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Petabase-scale sequence alignment catalyses viral discovery

Serratus alignment architecture

Serratus (v0.3.0) ( is an open-source cloud-infrastructure designed for ultra-high-throughput sequence alignment against a query sequence or pangenome (Extended Data Fig. 1). Serratus compute costs are dependent on search parameters (expanded discussion available: The nucleotide vertebrate viral pangenome search (bowtie2, database size: 79.8 MB) reached processing rates of 1.29 million SRA runs in 24 h at a cost of US$0.0062 per dataset (Extended Data Fig. 1). The translated-nucleotide RdRP search (DIAMOND14; database size: 7.1 MB) reached processing rates exceeding 0.5 million SRA runs in 12 h at a cost of US$0.0042 per dataset. All 5,686,715 runs analysed in the RdRP search were completed within 11 days for a total cost of US$23,980 or around US$2,350 per petabase. For a detailed breakdown of Serratus project costs and recommendations for managing cloud-computing costs, see Serratus wiki: Tutorials on how to find particular novel viruses using Serratus data are available at /wiki/Find_novel_viruses.

Computing cluster architecture

The processing of each sequencing library is split into three modules: ‘dl’ (download), ‘align’ and ‘merge’. The dl module acquires compressed data (.sra format) via prefetch (v2.10.4), from the Amazon Web Services (AWS) Simple Storage Service (S3) mirror of the SRA, decompresses to FASTQ with fastq-dump (v2.10.4) and splits the data into chunks of 1 million reads or read-pairs (‘fq-blocks’) into a temporary S3 cache bucket. To mitigate excessive disk usage caused by a few large datasets, a total limit of 100 million reads per dataset was imposed. The align module reads individual fq-blocks and aligns to an indexed database of user-provided query sequences using either bowtie2 (v2.4.1, –very-sensitive-local)51 for nucleotide search, or DIAMOND (v2.0.6 development version, –mmap-target-index –target-indexed –masking 0 –mid-sensitive -s 1 -c1 -p1 -k1 -b 0.75)14 for translated-protein search. Finally, the merge module concatenates the aligned blocks into a single output file (.bam for nucleotide, or .pro for protein) and generates alignment statistics with a Python script (see details about Summarizer in ‘Generating viral summary reports’ below).

Computing resource allocation

Each component is launched from a separate AWS autoscaling group with its own launch template, allowing the user to tailor instance requirements per task. This enabled us to minimize the use of costly block storage during compute-bound tasks such as alignment. We used the following Spot instance types; dl: 250 GB SSD block storage, 8 virtual CPUs (vCPUs), 32 GB RAM (r5.xlarge) around 1,300 instances; align: 10 GB SSD block storage, 8 vCPUs, 8 GB RAM (c5.xlarge) around 4,300 instances; merge: 150 GB SSD block storage, 4 vCPUs, 4 GB RAM (c5.large) around 60 instances. Users should note that it may be necessary to submit a service ticket to access more than the default EC2 instance limit.

AWS Elastic Compute Cloud (EC2) instances have higher network bandwidth (up to 1.25 GB s−1) than block storage bandwidth (250 MB s−1). To exploit this, we used S3 buckets as a data buffering and streaming system to transfer data between instances following methods developed in a previous cloud architecture ( This, combined with splitting of FASTQ files into individual blocks, effectively eliminated file input/output (i/o) as a bottleneck, as the available i/o is multiplied per running instance (conceptually analogous to a RAID0 configuration or a Hadoop distributed file system52).

Using S3 as a buffer also allowed us to decouple the input and output of each module. S3 storage is cheap enough that in the event of unexpected issues (for example, exceeding EC2 quotas) we could resolve system problems in real time and resume data processing. For example, shutting down the align modules to hotfix a genome indexing problem without having to re-run the dl modules, or if an alignment instance is killed by a Spot termination, only that block needs to be reprocessed instead of the entire sequencing run.

Work queue and scheduling

The Serratus scheduler node controls the number of desired instances to be created for each component of the workflow, based on the available work queue. We implemented a pull-based work queue. After boot-up, each instance launches a number of ‘worker’ threads equal to the number of CPUs available. Each worker independently manages itself via a boot script, and queries the ‘scheduler’ for available tasks. Upon completion of the task, the worker updates the scheduler of the result: success, or fail, and queries for a new task. Under ideal conditions, this allows for a worst-case response rate in the hundreds of milliseconds, keeping cluster throughput high. Each task typically lasts several minutes depending on the pangenome.

The scheduler itself was implemented using Postgres (for persistence and concurrency) and Flask (to pool connections and translate REST queries into SQL). The Flask layer allowed us to scale the cluster past the number of simultaneous sessions manageable by a single Postgres instance. The work queue can also be managed manually by the user, to perform operations such as re-attempting the downloading of an SRA accession after a failure or to pause an operation while debugging. Up to 300,000 SRA jobs can be processed in the work queue per batch process.

