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WAAFLE: a Workflow to Annotate Assemblies and Find LGT Events

Project description

WAAFLE

WAAFLE (a Workflow to Annotate Assemblies and Find LGT Events) is a method for finding novel LGT (Lateral Gene Transfer) events in assembled metagenomes, including those from human microbiomes. WAAFLE was developed in the Huttenhower Lab at the Harvard T.H. Chan School of Public Health by Tiffany Hsu and Eric A. Franzosa. Please direct questions to the WAAFLE user group.

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Citation

Tiffany Y. Hsu, Eric A. Franzosa, Dennis Wong, Chengwei Luo, Robert G. Beiko, Morgan Langille, Curtis Huttenhower. The landscape of novel lateral gene transfer events in the human microbiome. (In prep.)

(While WAAFLE is not yet published, if you use WAAFLE or the datasets provided below in your work, please cite this website: http://huttenhower.sph.harvard.edu/waafle.)

Installation

Install the WAAFLE software and Python dependencies with pip:

$ pip install waafle

You can also clone the WAAFLE package from bitbucket with mercurial (hg):

$ hg clone https://bitbucket.org/biobakery/waafle

Or download and inflate WAAFLE package directly:

$ wget https://bitbucket.org/biobakery/waafle/get/default.zip
$ unzip default.zip

Software requirements

  • Python 3+ or 2.7+
  • Python numpy (tested with v1.13.3)
  • NCBI BLAST+ (tested with v2.6.0)
  • bowtie2 (for performing read-level QC; tested with v2.2.3)

Database requirements

WAAFLE requires two input databases: 1) a BLAST-formatted nucleotide sequence database and 2) a corresponding taxonomy file. The versions used in the WAAFLE publication are available for download here:

Input data

An individual WAAFLE run requires one or two non-fixed inputs: 1) a file containing metagenomic contigs and (optionally) 2) a GFF file describing gene/ORF coordinates along those contigs.

Input contigs (required)

Contigs should be provided as nucleotide sequences in FASTA format. Contigs are expected to have unique, BLAST-compatible headers. WAAFLE is optimized for working with fragmentary contigs from partially assembled metagenomes (spanning 2-20 genes, or roughly 1-20 kb). WAAFLE is not optimized to work with extremely long contigs (100s of kbs), scaffolds, or closed genomes. The WAAFLE developers recommend MEGAHIT as a general-purpose metagenomic assembler.

Input ORF calls (optional)

  1. The optional GFF file, if provided, should conform to the GFF format. The WAAFLE developers recommend Prodigal as a general-purpose ORF caller with GFF output.

Performing a WAAFLE analysis

Analyzing a set of contigs with WAAFLE consistent of three steps, one of which is optional. These three steps are carried out by three independent scripts: waafle_search, waafle_genecaller (optional), and waafle_orgscorer.

Step 1 (required): running waafle_search

waafle_search is a light wrapper around blastn to help guide the nucleotide-level search of your metagenomic contigs against a WAAFLE-formatted database (for example, it ensures that all of the non-default BLAST output fields required for downstream processing are generated).

A sample call to waafle_search with input contigs contigs.fna and a blast database located in waafledb would be:

$ waafle_search contigs.fna waafledb/waafled

By default, this produces an output file contigs.blastout in the same location as the input contigs. See the --help menu for additional configuration options.

Step 1.5 (optional): Running waafle_genecaller

If the user chooses not to provide a GFF file along with their contigs, WAAFLE can identify gene coordinates of interest directly from the BLAST output produced in the previous step:

$ waafle_genecaller contigs.blastout

This produces a file in GFF format called contigs.gff for use in the next and last WAAFLE step. See the --help menu for additional configuration options.

Step 2 (required): Running waafle_orgscorer

The final and most critical step of a WAAFLE analysis is combining the BLAST output generated in Step 1 with a GFF file to 1) taxonomically score genes along the length of the input contigs and then 2) identify those contigs as derived from a single clade or a pair of clades (i.e. putative LGT). Assuming you have run steps 1 and 1.5 as described above, a sample call to waafle_orgscorer would be:

$ waafle_orgscorer \
  contigs.fna \
  contigs.blastout \
  contigs.gff \
  taxonomy.tsv

This will produce three output files which divide and describe your contigs as putative LGT contigs, single-clade (no-LGT) contigs, and unclassified contigs (e.g. those containing no genes):

  • contigs.lgt.tsv
  • contigs.no_lgt.tsv
  • contigs.unclassified.tsv

These files and their formats are described in more detailed below (see "Interpretting WAAFLE outputs").

