RAILS and Cobbler
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RAILS v1.4.1 and Cobbler v0.5.1

Rene L. Warren, 2014-2018

email: rwarren at bcgsc.ca

Name


RAILS: Radial Assembly Improvement by Long Sequence Scaffolding Cobbler: Gap-filling with long sequences

Description


RAILS and Cobbler are genomics application for scaffolding and automated finishing of genome assemblies with long DNA sequences. They can be used to scaffold & finish high-quality draft genome assemblies with any long, preferably high-quality, sequences such as scaftigs/contigs from another genome draft.

They both rely on accurate, long DNA sequences to patch gaps in existing genome assembly drafts.

Cobbler is a utility to automatically patch gaps (ambiguous regions in a draft assembly, represented by N's) It does so by first aligning the long sequences to the assembly, tallying the alignments and replacing N's with the sequences from these long DNA sequences.

RAILS is an all-in-one scaffolder and gap-filler. Its process is similar to that of Cobbler. It scaffolds your genome draft with the help of long DNA sequences (contig sequences are ordered/oriented using alignment information). The newly created gaps are automatically filled with the DNA sequence of the provided long DNA sequence.

You can test the software by executing "runme.sh" in the test folder. A simulated SARS genome assembly is provided to test the software.

What's new in v1.4.1

  1. Save in memory gap sequence from highest-matching read for both cobbler and RAILS
  2. Track the number of reads support in cobbler (-l) and RAILS, and allow cutoff when scaffolding (-l and -a), with latter (RAILS)
  3. Remove the hardcoded two-hit requirement for a read in RAILS. Instead, process two best hits for each read aligning different sequences
  4. Implement grace (-g) option, which effectively simulate read trimming (valuable for Nanopore read mapping (suggested -g 250 to -g 500))
  5. bug fixes (-list.tsv (cobbler) reported some instances of gap-fill regions not fixed in the assembly). cobbler gap-fill table now lists #supporting reads for each gap filled

Implementation and requirements


RAILS and Cobbler are implemented in PERL and run on any OS where PERL is installed. Both tools require samtools (tested with v1.8) to read sequence alignment bamfiles. The runRAILS.sh pipeline requires bwa (see Dependencies below for tested version).

Community guidelines:


I encourage the community to contribute to the development of this software, by providing suggestions for improving the code and/or directly contributing to the open source code for these tools. Users and developers may report software issues, bug fix requests, comments, etc, at https://github.com/warrenlr/RAILS

Install


Download the tar ball, gunzip and extract the files on your system using:

gunzip rails_v1-4-1.tar.gz
tar -xvf rails_v1-4-1.tar

Alternatively, individual tools are available within the github repository

Dependencies


Make sure you have installed bwa (Version: 0.7.15-r1140) and that is is in your path. Make sure you have installed samtools (Version: 1.8) and that is is in your path. Compatible tools may be used, but have not been tested fully (eg. minimap2, sambamba)

Test data


Go to ./test
(cd test)

You may need to change both runme.sh and runmeHuman.sh to specify the path of samtools on your system

1. SARS:
execute runme.sh
(./runme.sh)

2. Human:
execute runmeHuman.sh (will take a while to run)
(./runmeHuman.sh)

Citing RAILS/Cobbler


Thank you for using, developing and promoting this free software. If you use RAILS or Cobbler for you research, please cite:

Warren RL. 2016. RAILS and Cobbler: Scaffolding and automated finishing
of draft genomes using long DNA sequences. The Journal of Open Source
Software. doi: 10.21105/joss.00116

Usage


./runRAILS.sh
Usage: runRAILS.sh     

this pipeline will:
1. reformat the assembly file $1
2. rename the long sequence file $2
3. Build a database index with bwa
4. Align the reformatted long sequences to your re-formatted baseline assembly
5. Run Cobbler to gap-fill regions of ambiguity
6. Reformat Cobbler's .fa file
7. Build a database index of it with bwa
8. Align the reformatted long sequences to your re-formatted cobbler assembly
9. Run RAILS to generate a newly scaffolded assembly draft

Usage: ./cobbler.pl [v0.5.1]
-f  Assembled Sequences to further scaffold (Multi-FASTA format NO LINE BREAKS, required)
-q  File of filenames containing long Sequences queried (Multi-FASTA format NO LINE BREAKS, required)
-s  File of filenames containing full path to BAM file(s) (use v0.2 for reading SAM files)
-p  Full path to samtools (known to work/tested with v1.8, required)
-d  Anchoring bases on contig edges (ie. minimum required alignment size on contigs, default -d 1000, optional)
-i  Minimum sequence identity fraction (0 to 1), default -i 0.9, optional
-l  Minimum number of long sequence support per gap, default -l 1, optional
-g  Grace length (bp), default -g 1, optional
-t  LIST of names/header, long sequences to avoid using for merging/gap-filling scaffolds (optional)
-b  Base name for your output files (optional)
-v  Runs in verbose mode (-v 1 = yes, default = no, optional)
IMPORTANT: the order of files in -q and -s MUST match!