The system is designed to be fully self-scaling. An ‘autoscaling controller’ was implemented, which scales-in or scales-out the desired number of instances per task every five minutes on the basis of the work queue. As a backstop, when all workers on an instance fail to receive work instructions from the scheduler, the instance self shuts-down. Finally a ‘job cleaner’ component checks the active jobs against currently running instances. If an instance has disappeared owing to SPOT termination or manual shutdown, it resets the job allowing it to be processed up by the next available instance.

To monitor cluster performance in real-time, we used Prometheus (v2.5.0) and node exporter to retrieve CPU, disk, memory and networking statistics from each instance, to expose performance information about the work queue, and Python exporter to export information from the Flask server. This allowed us to identify and diagnose performance problems within minutes to avoid costly overruns.

Generating viral summary reports

We define a viral pangenome as the entire collection of reference sequences belonging to a taxonomic viral family, which may contain both full-length genomes and sequence fragments such as those obtained by RdRP amplicon sequencing.

We developed a Summarizer module written in Python to provide a compact, human- and machine-readable synopsis of the alignments generated for each SRA dataset. The method was implemented in for nucleotide alignment and for amino acid alignments. Reports generated by the Summarizer are text files with three sections described in detail online ( In brief, each contains a header section with alignment metadata and one-line summaries for each virus family pangenome, reference sequence and gene, respectively, with gene summaries provided for protein alignments only.

For each summary line we include descriptive statistics gathered from the alignment data such as the number of aligned reads, estimated read depth, mean alignment identity and coverage; that is, the distribution of reads across each reference sequence or pangenome. Coverage is measured by dividing a reference sequence into 25 equal bins and depicted as an ASCII text string of 25 symbols, one per bin; for example oaooomoUU:oWWUUWOWamWAAUW. Each symbol represents log2(n + 1), where n is the number of reads aligned to a bin in this order _.:uwaomUWAOM^. Thus, ‘_’ indicates no reads, ‘.’ exactly one read, ‘:’ two reads, ‘u’ 3–4 reads, ‘w’ 5–7 reads and so on; ‘^’ represents >213 = 8,192 reads in the bin. For a pangenome, alignments to its reference sequences are projected onto a corresponding set of 25 bins. For a complete genome, the projected pangenome bin number 1, 2, …, 25 is the same as the reference sequence bin number. For a fragment, a bin is projected onto the pangenome bin implied by the alignment of the fragment to a complete genome. For example, if the start of a fragment aligns halfway into a complete genome, bin 1 of the fragment is projected to bin floor(25/2) = 12 of the pangenome. The introduction of pangenome bins was motivated by the observation that bowtie2 selects an alignment at random when there are two or more top-scoring alignments, which tends to distribute coverage over several reference sequences when a single viral genome is present in the reads. Coverage of a single reference genome may therefore be fragmented, and binning to a pangenome better assesses coverage over a putative viral genome in the reads while retaining pangenome sequence diversity for detection.

Identification of viral families within a sequencing dataset

The Summarizer implements a binary classifier predicting the presence or absence of each virus family in the query on the basis of pangenome-aligned short reads. For a given family F, the classifier reports a score in the range [0,100] with the goal of assigning a high score to a dataset if it contains F and a low score if it does not. Setting a threshold on the score divides datasets into disjoint subsets representing predicted positive and negative detections of family F. The choice of threshold implies a trade-off between false positives and false negatives. Sorting by decreasing score ranks datasets in decreasing order of confidence that F is present in the reads.

Naively, a natural measure of the presence of a virus family is the number of alignments to its reference sequences. However, alignments may be induced by non-homologous sequence similarity, for example low-complexity sequence.

The score for a family was therefore designed to reflect the overall coverage of a pangenome because coverage across all or most of a pangenome is more likely to reflect true homology; that is, the presence of a related virus. Ideally, coverage would be measured individually for each base in the reference sequence, but this could add undesirable overhead in compute time and memory for a process that is executed in the Linux alignment pipe (FASTQ decompression → aligner → Summarizer → alignment file compression). Coverage was therefore measured by binning as described above, which can be implemented with minimal overhead.

A virus that is present in the reads with coverage too low to enable an assembly may have less practical value than an assembled genome. Also, genomes with lower identity to previously known sequences will tend to contain more novel biological information than genomes with high identity but highly diverged genomes will tend to have fewer aligned reads. With these considerations in mind, the classifier was designed to give higher scores when coverage is high, read depth is high and/or identity is low. This was accomplished as follows. Let H be the number of bins with at least 8 alignments to F, and L be the number of bins with from 1 to 7 alignments. Let S be the mean alignment percentage identity, and define the identity weight w = (S/100)−3, which is designed to give higher weight to lower identities, noting that w is close to 1 when identity is close to 100% and increases rapidly at lower identities. The classification score for family F is calculated as ZF = max(w(4H + L)),100). By construction, ZF has a maximum of 100 when coverage is consistently high across a pangenome, and is also high when identity is low and coverage is moderate, which may reflect high read depth but many false negative alignments due to low identity. Thus, ZF is greater than zero when there is at least one alignment to F and assigns higher scores to SRA datasets that are more likely to support successful assembly of a virus belonging to F.

