GraphAligner

Seed-and-extend program for aligning long error-prone reads to genome graphs. For a description of the bitvector alignment extension algorithm, see https://academic.oup.com/bioinformatics/advance-article/doi/10.1093/bioinformatics/btz162/5372677

Installation

Install via bioconda:

• Install miniconda https://conda.io/projects/conda/en/latest/user-guide/install/index.html
• conda install -c bioconda graphaligner

Compilation

Bioconda is the recommended installation method. If you however want to compile the aligner yourself, run these:

• Install miniconda https://conda.io/projects/conda/en/latest/user-guide/install/index.html
• git clone https://github.com/maickrau/GraphAligner.git
• cd GraphAligner
• git submodule update --init --recursive
• conda env create -f CondaEnvironment.yml
• source activate GraphAligner
• make bin/GraphAligner

Running

Quickstart: GraphAligner -g input_graph.gfa -f input_reads.fa -a output_alignments.json

See Parameters, the option GraphAligner --help and the subsections below for more information and options

File formats

The aligner's file formats are interoperable with vg's file formats. Graphs can be inputed either in .gfa format or .vg format. Reads are inputed as .fasta or .fastq, either gzipped or uncompressed. Alignments are outputed in vg's alignment format, either as a binary .gam or JSON depending on the file name. Custom seeds can be inputed in .gam format.

Seed hits

The aligner has three built-in methods for finding seed hits: minimizers (default), maximal unique matches (MUMs) and maximal exact matches (MEMs). Only matches entirely within a node are found. Minimizers (default) are faster and MUM/MEMs can be more sensitive. MUM/MEM modes use MUMmer4 to find matches between the read and nodes. Use the parameter --seeds-mum-count n to use the n longest MUMs as seeds (or -1 for all MUMs), and --seeds-mem-count n for the n longest MEMs (or -1 for all MEMs). Use --seeds-mxm-length n to only use matches at least n characters long. If you are aligning multiple files to the same graph, use --seeds-mxm-cache-prefix file_name_prefix to store the MUM/MEM index to disk for reuse instead of rebuilding it each time.

Alternatively you can use any method to find seed hits and then import the seeds in .gam format with the parameter -s seedfile.gam. The seeds must be passed as an alignment message, with path.mapping[0].position describing the position in the graph, name the name of the read and query_position the position in the forward strand of the read. Match length (path.mapping[0].edit[0].from_length) is only used to order the seeds, with longer matches tried before shorter matches.

Alternatively you can use the parameter --seeds-first-full-rows to use the dynamic programming alignment algorithm on the entire first row instead of using seeded alignment. This is very slow except on tiny graphs, and not recommended.

Extension

The aligner uses a bitvector banded DP alignment algorithm to extend the seed hits. The DP matrix is calculated inside a certain area (the band), which depends on the extension parameters. Note that "bandwidth" in graph alignment does NOT directly correspond to bandwidth in linear alignment. The bandwidth parameter describes the maximum allowed score difference between the minimum score in a row and a cell, with cells whose score is higher than that falling outside the band. Generally the bandwidth parameters should be between 1-35.

The algorithm starts using the initial bandwidth. Should it detect that the alignment is incorrect, it will rewind and rerun with the ramp bandwidth parameter, aligning high-error parts of the read without slowing down alignment in low-error parts. The tangle effort parameter determines how much time the aligner spends inside complex cyclic subgraphs. If the size of the band grows beyond the tangle effort parameter, the aligner will use the current best alignment for the aligned prefix and move forward along the read. This might miss the optimal alignment.

Parameters

• -g input graph. Format .gfa / .vg
• -f input reads. Format .fasta / .fastq / .fasta.gz / .fastq.gz. You can input multiple files with -f file1 -f file2 ... or -f file1 file2 ...
• -t number of aligner threads. The program also uses two IO threads in addition to these.
• -a output file name. Format .gam or .json
• --try-all-seeds extend from all seeds. Normally a seed is not extended if it looks like a false positive.
• --all-alignments output all alignments. Normally only a set of non-overlapping partial alignments is returned. Use this to also include partial alignments which overlap each others. This also forces --try-all-seeds.
• --global-alignment force the read to be aligned end-to-end. Normally the alignment is stopped if the score gets too poor. This forces the alignment to continue to the end of the read regardless of score. If you use this you should do some other filtering on the alignments to remove false alignments.

Seeding:

• -s External seeds. Load seeds from a .gam file. You can input multiple files with -s file1 -s file2 ... or -s file1 file2 ...
• --seeds-minimizer-chunksize Minimizer seeds are grouped into chunks based on their position in the read. Chunk size in base pairs
• --seeds-minimizer-count Minimizer seeds. Use the n least common minimizers from each chunk in the read. -1 for all minimizers
• --seeds-minimizer-length k-mer size for minimizer seeds
• --seeds-minimizer-windowsize Window size for minimizer seeds
• --seeds-mum-count MUM seeds. Use the n longest maximal unique matches. -1 for all MUMs
• --seeds-mem-count MEM seeds. Use the n longest maximal exact matches. -1 for all MEMs
• --seeds-mxm-length MUM/MEM minimum length. Don't use MUMs/MEMs shorter than n
• --seeds-mxm-cache-prefix MUM/MEM file cache prefix. Store the MUM/MEM index into disk for reuse. Recommended unless you are sure you won't align to the same graph multiple times
• --seeds-first-full-rows Don't use seeds. Instead use the DP alignment on the first row. The runtime depends on the size of the graph so this is very slow. Not recommended

Defaults are --seeds-minimizer-count 5 --seeds-minimizer-length 19 --seeds-minimizer-windowsize 30 --seeds-minimizer-chunksize 100

Extension:

• -b alignment bandwidth. Unlike in linear alignment, this is the score difference between the minimum score in a row and the score where a cell falls out of the band. Values should be between 1-35.
• -B ramp bandwidth. If a read cannot be aligned with the alignment bandwidth, switch to the ramp bandwidth at the problematic location. Values should be between 1-35.
• -C tangle effort. Determines how much effort the aligner spends on tangled areas. Higher values use more CPU and memory and have a higher chance of aligning through tangles. Lower values are faster but might return an inoptimal or a partial alignment. Use for complex graphs (eg. de Bruijn graphs of mammalian genomes) to limit the runtime in difficult areas. Values should be between 1'000 - 500'000.
• --high-memory high memory mode. Runs a bit faster but uses a LOT more memory

Defaults are -b 5 -B 10 -C 10000