waafle_orgscorer offers many options for fine-tuning your analysis. The various analysis parameters have been pre-optimized for maximum specificity on both short contigs (containing as little as two partial genes) and longer contigs (10s of genes). These options are detailed in the --help menu:

WAAFLE outputs

The contigs.lgt.tsv output file lists the details of putative LGT contigs. Its fields are a superset of the types of fields included in the other output files. The following represents the first two lines/rows of a contigs.lgt.tsv file transposed such that first line (column headers) is shown as the first column and the details of the first LGT contig (second row) are shown as the second column:

CONTIG_NAME          12571
CALL                 lgt
CONTIG_LENGTH        9250
MIN_MAX_SCORE        0.856
AVG_MAX_SCORE        0.965
SYNTENY              AABAAAA
DIRECTION            B>A
CLADE_A              s__Ruminococcus_bromii
CLADE_B              s__Faecalibacterium_prausnitzii
LCA                  f__Ruminococcaceae
MELDED_A             --
MELDED_B             --
TAXONOMY_A           r__Root|k__Bacteria|p__Firmicutes|...|g__Ruminococcus|s__Ruminococcus_bromii
TAXONOMY_B           r__Root|k__Bacteria|p__Firmicutes|...|g__Faecalibacterium|s__Faecalibacterium_prausnitzii
LOCI                 252:668:-|792:1367:-|1557:2360:-|2724:3596:-|4540:5592:+|5608:7977:+|8180:8425:+
ANNOTATIONS:UNIPROT  R5E4K6|D4L7I2|D4JXM0|D4L7I1|D4L7I0|None|D4L7H8

The fields in detail:

  • CONTIG_NAME: the name of the contig from the input FASTA file.
  • CALL: indicates that this was an LGT contig.
  • CONTIG_LENGTH: the length of the contig in nucleotides.
  • MIN_MAX_SCORE: the minimum score for the pair of clades explaining the contig along the length of the contig. (i.e. the score for identifying this as a putative LGT contig, with a default threshold of 0.8.)
  • AVG_MAX_SCORE: the average score for the pair of clades explaining the contig (used for ranking multiple potential explanations of the contig).
  • SYNTENY: the pattern of genes assigned to the A or B clades along the contig. * indicates that the gene could be contributed by either clade; ~ indicates an ignored gene; ! indicates a problem (should not occur).
  • DIRECTION: indicates this as a putative B-to-A transfer event, as determined from synteny (A genes flank the inserted B gene). A?B indicates that the direction was uncertain.
  • CLADE_A: the name of clade A.
  • CLADE_B: the name of clade B.
  • LCA: the lowest common ancestor of clades A and B. A higher-level LCA indicates a more remote LGT event.
  • MELDED_A: if using a meld reporting option, the individual melded clades will be listed here. For example, if a contig could be explained by a transfer from Genus_1 species_1.1 to either Genus_2 species_2.1 or Genus_2 species_2.2, this field would list species_2.1; species 2.2 and Genus 2 would be given as CLADE_A.
  • MELDED_B: see above.
  • TAXONOMY_A: the full taxonomy of CLADE_A.
  • TAXONOMY_B: the full taxonomy of CLADE_B.
  • LOCI: Ccordinates of the loci (genes) that were considered for this contig in format START:STOP:STRAND.
  • ANNOTATIONS:UNIPROT: indicates that UniProt annotations were provided for the genes in the input sequence database (in format UNIPROT=IDENTIFIER). The best-scoring UniProt annotation for each gene is given here. (Additional annotations would appear as additional, similarly-formatted columns in the output.)

Contig-level quality control

The WAAFLE workflow described above has been optimized to distinguish LGTs from other biological events (e.g. gene deletion). However, it cannot intrinsically identify spurious LGTs resulting from misassembly (e.g. chimerism). For this, we provide a separate method, the scripts waafle_junctions and waafle_qc.

Quantifying junction support with waafle_junctions

waafle_junctions maps reads to contigs to quantify support for individiual gene-gene junctions. Specifically, two forms of support are considered/computed:

  1. Sequencing fragments (paired reads) that span the gene-gene junction. These are less common for junctions that are larger than typical sequencing insert sizes (~300 nts).

  2. Relative junction coverage compared to the mean coverage of the two flanking genes. If the junction is not well covered relative to its flanking genes, it may represent a non-biological overlap.