Sensitivity to novel viruses as a function of identity

We aimed to assess the sensitivity of our pipeline as a function of sequence identity by asking what fraction of novel viruses is detected at increasingly low identities compared to the reference sequences used for the search. Several variables other than identity affect sensitivity, including read length, whether reads are mate-paired, sequencing error rate, coverage bias and the presence of other similar viruses that may cause some variants to be unreported in the contigs. Coverage bias can render a virus with high average read depth undetectable, in particular if the query is RdRP-only and the RdRP gene has low coverage or is absent from the reads. Successful detection might be defined in different ways, depending on the goals of the search; for example, a single local alignment of a reference to a read (maximizing sensitivity, but not always useful in practice); a microassembled palmprint; a full assembly contig that contains a complete palmprint or otherwise classifiable fragment of a marker gene; or an assembly of a complete genome. We assessed alignment sensitivity of bowtie2 –very-sensitive-local and Serratus-optimized DIAMOND14 as a function of identity by simulating typical examples in a representative scenario: unpaired reads of length 100 with a base call error rate of 1%. We manually selected test-reference pairs of RefSeq complete Ribovirus genomes at RdRP amino acid identities 100%, 95% … 20%, generating simulated length-100 reads at uniformly distributed random locations in the test genome with a mean coverage of 1,000×. For bowtie2, the complete reference genome was used as a reference; for DIAMOND the reference was the translated amino acid sequence of the RdRP gene (400 amino acids), which was identified by aligning to the ‘wolf18’ dataset. These choices model the coronavirus pangenome used as a bowtie2 query and the rdrp1 protein reference used as a DIAMOND query, respectively. Sensitivity was assessed as the fraction of reads aligned to the reference. With bowtie2, the number of unmapped reads reflects a combination of lack of alignment sensitivity and divergence in gene content as some regions of the genome may lack homology to the reference. With DIAMOND, the number of unmapped reads reflects a combination of lack of alignment sensitivity and the fraction of the genome that is not RdRP, which varies by genome length 1g. They show that the fraction of aligned reads by bowtie2 drops to around 2% to 4% at 90% RdRP amino acid identity, and maps no reads for most of the lower identity test–reference pairs. DIAMOND maps around 5% to 10% of reads down to 50% RdRP amino acid identity, then less than 1% at lower identities; around 30% to 35% is the lower limit of practical detection.

Defining viral pangenomes and the SRA search space

Nucleotide search pangenomes

To create a collection of viral pangenomes, a comprehensive set of complete and partial genomes representing the genetic diversity of each viral family, we used two approaches.

For Coronaviridae, we combined all RefSeq (n = 64) and GenBank (n = 37,451) records matching the NCBI Nucleotide53 server query “txid11118[Organism:exp]” (date accessed: 1 June 2020). Sequences of fewer than 200 nt were excluded as well as sequences identified to contain non-CoV contaminants during preliminary testing (such as plasmid DNA or ribosomal RNA fragments). Remaining sequences were clustered at 99% identity with UCLUST (USEARCH: v11.0.667)54 and masked by Dustmasker (ncbi-blast:2.10.0) (–window 30 and –window 64)55. The final query contained 10,101 CoV sequences (accessions in Supplementary Table 1a; masked coordinates in Supplementary Table 1b). SeqKit (v0.15) was used for working with fasta files56.

For all other vertebrate viral family pangenomes, RefSeq sequences (n = 2,849) were downloaded from the NCBI Nucleotide server with the query “Viruses[Organism] AND srcdb refseq[PROP] NOT wgs[PROP] NOT cellular organisms[ORGN] NOT AC 000001:AC 999999[PACC] AND (“vhost human”[Filter] AND “vhost vertebrates”[Filter])” (date accessed: 17 May 2020). Retroviruses (n = 80) were excluded as preliminary testing yielded excessive numbers of alignments to transcribed endogenous retroviruses. Each sequence was annotated with its taxonomic family according to its RefSeq record; those for which no family was assigned by RefSeq (n = 81) were designated as ‘unknown’.

The collection of these pangenomes was termed ‘cov3m’, and was the nucleotide sequence reference used for this study.

Amino acid viral RdRP search panproteome

For the translated-nucleotide search of viral RNA-dependent RNA polymerase (RdRP; hereinafter viral RdRP is implied) we combined sequences from several sources. (1) The ‘wolf18’ collection is a curated snapshot (around 2018) of RdRP from GenBank (ref. 19 accessed: (2) The ‘wolf20’ collection is RdRPs from assembled from marine metagenomes (ref. 7 accessed: (3) All viral GenBank protein sequences were aligned with DIAMOND –ultra-sensitive14 against the combined wolf18 and wolf20 sequences (E-value < 1 × 10−6). These produced local alignments that contained truncated RdRP, so each RdRP-containing GenBank sequence was then re-aligned to the wolf18 and wolf20 collection to ‘trim’ them to ‘wolf’ RdRP boundaries. (4) The above algorithm was also applied to all viral GenBank nucleotide records to capture additional RdRP not annotated as such by GenBank. A region of HCV capsid protein shares similarity to HCV RdRP; sequences annotated as HCV capsid were therefore removed. Eight novel coronavirus RdRP sequences identified in a pilot experiment were added manually. The combined RdRP sequences from the above collections were clustered (UCLUST) at 90% amino acid identity and the resulting representative sequences (centroids, n = 14,653) used as the rdrp1 search query.

