annotation quality metrics include number of gene models, exons per gene model, and the average lengths of genes, exons and transcripts . Such length and count metrics are useful, but they do not fully capture the completeness of assemblies.
Completeness is better gauged using a set of genes that are universally distributed as orthologs across particular clades of species . A summary of complete single-copy, duplicated, fragmented, and missing Benchmarking Universal Single-Copy Orthologs (BUSCO) genes is often provided as a quantitative measure of genome completeness based on expected gene content. While BUSCO is limited to assessment of the gene space, the LTR Assembly Index (LAI ;) is capable of gauging completeness in more repetitive genomic regions by estimating the percentage of intact LTR retroelements. LAI is particularly useful for assessing plant genome assemblies, which are often largely comprised of repeats. Recently, dramatic increases in the completeness of repetitive portions of plant genomes have been achieved due to improvements in long-read data .
Here, we describe an easy-to-use and interactive web framework based on the R/Shiny package  that integrates a suite of quantitative measures to characterize genome assemblies and annotations. Our application, named GenomeQC, provides researchers with a summary of these statistics and allows for benchmarking against gold standard reference assemblies. We have also developed a Docker container of the GenomeQC pipeline that calculates these metrics and supports analysis of large (> 2.5Gb) genomes.
Comparison with similar software programs
Although several tools exist for evaluating and visualizing the quality of genome assemblies, they are often challenging to install and configure, do not support assessment of gene structure annotations, and do not determine the completeness of the repetitive fraction of the genome based on LTR retrotransposon content. We tested the GenomeQC tool along with two other genome assembly evaluation tools, QUAST-LG  and REAPR  on three maize genome datasets (B73_v4, Mo17 and W22) as input. Table 1 shows the comparison of the output metrics generated by each tool along with their run time on the test datasets. The full details of the outputs and the datasets used for benchmarking the tools are included in the Additional file 2 (Table S1, Table S2, Table S3, Table S4, Table S5, Table S6) along with parameters and commands used in running these tools.
Workflow of the web application
The web-application of the GenomeQC tool (Fig. 1) has three sections:
Analyze Genome Assembly
This allows the user to upload a maximum of two genome assemblies for analysis. Users also have the option to benchmark the quality of the uploaded genome assembly with the gold reference genomes by selecting the names from the drop-down list. Other inputs that the user needs to provide for this section include: the label for the genome assembly plots, estimated genome size, datasets and species name for BUSCO analysis and email address to which the plots will be sent.
This calculates standard length and number metrics like N50, L50, vector contamination check and gene set completeness.
To calculate the standard length metrics N50, L50, NG(X) values for the user uploaded assembly, two custom python scripts NG.py and assembly_stats.py are employed. The gene space completeness analysis of the genome assembly is performed using the BUSCO package version 3.0.2  with genome mode. For vector contamination check, custom script contamination.py is used which implements a python wrapper for the NCBI BLAST+ program  blastn and a modified version of the taxify script from the blobtools package v1.1  to blast (with parameters: task = “megablast”, max_target_seqs = 1, max_hsps = 1, evalue = 1e-25) the input contigs/scaffold sequences in the uploaded genome assembly against the UniVec Database  and add the taxon ids to the blast hits. All the plots are generated using the R package ggplot2 and python modules pandas and plotly.
The output layer of the interface displays the NG(X) plot and the interactive assembly metrics table. The BUSCO and contamination plots are emailed to the user at the provided email address.
Compare Reference Genomes
The input widget of this section takes two parameters: name of one or more reference genome assemblies and the user’s email address.
The reference genome metrics are pre-calculated using the same custom scripts NG.py and assembly_stats.py and the BUSCO package version 3.0.2. The R package ggplot2 and a custom python script (modules pandas and plotly) are used to plot the pre-computed reference metrics.
The parameters used for the computation of metrics for the reference genomes of the different plant species are provided in the GenomeQC user guide accessible at the GitHub repository.
The output layer of the interface displays the NG(X) plot and the interactive assembly metrics table. The BUSCO assembly and annotation plots are emailed to the user at the provided email address.
Analyze Genome Annotation
This allows the user to upload a gene structure annotation set, genome assembly and transcript file (optional) for analysis. Users also have the option to benchmark the quality of the uploaded gene annotations with the gold reference genomes by selecting the names from the drop-down list. Other inputs that the user needs to provide for this analysis section include: labels for the plots and table, dataset names for BUSCO analysis and email address to which the plots will be sent.
Once the required files and parameters are provided to the tool, it computes the length and count metrics for different features of the GFF file using the custom python script gff_stats.py and assesses the completeness of the gene set based on a conserved set of orthologs using the BUSCO package version 3.0.2 with transcriptome mode.
