soapdenovo-31mer - Online in the Cloud

PROGRAM:

NAME

soapdenovo - Short-read assembly method that can build a de novo draft assembly

SYNOPSIS

soapdenovo_31mer soapdenovo_63mer soapdenovo_127mer

Introduction

SOAPdenovo is a novel short-read assembly method that can build a de novo draft assembly
for the human-sized genomes. The program is specially designed to assemble Illumina GA
short reads. It creates new opportunities for building reference sequences and carrying
out accurate analyses of unexplored genomes in a cost effective way.

1) Support large kmer up to 127 to utilize long reads. Three version are provided.
I. The 31mer version support kmer only <=31.
II. The 63mer version support kmer only <=63 and doubles the memory consumption than
31mer version, even being used with kmer <=31.
III. The 127mer version support kmer only <=127 and double the memory consumption than
63mer version, even being used with kmer <=63.

Please notice that, with longer kmer, the quantity of nodes would decrease significantly,
thus the memory consumption is usually smaller than double with shifted version.

2) New parameter added in "pregraph" module. This parameter initiates the memory
assumption to avoid further reallocation. Unit of the parameter is GB. Without further
reallocation, SOAPdenovo runs faster and provide the potential to eat up all the memory of
the machine. For example, if the workstation provides 50g free memory, use -a 50 in
pregraph step, then a static amount of 50g memory would be allocated before processing
reads. This can also avoid being interrupted by other users sharing the same machine.

3) Gap filled bases now represented by lowercase characters in 'scafSeq' file.

4) Introduced SIMD instructions to boost the performance.

Configuration file

For big genome projects with deep sequencing, the data is usually organized as multiple
read sequence files generated from multiple libraries. The configuration file tells the
assembler where to find these files and the relevant information. “example.config” is an
example of such a file.

The configuration file has a section for global information, and then multiple library
sections. Right now only “max_rd_len” is included in the global information section. Any
read longer than max_rd_len will be cut to this length.

The library information and the information of sequencing data generated from the library
should be organized in the corresponding library section. Each library section starts
with tag [LIB] and includes the following items:

avg_ins
This value indicates the average insert size of this library or the peak value
position in the insert size distribution figure.

reverse_seq
This option takes value 0 or 1. It tells the assembler if the read sequences need
to be complementarily reversed. Illumima GA produces two types of paired-end
libraries: a) forward-reverse, generated from fragmented DNA ends with typical
insert size less than 500 bp; b) forward-forward, generated from circularizing
libraries with typical insert size greater than 2 Kb. The parameter “reverse_seq”
should be set to indicate this: 0, forward-reverse; 1, forward-forward.

asm_flags=3
This indicator decides in which part(s) the reads are used. It takes value 1(only
contig assembly), 2 (only scaffold assembly), 3(both contig and scaffold assembly),
or 4 (only gap closure).

rd_len_cutoff
The assembler will cut the reads from the current library to this length.

rank It takes integer values and decides in which order the reads are used for scaffold
assembly. Libraries with the same “rank” are used at the same time during scaffold
assembly.

pair_num_cutoff
This parameter is the cutoff value of pair number for a reliable connection between
two contigs or pre-scaffolds.

map_len
This takes effect in the “map” step and is the minimun alignment length between a
read and a contig required for a reliable read location.

The assembler accepts read file in two formats: FASTA or FASTQ. Mate-pair relationship
could be indicated in two ways: two sequence files with reads in the same order belonging
to a pair, or two adjacent reads in a single file (FASTA only) belonging to a pair.

In the configuration file single end files are indicated by “f=/path/filename” or
“q=/pah/filename” for fasta or fastq formats separately. Paired reads in two fasta
sequence files are indicated by “f1=” and “f2=”. While paired reads in two fastq sequences
files are indicated by “q1=” and “q2=”. Paired reads in a single fasta sequence file is
indicated by “p=” item.

All the above items in each library section are optional. The assembler assigns default
values for most of them. If you are not sure how to set a parameter, you can remove it
from your configuration file.

Get it started

Once the configuration file is available, a typical way to run the assembler is: ${bin}
all –s config_file –K 63 –R –o graph_prefix

User can also choose to run the assembly process step by step as: ${bin} pregraph
\[u2013]s config_file \[u2013]K 63 [\[u2013]R -d \[u2013]p -a] \[u2013]o graph_prefix
${bin} contig \[u2013]g graph_prefix [\[u2013]R \[u2013]M 1 -D] ${bin} map \[u2013]s
config_file \[u2013]g graph_prefix [-p] ${bin} scaff \[u2013]g graph_prefix [\[u2013]F -u
-G -p]

Options

-a INT Initiate the memory assumption (GB) to avoid further reallocation

-s STR configuration file

-o STR output graph file prefix

-g STR input graph file prefix

-K INT K-mer size [default 23, min 13, max 127]

-p INT multithreads, n threads [default 8]

-R use reads to solve tiny repeats [default no]

-d INT remove low-frequency K-mers with frequency no larger than [default 0]

-D INT remove edges with coverage no larger that [default 1]

-M INT strength of merging similar sequences during contiging [default 1, min 0, max
3]

-F intra-scaffold gap closure [default no]

-u un-mask high coverage contigs before scaffolding [defaut mask]

-G INT allowed length difference between estimated and filled gap

-L minimum contigs length used for scaffolding

Output files

These files are output as assembly results:

a. *.contig

contig sequences without using mate pair information

b. *.scafSeq

scaffold sequences (final contig sequences can be extracted by breaking down scaffold
sequences at gap regions)

There are some other files that provide useful information for advanced users, which are
listed in Appendix B.