In addition, we added delta virus antigen proteins from NC 001653, M21012, X60193, L22063, AF018077, AJ584848, AJ584847, AJ584844, AJ584849, MT649207, MT649208, MT649206, NC 040845, NC 040729, MN031240, MN031239, MK962760, MK962759 and eight additional homologues we identified in a pilot experiment.

SRA search space and queries

To run Serratus, a target list of SRA run accessions is required. We defined 11 (not-mutually exclusive) queries as our search space, which were named human, mouse, mammal, vertebrate, invertebrate, eukaryotes, prokaryotes/others, bat (including genomic sequences), virome, metagenome and mammalian genome (Supplementary Table 1c). Our search was restricted to Illumina sequencing technologies and to RNA-seq, meta-genomic and meta-transcriptome library types for these organisms (except for the mammalian genome query, which was genome or exome). Before each Serratus deployment, target lists were depleted of accessions already analysed. Reprocessing of a failed accession was attempted at least twice. In total, we aligned 3,837,755/4,059,695 (94.5%) of the runs in our nucleotide-pangenome search (around May 2020) and 5,686,715/5,780,800 (98.37%) of the runs in our translated-nucleotide RdRP search (around January 2021).

User interfaces for the Serratus databases

We implemented an on-going, multi-tiered release policy for code and data generated by this study, as follows. All code, electronic notebooks and raw data are immediately available at and on the s3://serratus-public bucket, respectively. Upon completion of a project milestone, a structured data release is issued containing raw data into our viral data warehouse s3://lovelywater/. For example, the .bam nucleotide alignment files from 3.84 million SRA runs are stored in s3://lovelywater/bam/X.bam; and the protein .summary files are in s3://lovelywater/psummary/X.psummary, where X is a SRA run accession. These structured releases enable downstream and third-party programmatic access to the data.

Summary files for every searched SRA dataset are parsed into a publicly accessible AWS Relational Database (RDS) instance that can be queried remotely via any PostgreSQL client. This enables users and programs to perform complex operations such as retrieving summaries and metadata for all SRA runs matching a given reference sequence with above a given classifier score threshold. For example, one can query for all records containing at least 20 aligned reads to hepatitis delta virus (NC 001653.2) and the associated host taxonomy for the corresponding SRA datasets:

SELECT sequence_accession, run_id, tax_id, n_reads
FROM nsequence JOIN
srarun ON (nsequence.run_id = WHERE n_reads >= 20

For users unfamiliar with SQL, we developed Tantalus (, an R programming-language package that directly interfaces the Serratus PostgreSQL database to retrieve summary information as data-frames. Tantalus also offers functions to explore and visualize the data.

Finally, the Serratus data can be explored via a graphical web interface by accession, virus or viral family at Under the hood, we developed a REST API to query the database from the website. The website uses React+D3.js to serve graphical reports with an overview of viral families found in each SRA accession matching a user query.

All four data access interfaces are under ongoing development, receiving community feedback via their respective GitHub issue trackers to facilitate the translation of this data collection into an effective viral discovery resource. Documentation for data access methods is available at

Geocoding BioSamples

To generate the map in Fig. 1c, we parsed and extracted geographical information from all 16 million BioSample XML submissions57. Geographic information is either in the form of coordinates (latitude and longitude) or freeform text (for example, ‘France’, ‘Great Lakes’). For each BioSample, coordinate extraction was attempted using regular expressions. If that failed, text extraction was attempted using a manually curated list of keywords that capture BioSample attribute names that are likely to contain geographical information. If that failed, then we were unable to extract geographical information for that BioSample. Geocoding the text to coordinates was done using Amazon Location Service on a reduced set of distinct filtered text values (52,028 distinct values from 2,760,241 BioSamples with potential geographical text). BioSamples with geocoded coordinates were combined with BioSamples with submitted coordinate information to form a set of 5,325,523 geospatial BioSamples. This is then cross-referenced with our subset of SRA accessions with an RdRP match to generate the figure.

All intermediate and resulting data from this step are stored on the SQL database described above. Development work is public at

Viral alignment, assembly and annotation

Upon identification of CoV reads in a run from alignment, we assembled 52,772 runs containing at least 10 reads that aligned to our CoV pangenome or at least 2 reads with CoV-positive k-mers16. A total of 11,120 of the resulting assemblies contained identifiable CoV contigs, of which only 4,179 (37.58%) contained full-length CoV RdRP (Supplementary Table 1d). The discrepancy between alignment-positive, assembly-positive and RdRP-positive libraries arises owing to random sampling of viral reads and assembly fragmentation. In this respect, alignment or k-mer based methods are more sensitive than assembly in detecting for the presence of low-abundance viruses (genome coverage < 1) with high identity to a reference sequence. Scoring libraries for genome coverage and depth is a good predictor of ultimate assembly success (Extended Data Fig. 3); thus, it can be used to efficiently prioritize computationally expensive assembly in the future, as has been previously demonstrated for large-scale SRA alignment analyses58.