The output layer displays the interactive annotation metrics table file. The BUSCO stack plots are emailed to the user at the provided email address.
Workflow of the docker application
There are two docker files: one for analyzing the genome assembly and a second for analyzing the genome annotation file (Fig. 2).
Analyze Genome Assembly
The docker pipeline takes as input: genome assembly in FASTA format, estimated genome size in Mb, BUSCO datasets and species name, email address and the name of the output files and directory.
The pipeline computes the various relevant assessment metrics like N50, L50, NG(X) values, BUSCO gene space completeness metrics and vector contamination check. In addition to these metrics, the docker pipeline provides the functionality to compute LTR Assembly Index (LAI) of the input genome assembly to assess the repeat space completeness of the assembled genome sequence. To calculate the standard length and count metrics N50, L50 etc. and the NG(X) values for the user input assembly, two custom python scripts NG.py and assembly_stats.py are employed. The gene space completeness analysis of the genome assembly is performed using the BUSCO package version 3.0.2 with genome mode. For vector contamination check, custom script contamination.py is used that implements the python wrapper for the NCBI BLAST+ program blastn (with parameters: task = “megablast”, max_target_seqs = 1, max_hsps = 1, evalue = 1e-25) to blast the input contigs/scaffold sequences in the uploaded genome assembly against the UniVec Database and a modified version of the taxify script (from the blobtools package v1.1) to add the taxon ids to the blast hits. To calculate the LAI score for the input genome assembly, the pipeline uses the software package LTR retriever v2.8.2 . This program is designed to identify intact LTR retrotransposons with high accuracy and sensitivity. This set of high confidence LTR retrotransposons are then used to assess the repeat space completeness of the assembly by calculating the percentage of fully-assembled LTR retrotransposons in the assembled genome sequence.
The pipeline generates the following output files and directories:
Output file (text file format) containing the NG(X) values which could be easily plotted in R or Excel to generate the NG(X) graph.
Assembly metrics output file (text file format) contains all the standard metrics like N50, L50, total number of bases, %N, etc.
Vector contamination plot in HTML format and the associated blast hits.
BUSCO output directory containing the summary text file for the number of complete, fragmented and missing BUSCO genes identified in the input genome assembly.
LAI output file (.out. LAI) containing the LAI score for the input assembly.
Analyze Genome Annotation
The docker pipeline takes as input: genome annotation file in GFF format, and transcripts file in FASTA format BUSCO datasets, and the name of the output files and directory.
The pipeline computes the various relevant assessment metrics as computed by the web-server including number and length of gene models, exons, etc. and the BUSCO gene space completeness metrics. Custom python script gff_stats.py is employed to calculate the different gene model statistics for the input annotation GFF file. The gene space completeness analysis of the input genome annotations is performed using the BUSCO package version 3.0.2 with transcriptome mode.
The pipeline generates the following output files and directories:
Annotation metrics output file (text file format) that contains the relevant statistics on the different features of the GFF file like number of gene models, exons, transcripts etc.
BUSCO output directory containing the summary text file for the number of complete, fragmented and missing BUSCO genes identified in the input genome annotation set.
Two files are required as input for GenomeQC analysis.
“Genome Assembly File” is a sequence file in the standard FASTA format. The file should be gunzipped compressed (.gz) before uploading it to the web-application. The maximum upload limit for the assembly file is 1Gb.
“Genome Structure Annotation File” is a tab separated text file in GFF/GTF format . The file should be gunzipped compressed (.gz) before uploading it to the web-application.
“Transcript FASTA file”: BUSCO analysis of structural annotations requires a transcript file in FASTA format as input. Thus, the user could either directly upload a transcript (DNA nucleotide sequences) file in compressed (.gz) FASTA format or the tool could extract the transcript sequences from the uploaded assembly and annotation files using the gffread utility v0.9.12 . Currently the tool is configured to first use the information from a transcripts file if provided by the user. If the user does not upload the transcripts file, the tool will check whether the sequence IDs in the first column of the GFF file correspond to the headers in the FASTA file. If there is a discrepancy, the tool will print an error message. Otherwise, the BUSCO job will be submitted.
The tool’s analysis interface is organized into three sections for three types of analysis.
The “Compare reference genomes” section outputs various pre-computed assembly and annotation metrics from a user-selected list of reference genomes.
The “Analyze your genome assembly” section provides the user the option to perform analysis on their genome assembly as well as benchmark the quality of their genome assembly using pre-computed metrics from gold standard reference genomes.