Usage: ./RAILS [v1.4.1]
-f  Assembled Sequences to further scaffold (Multi-Fasta format, required)
-q  File of filenames containing long Sequences queried (Multi-Fasta format, required)
-s  File of filenames containing full path to BAM file(s)
-p  Full path to samtools (known to work/tested with v1.8, required)
-d  Anchoring bases on contig edges (ie. minimum required alignment size on contigs, default -d 1000, optional)
-i  Minimum sequence identity fraction (0 to 1), default -i 0.9, optional
-t  LIST of names/header, long sequences to avoid using for merging/gap-filling scaffolds (optional)
-l  Minimum number of links to compute scaffold (default -l 1, optional)
-a  Maximum link ratio between two best contig pairs *higher values lead to least accurate scaffolding* (default -a 0.99, optional)
-g  Grace length (bp), default -g 1, optional
-b  Base name for your output files (optional)
-v  Runs in verbose mode (-v 1 = yes, default = no, optional)
IMPORTANT: the order of files in -q and -s MUST match!

How it works


The pipeline is detailed in the provided script runRAILS.sh. PLEASE ensure the draft assembly is FASTA-formatted with one sequence per line (NO LINE BREAKS)

Cobbler's process:

The assembly draft sequence supplied to Cobbler is first broken up at the ambiguous regions of the assembly (Ns) to create scaftigs. In the runRAILS.sh, these scaftigs are renamed, tracking their scaffold of origin (renumbered incrementally) and their position within it (also numbered incrementally). A bwa index is created and the long sequence file, also re-numbered, is aligned to the scaftigs. Cobbler is supplied with the alignment file (-s sam file) and the long reads files (-q option), specifying the minimum length of anchoring bases (-d) aligning at the edge of scaftigs and the minimum sequence identity of the alignment (-i). When 1 or more long sequences align unambiguously to the 3'end of a scaftig and the 5'end of its neighbour, the gap is patched with the sequence of that long sequence. If no long sequences are suitable, or the -d and -i conditions are not met, the original Ns are placed back between those scaftigs.

RAILS process:

In RAILS, the process is similar as for Cobbler, except that the draft assembly is not broken up at Ns, since the goal is to merge distinct sequences into larger ones. Long sequences are aligned to the draft assembly sequences, orienting and ordering sequences and simulateneously filling the gaps between them, using DNA bases from the long sequences.

Scaffolding in RAILS is done using the LINKS scaffolder code (Warren et al. 2015), the unpublished scaffolding engine in the widely-used SSAKE assembler (Warren et al. 2007), and foundation of the SSPACE-LongRead scaffolder (Boetzer and Pirovano, 2014).

Output: For both Cobbler and RAILS, a summary of the gaps closed and their lengths is provided (.tsv) as a text file. A fasta file (.fa) of the finished and/or scaffolded draft is generated for both along with a log file reporting basic success statistics.

Boetzer M, Pirovano W. 2014. SSPACE-LongRead: scaffolding bacterial draft genomes using long read sequence information. BMC Bioinformatics.15:211. DOI: 10.1186/1471-2105-15-211

Warren RL, Yang C, Vandervalk BP, Behsaz B, Lagman A, Jones SJ, Birol I. 2015. LINKS: Scalable, alignment-free scaffolding of draft genomes with long reads. GigaScience 4:35. DOI: 10.1186/s13742-015-0076-3

Warren RL, Sutton GG, Jones SJM, Holt RA.  2007.  Assembling millions of short DNA sequences using SSAKE.  Bioinformatics. 23(4):500-501. DOI: 10.1093/bioinformatics/btl629

Runs on the human genome


On a human draft assembly, cobbler patched over 65% of the gaps using 1, 2.5, 5, 15 kb long DNA sequences simulated from the human genome reference. The Pearson correlation between the predicted gap sizes and the size of patched gaps is R=0.8150

Table 1. Patching gaps with Cobbler (v0.2) using simulated 1, 2.5, 5, 15kbp simulated long sequences from human genome reference GRCh38.

Metric Value
Total gaps 148,091
Number of gaps patched 95,523
Proportion of gaps patched 65.1%
Average length (bp) 343.39
Length st.dev +/- 931.12
Total bases added 32,801,755
Largest gap resolved (bp) 13,662
Shortest gap resolved (bp) 1

RAILS (v1.1) was used to further contiguate the human baseline assembly draft and automatically close gaps within in:

Table 2. RAILS scaffolding and gap-filling summary on a human assembly baseline, using simulated 1, 2.5, 5, 15kbp simulated long sequences from human genome reference GRCh38.

Metric Value
Number of merges induced 6,029
Average closed gap length (bp) 1,136.71
Closed gap length st.dev +/- 2,511.69
Total bases added 6,853,222
Largest gap resolved (bp) 14,471
Shortest gap resolved (bp) 1

6,029 merges resulted from RAILS scaffolding of the baseline human assembly draft (1,695 >= 500bp) The scaffold N50 length increased from 5.6 to 7.3 Mbp, a 30% increase in N50 length.

Table 3. Assembly statistics on human genome scaffolding and finishing post Cobbler and RAILS (reporting sequences 500 bp and larger).

Stage n:500 n:N50 n:NG50 NG50 (bp) N50 (bp) max (bp) sum (bp)
Baseline 65,905 145 164 5,144,025 5,597,244 26.41e6 2.794e9
Cobbler 65,905 145 161 5,312,196 5,658,133 26.66e6 2.827e9
RAILS 64,210 113 125 6,935,685 7,266,542 32.14e6 2.836e9

License Preamble


RAILS and Cobbler Copyright (c) 2014-2018 British Columbia Cancer Agency Branch. All rights reserved.

RAILS and Cobbler are released under the GNU General Public License v3

This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, version 3.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program. If not, see http://www.gnu.org/licenses/.