DIAMOND optimization and output

To optimize DIAMOND14 for small (<10 MB) databases such as the RdRP search database, we built a probabilistic hash set that stores 8-bit hash values for the database seeds, using SIMD instructions for fast probing. This index is loaded as a memory mapped file to be shared among processes and allows us to filter the query reads for seeds contained in the database, thus omitting the full construction of the query seed table. We also eliminated the overhead of building seed distribution histograms that is normally required to allocate memory and construct the query table in a single pass over the data using a deque-like data structure. In addition, query reads were not masked for simple repeats, as the search database is already masked. These features are available starting from DIAMOND v2.0.8 with the command line flags –target-indexed –masking 0. In a benchmark of 4 sets of 1 million reads from a bat metagenome (ERR2756788), the implemented optimization produced a speed-up of ×1.47 and reduced memory use by 64%, compared to the public unmodified DIAMOND v2.0.6, using our optimized set of parameters in both cases (see 1.1.1). Together, the optimized parameters and implementation reduced DIAMOND runtime against RdRP search from 197.96 s (s.d. = 0.18 s), to 21.29 s (s.d.  = 0.23 s) per million reads, a speed-up of a factor of 9.3. This effectively reduced the computational cost of translated-nucleotide search for Serratus from US$0.03 to US$0.0042 per library.

DIAMOND output files (we label .pro) were specified with the command -f 6 qseqid qstart qend qlen qstrand sseqid sstart send slen pident evalue cigar qseq_translated full_qseq full_qseq_mate.


RNA viral genome assembly faces several distinct challenges stemming from technical and biological bias in sequencing data. During library preparation, reverse transcription introduces 50 end coverage bias, and GC-content skew and secondary structures lead to unequal PCR amplification59. Technical bias is confounded by biological complexity such as intra-sample sequence variation due to transcript isoforms and/or to the presence of multiple strains.

To address the assembly challenges specific to RNA viruses, we developed coronaSPAdes (v3.15.3), which is described in detail in a companion manuscript25. In brief, rnaviralSPAdes and the more specialized variant, coronaSPAdes, combines algorithms and methods from several previous approaches based on metaSPAdes60, rnaSPAdes61 and metaviralSPAdes62 with a HMMPathExtension step. coronaSPAdes constructs an assembly graph from an RNA-seq dataset (transcriptome, meta-transcriptome, and meta-virome are supported), removing expected sequencing artifacts such as low complexity (poly-A/poly-T) tips, edges, single-strand chimeric loops or double-strand hairpins61 and subspecies-bases variation62.

To deal with possible misassemblies and high-covered sequencing artefacts, a secondary HMMPathExtension step is performed to leverage orthogonal information about the expected viral genome. Protein domains are identified on all assembly graphs using a set of viral hidden Markov models (HMMs), and similar to biosyntheticSPAdes63, HMMPathExtension attempts to find paths on the assembly graph that pass through significant HMM matches in order.

coronaSPAdes is bundled with the Pfam SARS-CoV-2 set of HMMs64, although these may be substituted by the user. This latter feature of coronaSPAdes was used for HDV assembly, in which the HMM model of HDAg, the hepatitis delta antigen, was used instead of the Pfam SARS-CoV-2 set. Note that despite the name, the HMMs from this set are quite general, modelling domains found in all coronavirus genera in addition to RdRP, which is found in many RNA virus families. Hits from these HMMs cover most bases in most known coronavirus genomes, enabling the recovery of strain mixtures and splice variants.

Microassembly of RdRP-aligned reads

Reads aligned by DIAMOND14 in the translated-nucleotide RdRP search are stored in the .pro alignment file. All sets of mapped reads (3,379,127 runs) were extracted, and each non-empty set was assembled with rnaviralSPAdes (v3.15.3)25 using default parameters. This process is referred to as ‘microassembly’, as a collection of DIAMOND hits is orders of magnitude smaller than the original SRA accession (40 ± 534 KB compressed size, ranging from a single read up to 53 MB). Then bowtie251 (default parameters) was used to align the DIAMOND read hits of an accession back to the microassembled contigs of that accession. Palmscan (v1.0.0, -rdrp -hicon)18 was run on microassembled contigs, resulting in high-confidence palmprints for 337,344 contigs. Finally mosdepth (v0.3.1)65 was used to calculate a coverage pileup for each palmprint hit region within microassembled contigs.