FAQ

How to set K-mer size?
The program accepts odd numbers between 13 and 31. Larger K-mers would have higher rate of
uniqueness in the genome and would make the graph simpler, but it requires deep sequencing
depth and longer read length to guarantee the overlap at any genomic location.

How to set library rank?
SOAPdenovo will use the pair-end libraries with insert size from smaller to larger to
construct scaffolds. Libraries with the same rank would be used at the same time. For
example, in a dataset of a human genome, we set five ranks for five libraries with insert
size 200-bp, 500-bp, 2-Kb, 5-Kb and 10-Kb, separately. It is desired that the pairs in
each rank provide adequate physical coverage of the genome.

APPENDIX A: an example.config

#maximal read length
max_rd_len=50
[LIB]
#average insert size
avg_ins=200
#if sequence needs to be reversed
reverse_seq=0
#in which part(s) the reads are used
asm_flags=3
#use only first 50 bps of each read
rd_len_cutoff=50
#in which order the reads are used while scaffolding
rank=1
# cutoff of pair number for a reliable connection (default 3)
pair_num_cutoff=3
#minimum aligned length to contigs for a reliable read location (default 32)
map_len=32
#fastq file for read 1
q1=/path/**LIBNAMEA**/fastq_read_1.fq
#fastq file for read 2 always follows fastq file for read 1
q2=/path/**LIBNAMEA**/fastq_read_2.fq
#fasta file for read 1
f1=/path/**LIBNAMEA**/fasta_read_1.fa
#fastq file for read 2 always follows fastq file for read 1
f2=/path/**LIBNAMEA**/fasta_read_2.fa
#fastq file for single reads
q=/path/**LIBNAMEA**/fastq_read_single.fq
#fasta file for single reads
f=/path/**LIBNAMEA**/fasta_read_single.fa
#a single fasta file for paired reads
p=/path/**LIBNAMEA**/pairs_in_one_file.fa
[LIB]
avg_ins=2000
reverse_seq=1
asm_flags=2
rank=2
# cutoff of pair number for a reliable connection
#(default 5 for large insert size)
pair_num_cutoff=5
#minimum aligned length to contigs for a reliable read location
#(default 35 for large insert size)
map_len=35
q1=/path/**LIBNAMEB**/fastq_read_1.fq
q2=/path/**LIBNAMEB**/fastq_read_2.fq
q=/path/**LIBNAMEB**/fastq_read_single.fq
f=/path/**LIBNAMEB**/fasta_read_single.fa

Appendix B: output files

1. Output files from the command “pregraph”

a. *.kmerFreq

Each row shows the number of Kmers with a frequency equals the row number.

b. *.edge

Each record gives the information of an edge in the pre-graph: length, Kmers on both ends,
average kmer coverage, whether it’s reverse-complementarily identical and the sequence.

c. *.markOnEdge & *.path

These two files are for using reads to solve small repeats

e. *.preArc

Connections between edges which are established by the read paths.

f. *.vertex

Kmers at the ends of edges.

g. *.preGraphBasic

Some basic information about the pre-graph: number of vertex, K value, number of edges,
maximum read length etc.

2. Output files from the command “contig”

a. *.contig

Contig information: corresponding edge index, length, kmer coverage, whether it’s tip and
the sequence. Either a contig or its reverse complementry counterpart is included. Each
reverse complementary contig index is indicated in the *.ContigIndex file.

b. *.Arc

Arcs coming out of each edge and their corresponding coverage by reads

c. *.updated.edge

Some information for each edge in graph: length, Kmers at both ends, index difference
between the reverse-complementary edge and this one.

d. *.ContigIndex

Each record gives information about each contig in the *.contig: it’s edge index, length,
the index difference between its reverse-complementary counterpart and itself.

3. Output files from the command “map”

a. *.peGrads

Information for each clone library: insert-size, read index upper bound, rank and pair
number cutoff for a reliable link.

This file can be revised manually for scaffolding tuning.

b. *.readOnContig

Read locations on contigs. Here contigs are referred by their edge index. Howerver about
half of them are not listed in the *.contig file for their reverse-complementary
counterparts are included already.

c. *.readInGap

This file includes reads that could be located in gaps between contigs. This information
will be used to close gaps in scaffolds.

4. Output files from the command “scaff”

a. *.newContigIndex

Contigs are sorted according their length before scaffolding. Their new index are listed
in this file. This is useful if one wants to corresponds contigs in *.contig with those
in *.links.

b. *.links

Links between contigs which are established by read pairs. New index are used.

c. *.scaf_gap

Contigs in gaps found by contig graph outputted by the contiging procedure. Here new index
are used.

d. *.scaf

Contigs for each scaffold: contig index (concordant to index in *.contig), approximate
start position on scaffold, orientation, contig length, and its links to others.

e. *.gapSeq

Gap sequences between contigs.

f. *.scafSeq

Sequence of each scaffold.

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