A sample call to waafle_junctions looks like:

$ waafle_qc \
  contigs.fna \
  contigs.gff \
  --reads1 contigs_reads.1.fq \
  --reads2 contigs_reads.2.fq \

With this call, waafle_junctions will use bowtie2 to index the contigs and then align the input reads (pairwise) against the index to produce a SAM file. (waafle_junctions can also interpret a mapping from an existing SAM file.) The alignment results are then interpreted to score individual junctions, producing an output file for each.

  • contigs.junctions.tsv

A sample report for an individual junction looks like:

CONTIG             SRS011086_k119_10006
GENE1              1:1363:-
GENE2              1451:2382:-
LEN_GENE1          1363
LEN_GENE2          932
GAP                87
JUNCTION_HITS      5
COVERAGE_GENE1     8.1079
COVERAGE_GENE2     11.3573
COVERAGE_JUNCTION  10.9101
RATIO              1.1210

This report indicates that the junction between genes 1 and 2 (which may or may not be an LGT junction) was well supported: it was spanned by 5 mate-pairs (JUNCTION_HITS=5) and had coverable coverage (RATIO=1.12) to the mean of its flanking genes (8.11 and 11.4).

waafle_junctions can be tuned to produce additional gene- and nucleotide-level quality reports. Consult the --help menu for a full list of options.

Using junction data for contig QC with waafle_qc

The waafle_qc script interprets the output of waafle_junctions to remove contigs with weak read support at one or more junctions. Currently, the script focuses on the junctions flanking LGT'ed genes among putative LGT-containing contigs.

A sample call to waafle_qc looks like:

$ waafle_qc contigs.lgt.tsv contigs.junctions.tsv

Where contigs.junctions.tsv is the output of waafle_junctions on this set of contigs and its underlying reads. This produces a file contigs.lgt.tsv.qc_pass: a subset of the original LGT calls that were supported by read-level evidence.

By default, a junction is supported if it was contained in 2+ mate-pairs or had >0.5x the average coverage of its two flanking genes. These thresholds are tunable with the --min-junction-hits and --min-junction-ratio parameters of waafle_qc, respectively. Consult the --help menu for a full list of options.

Advanced topics

Formatting a sequence database for WAAFLE

WAAFLE performs a nucleotide-level search of metagenomic contigs against a collection of taxonomically annotated protein-coding genes (not complete genomes). A common way to build such a database is to combine a collection of microbial pangenomes of interest. The protein-coding genes should be organized in FASTA format and then indexed for use with blastn. For example, the FASTA file waafledb.fnt would be indexed as:

$ makeblastdb -in waafledb.fnt -dbtype nucl

WAAFLE expects the input FASTA sequence headers to follow a specific format. At a minimum, the headers must contain a unique sequence ID (GENE_123 below) followed by | (pipe) followed by a taxon name or taxonomic ID (SPECIES_456 below):

>GENE_123|SPECIES_456

Headers can contain additional |-delimited fields corresponding to functional annotations of the gene. These fields have the format SYSTEM=IDENTIFIER and multiple such fields can be included per header, as in:

>GENE_123|SPECIES_456|PFAM=P00789|EC=1.2.3.4

Headers are allowed to contain different sets of functional annotation fields. WAAFLE currently expects at most one annotation per annotation system per gene; this will be improved in future versions. (We currently recommend annotating genes against the UniRef90 and UniRef50 databases to enable link-outs to more detailed functional annotations in downstream analyses.)

WAAFLE assumes that the taxa listed in the sequence database file are all at the same taxonomic level (for example, all genera or all species or all strains).

Formatting a WAAFLE taxonomy file

WAAFLE requires a taxonomy file to understand the taxonomic relationships among the taxa whose genes are included in the sequence database. The taxonomy file is a tab delimited list of child-parent relationships, for example:

k__Animalia      r__Root
p__Chordata      k__Animalia
c__Mammalia      p__Chordata
o__Primates      c__Mammalia
f__Hominidae     o__Primates
g__Homo          f__hominidae
s__Homo sapiens  g__Homo

While the format of this file is relatively simple, it has a number of critical structural constraints that must be obeyed:

  • All taxon names/IDs used in the sequence database must appear is the taxonomy file.

  • The file must contain a root taxon from which all other taxa descend (the root taxon should be named r__Root, as above).

  • All taxon names/IDs used in the sequence database must be the same distance from the root.

The following properties of the taxonomy file are optional:

  • Additional taxa below the level of the taxa in the sequence file can be included in the taxonomy file. For example, a species-level sequence database could contain isolate genomes as children of the species-level clades in the taxonomy file. (WAAFLE can use such entries as "leaf support" for LGT events.)

  • We recommend prefixing taxonomic clades to indicate their level. For example, g__Homo identifies Homo as a genus above.

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