Classification of assembled RdRP sequences

Our methods for RdRP classification are described and validated in a companion paper18. In brief, we defined a barcode sequence, the polymerase palmprint (PP),as an approximately 100-amino-acid segment of the RdRP palm subdomain delineated by well-conserved catalytic motifs. We implemented an algorithm, Palmscan, to identify palmprint sequences and discriminate RdRPs from reverse transcriptases. The combined set of RdRP palmprints from public databases and our assemblies was classified by clustering into operational taxonomic units (OTUs) at 90%, 75% and 40% identity, giving species-like, genus-like and family-like clusters (sOTUs, gOTUs and fOTUs), respectively. Tentative taxonomy of novel OTUs was assigned by aligning to palmprints of named viruses and taking a consensus of the top hits above the identity threshold for each rank.

Quality control of assembled RdRP sequences

Our goal was to identity novel viral RdRP sequences and novel sOTUs in SRA libraries. From this perspective, we considered the following to be erroneous to varying degrees: sequences that are (a) not polymerases; (b) not viral; (c) with differences due to experimental artefacts; or (d) with sufficient differences to cause a spurious inference of a novel sOTU. We categorized potential sources of such errors and implemented quality control procedures to identify and mitigate them, as follows.

Point errors are single-letter substitution and indel errors that may be caused by PCR or sequencing per se. Random point errors are not reproduced in multiple non-PCR duplicate reads and are unlikely to assemble because such errors almost always induce identifiable structures in the assembly graph (tips and bubbles) that are pruned during graph simplification. In rare cases, a contig may contain a read with random point errors. Such contigs will have low coverage of around 1, and we therefore recorded coverage as a quality control metric and assessed whether low-coverage assemblies were anomalous compared to high-coverage assemblies by measures such as the frequencies with which they are reproduced in multiple libraries compared to exactly one library, finding no noticeable difference when coverage is low.

Chimeras of polymerases from different species could arise from PCR amplification or assembly. We used the UCHIME2 (usearch v8.0.1623) algorithm66 to screen assembled palmprint sequences, finding no high-scoring putative chimeras. Mosaic sequences formed by joining a polymerase to unrelated sequence would either have an intact palmprint, in which case the mosaic would be irrelevant to our analysis, or would be rejected by Palmscan owing to the lack of delimiting motifs.

Reverse transcriptases are homologous to RdRP. Retroviral insertions into host genomes induce ubiquitous sequence similarity between host genomes and viral RdRP. Palmscan was designed to discriminate RdRP from sequences of reverse transcriptase origin. Testing on a large decoy set of non-RdRP sequences with recognizable sequence similarity showed that the Palmscan false discovery rate for RdRP identification is 0.001. We estimated the probability of false positive matches in unrelated sequence by generating sufficient random nucleotide and amino acid sequences to show that the expected number of false positive palmprint identifications is zero in a dataset of comparable size to our assemblies. We also regard the low observed frequency of palmprints in DNA whole-genome sequencing data (in 2.6 Pbp or 25.8% of reads, accounted for 100 known palmprints and 95 novel palmprints or 0.13% of the total identified) as a de facto confirmation of the low probability false positives in unrelated sequence.

Endogenous viral elements (EVEs; that is, insertions of viral sequence into host genomes that are potentially degraded and non-functional) cannot be distinguished from viral genomes on the basis of the palmprint sequence alone. To assess the frequency of EVEs in our data, we re-assembled 890 randomly chosen libraries yielding one or more palmprints using all reads, extracted the 23,530 resulting contigs with a positive palmprint hit by Palmscan, and classified them using Virsorter2 (v2.1)67. Of these contigs, 11,914 were classified as viral, confirming the Palmscan identification; 49 as Viridiplantae (green plants); 46 as Metazoa; 25 as Fungi and the remainder were unclassified. Thus, 120/12,034 = 1% of the classified contigs were predicted as non-viral, suggesting that the frequency of EVEs in the reported palmprints is around 1%.

Annotation of CoV assemblies

Accurate annotation of CoV genomes is challenging owing to ribosomal frameshifts and polyproteins that are cleaved into maturation proteins68, and thus previously annotated viral genomes offer a guide to accurate gene-calls and protein functional predictions. However, although many of the viral genomes we were likely to recover would be similar to previously annotated genomes in Refseq or GenBank, we anticipated that many of the genomes would be taxonomically distant from any available reference. To address these constraints, we developed an annotation pipeline called DARTH (version maul)69 which leverages both reference-based and ab initio annotation approaches.