The “Analyze your genome annotation” section provides the user the option to perform analysis on their genome annotations as well as benchmark their analysis versus pre-computed reference genomes.
The “Assembly NG(X) Plot” tab calculates NG values for an uploaded assembly based on the input estimated genome size at different integer thresholds (1–100%) and generates a plot showing the thresholds on the x-axis and the corresponding log-scaled scaffold or contig lengths on the y-axis. Genome assemblies with larger scaffold/contig lengths across NG(X) thresholds are more contiguous.
The NG(X) values can be downloaded as a .csv file and the plot can be saved in png format by right clicking on the plot.
The “Assembly Metrics Table” and the “Annotation Metrics Table” tabs calculate various length and count metrics for the uploaded assembly and annotation files and outputs interactive tables with pop-up plots based on row selection. These tabs provide the user with quick summaries of standard assembly and annotation metrics. These tables can be downloaded as comma separated files.
The “Assembly BUSCO and Contamination Plots” tab: calculates and emails BUSCO scores for the uploaded genome assembly and compares it with the pre-computed values of the user-selected reference genomes. A high quality genome assembly is expected to contain a higher number of complete and single copy BUSCO genes (C&S) and a lower number of missing (M) or fragmented (F) BUSCO genes . These plots are emailed as png and html files. The HTML file can be opened in a chart studio and customized.
The “Annotation BUSCO plot” tab calculates and emails the BUSCO scores for the uploaded genome annotations and compares it with pre-computed values of the user-selected reference genomes. BUSCO and contamination plots are also emailed as html files. Figure 3 shows the summaries and graphical outputs generated by the GenomeQC web application.
GenomeQC provides a user-friendly web framework for calculating contiguity and completeness metrics for genome assemblies and annotations. The web application is optimized to compute metrics for small to medium-sized genomes with an upper limit of 2.5 Gb (the approximate size of the maize genome). However, the containerized version of the pipeline available through our GitHub repository can be used to calculate metrics for larger genomes. In addition to standard length metrics such as N50 and L50, GenomeQC assesses gene- and repeat-space completeness of an input genome assembly and screens for both vector and adapter contamination, a standard check implemented by NCBI before accepting new assemblies into the database. The web application of GenomeQC calculates standard metrics on the fly with just the genome assembly as input, requiring no additional computational resources or software installation. Optional annotation assessments are also performed by GenomeQC when gene predictions are provided as input (in GFF format). The report includes standard statistics of the gene model features like the number and size of gene models, exons, transcripts, etc. and performs quality assessment of the gene structure annotations using the BUSCO ortholog gene set. Finally, GenomeQC allows researchers to benchmark their metrics relative to gold standard reference genomes. These utilities should prove useful as the practice of genome assembly increasingly becomes a routine component of a biologists’ toolbelt.
Availability of source code and requirements
Project name: GenomeQC.
Project home page:
Operating system(s): platform independent
Programming language: R, R shiny, Python, Shell script
Other requirements: Docker engine
License: Any restrictions to use by non-academics: None
Availability of data and materials
Benchmark Universal Single Copy Orthologs
LTR Assembly Index
Long Terminal Repeats
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The authors would like to thank Levi Baber (Iowa State University Director of Research IT) for technical help and Jack Gardiner (Curator at MaizeGDB) for testing the web application and providing helpful suggestions.
This work was supported by United States Department of Agriculture-Agricultural Research Service (Project Number 5030–21000–068-00-D) to CMA, Specific Coorperative Agreement 58–5030–8-064 to MBH and CJLD, and Iowa State University Plant Sciences Institute Faculty Scholar support to CJLD. The content is solely the responsibility of the authors and does not necessarily represent the official views of the funding agency. Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. USDA is an equal opportunity provider and employer.
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Exponential growth in the number of plant genome assemblies deposited in the NCBI Assembly database from November 2004 through December 2018. Figure S2. Number of plant genome assemblies in the NCBI Assembly Database at each level of assembly contiguity.
Information on the input data, parameters and output from the three tools: QUAST-LG, REAPR and GenomeQC. Table S1. QUAST-LG output with just the genome fasta file as input. Table S2. QUAST-LG output with reference genome as input. Table S3. QUAST-LG output with reads as input. Table S4. REAPR output. Table S5. GenomeQC assembly output. Table S6. GenomeQC annotation output.
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Manchanda, N., Portwood, J.L., Woodhouse, M.R. et al. GenomeQC: a quality assessment tool for genome assemblies and gene structure annotations. BMC Genomics 21, 193 (2020). https://doi.org/10.1186/s12864-020-6568-2
- Genome assembly
- Gene annotations
- Web interface
- Docker containers