In brief, DARTH consists of the following phases: standardize the ordering and orientation of assembly contigs using conserved domain alignments, perform reference-based annotation of the contigs, annotate RNA secondary structure, ab initio gene-calling, generate files for aiding assembly and annotation diagnostics, and generate a master annotation file. It is important to put the contigs in the ‘expected’ orientation and ordering to facilitate comparative analysis of synteny and as a requirement for genome deposition. To perform this standardization, DARTH generates the six-frame translation of the contigs using the transeq (EMBOSS: and uses HMMER3 (v3.3.2)71 to search the translations for Pfam domain models specific to CoV64. DARTH compares the Pfam accessions from the HMMER alignment to the NCBI SARS-CoV-2 reference genome (NCBI Nucleotide accession NC_045512.2) to determine the correct ordering and orientation, and produces an updated assembly FASTA file. DARTH performs reference-based annotation using VADR (v1.1)72, which provides a set of genome models for all CoV RefSeq genomes73. VADR provides annotations of gene coordinates, polyprotein cleavage sites, and functional annotation of all proteins. DARTH supplements the VADR annotation by using Infernal74 to scan the contigs against the SARS-CoV-2 Rfam release75 which provides updated models of CoV 50 and 30 untranslated regions (UTRs) along with stem-loop structures associated with programmed ribosomal frame-shifts. Although VADR provides reference-based gene-calling, DARTH also provides ab initio gene-calling by using FragGeneScan (v1.31)76, a frameshift-aware gene caller. DARTH also generates auxiliary files that are useful for assembly quality and annotation diagnostics, such as indexed BAM files created with SAMtools (v1.7)77 representing self-alignment of the trimmed reads to the canonicalized assembly using bowtie251, and variant-calls using bcftools from SAMtools. DARTH generates these files so that the can be easily loaded into a genome browser such as JBrowse78 or IGV79. As the final step DARTH generates a single Generic Feature Format (GFF) 3.0 file80 containing combined set of annotation information described above, ready for use in a genome browser, or for submitting the annotation and sequence to a genome repository.

Phage assembly

Each metagenomic dataset was individually de-novo-assembled using MEGAHIT (v1.2.9)81, and filtered to remove contigs smaller than 1 kb in size. ORFs were then predicted on all contigs using Prodigal (v2.6.3)82 with the following parameters: -m -p meta. Predicted ORFs were initially annotated using USEARCH54 to search all predicted ORFs against UniProt83, UniRef90 and KEGG84. Sequencing coverage of each contig was calculated by mapping raw reads back to assemblies using bowtie251. Terminase sequences from Al-Shayeb et al.42 were clustered at 90% amino acid identity to reduce redundancy using CD-HIT (v4.8.1)85, and HMM models were built with hmmbuild (from the HMMER3 suite71) from the resulting set. Terminases in the assemblies from Serratus were identified using hmmsearch, retaining representatives from contigs greater than 140 kb in size. Some examples of prophage and large phages that did not co-cluster with the sequences from Al-Shayeb et al. were also recovered because they were also present in a sample that contained the expected large phages. The terminases were aligned using MAFFT (v7.407)86 and filtered by TrimAL (v1.14)87 to remove columns comprising more than 50% gaps, or 90% gaps, or using the automatic gappyout setting to retain the most conserved residues. Maximum likelihood trees were built from the resulting alignments using IQTREE (v1.6.6)88.

Deploying the assembly and annotation workflow

The Serratus search for known or closely related viruses identified 37,131 libraries (14,304 by nucleotide and 23,898 by amino acid) as potentially positive for CoV (score ≥ 20 and ≥10 reads). To supplement this search we also used a recently developed index of the SRA called STAT16, which identified an additional 18,584 SRA datasets not in the defined SRA search space. The STAT BigQuery (accessed 24 June 2020) was: WHERE tax id=11118 AND total count >1.

We used AWS Batch to launch thousands of assemblies of NCBI accessions simultaneously. The workflow consists of four standard parts: a job queue, a job definition, a compute environment, and finally, the jobs themselves. A CloudFormation template ( was created for building all parts of the cloud infrastructure from the command line. The job definition specifies a Docker image, and asks for 8 virtual CPUs (vCPUs, corresponding to threads) and 60 GB of memory per job, corresponding to a reasonable allocation for coronaSPAdes. The compute environment is the most involved component. We set it to run jobs on cost-effective Spot instances (optimal setting) with an additional cost-optimization strategy (SPOT_CAPACITY_OPTIMIZED setting), and allowing up to 40,000 vCPUs total. In addition, the compute environment specifies a launch template which, on each instance, (i) automatically mounts an exclusive 1 TB EBS volume, allowing sufficient disk space for several concurrent assemblies, and (ii) downloads the 5.4 GB CheckV (v0.6.0)89 database, to avoid bloating the Docker image.

The peak AWS usage of our Batch infrastructure was around 28,000 vCPUs, performing around 3,500 assemblies simultaneously. A total of 46,861 accessions out of 55,715 were assembled in a single day. They were then analysed by two methods to detect putative CoV contigs. The first method is CheckV89, followed selecting contigs associated to known CoV genomes. The second method is a custom script ( that parses coronaSPAdes BGC candidates and keeps contigs containing CoV domain(s). For each accession, we kept the set of contigs obtained by the first method (CheckV) if it is non-empty, and otherwise we kept the set of contigs from the second method (BGC).

A majority (76%) of the assemblies were discarded for one of the following reasons: (i) no CoV contigs were found by either filtering method; (ii) reads were too short to be assembled; (iii) Batch job or SRA download failed; or (iv) coronaSPAdes ran out of memory. A total of 11,120 assemblies were considered for further analysis.

The average cost of assembly was between US$0.30 and US$0.40 per library, varying depending on library type (RNA-seq versus metagenomic). This places an estimate of 46–95-fold higher cost for assembly alone compared to a cost of US$0.0042 or US$0.0065 for an alignment-based search.

Taxonomic and phylogenetic analyses

Taxonomy prediction for coronavirus genomes

We developed a module, SerraTax, to predict taxonomy for CoV genomes and assemblies ( SerraTax was designed with the following requirements in mind: provide taxonomy predictions for fragmented and partial assemblies in addition to complete genomes; report best-estimate predictions balancing over-classification and under-classification (too many and too few ranks, respectively); and assign an NCBI Taxonomy Database90 identifier (TaxID).

Assigning a best-fit TaxID was not supported by any previously published taxonomy prediction software to the best of our knowledge; this requires assignment to intermediate ranks such as sub-genus and ranks below species (commonly called strains, but these ranks are not named in the Taxonomy database), and to unclassified taxa, for example, TaxID 2724161, unclassified Buldecovirus, in cases in which the genome is predicted to fall inside a named clade but outside all named taxa within that clade.

SerraTax uses a reference database containing domain sequences with TaxIDs. This database was constructed as follows. Records annotated as CoV were downloaded from UniProt83, and chain sequences were extracted. Each chain name, for example Helicase, was considered to be a separate domain. Chains were aligned to all complete coronavirus genomes in GenBank using UBLAST (usearch: v11.0.667)54 to expand the repertoire of domain sequences. The reference sequences were clustered using UCLUST54 at 97% sequence identity to reduce redundancy.

For a given query genome, ORFs are extracted using the getorf (EMBOSS:6.6.0) software70. ORFs are aligned to the domain references and the top 16 reference sequences for each domain are combined with the best-matching query ORF. For each domain, a multiple alignment of the top 16 matches plus query ORF is constructed on the fly by MUSCLE (v3.8.3191) and a neighbour-joining tree is inferred from the alignment, also using MUSCLE. Finally, a consensus prediction is derived from the placement of the ORF in the domain trees. Thus, the presence of a single domain in the assembly suffices to enable a prediction; if more domains are present they are combined into a consensus.

Taxonomic assignment by phylogenetic placement

To generate an alternate taxonomic annotation of an assembled genome, we created a pipeline based on phylogenetic placement, SerraPlace.

To perform phylogenetic placement, a reference phylogenetic tree is required. To this end, we collected 823 reference amino acid RdRP sequences, spanning all Coronaviridae. To this set we added an outgroup RdRP sequence from the Torovirus family (NC 007447). We clustered the sequences to 99% identity using USEARCH (ref. 54, UCLUST algorithm, v11.0.667), resulting in 546 centroid sequences. Subsequently, we performed multiple sequence alignment on the clustered sequences using MUSCLE. We then performed maximum likelihood tree inference using RAxML-NG (ref. 92, ‘PROTGTR+FO+G4’, v0.9.0), resulting in our reference tree.

To apply SerraPlace to a given genome, we first use HMMER (ref. 71, v3.3) to generate a reference HMM, based on the reference alignment. We then split each contig into ORFs using esl-translate, and use hmmsearch (P value cut-off 0.01) and seqtk (commit 7c04ce7) to identify those query ORFs that align with sufficient quality to the previously generated reference HMM. All ORFs that pass this test are considered valid input sequences for phylogenetic placement. This produces a set of likely placement locations on the tree, with an associated likelihood weight. We then use Gappa (v0.6.1,93) to assign taxonomic information to each query, using the taxonomic information for the reference sequences. Gappa assigns taxonomy by first labelling the interior nodes of the reference tree by a consensus of the taxonomic labels of all descendant leaves of that node. If 66% of leaves share the same taxonomic label up to some level, then the internal node is assigned that label. Then, the likelihood weight associated with each sequence is assigned to the labels of internal nodes of the reference tree, according to where the query was placed.

From this result, we select that taxonomic label that accumulated the highest total likelihood weight as the taxonomic label of a sequence. Note that multiple ORFs of the same genome may result in a taxonomic label, in which case, we select the longest sequence as the source of the taxonomic assignment of the genome.

Phylogenetic inference

We performed phylogenetic inferences using a custom snakemake (v6.6.0) pipeline (available at, using ParGenes (v1.1.2)94. ParGenes is a tree search orchestrator, combining ModelTestNG (v0.1.3)95 and RAxML-NG, and enabling higher levels of parallelization for a given tree search.

To infer the maximum likelihood phylogenetic trees, we performed a tree search comprising 100 distinct starting trees (50 random, 50 parsimony), as well as 1,000 bootstrap searches. We used ModelTest-NG to automatically select the best evolutionary model for the given data. The pipeline also automatically produces versions of the best maximum likelihood tree annotated with Felsenstein’s Bootstrap96 support values, and Transfer Bootstrap Expectation values97.

Reporting summary

Further information on research design is available in the Nature Research Reporting Summary linked to this paper.