group

Category: GROUP

Group reads by UMI to identify reads from the same original molecule

Description

Groups reads together that appear to have come from the same original molecule. Reads are grouped by template, and then templates are sorted by the 5’ mapping positions of the reads from the template, used from earliest mapping position to latest. Reads that have the same end positions are then sub-grouped by UMI sequence.

Accepts reads in any order (including unsorted) and outputs reads sorted by:

1. The lower genome coordinate of the two outer ends of the templates (strand-aware)
2. The sequencing library
3. The cell barcode (CB tag, if present)
4. The assigned UMI tag
5. Read Name

It is recommended to sort the reads into template-coordinate order prior to running this tool to avoid re-sorting the input. Use fgumi sort --order template-coordinate for the pre-sorting. The output will always be written in template-coordinate order.

During grouping, reads and templates are filtered out as follows:

1. Templates are filtered if all reads for the template are unmapped
2. Templates are filtered if any non-secondary, non-supplementary read has mapping quality < min-map-q
3. Templates are filtered if any UMI sequence contains one or more N bases
4. Templates are filtered if –min-umi-length is specified and the UMI does not meet the length requirement
5. Records are filtered out if flagged as either secondary or supplementary

Grouping of UMIs is performed by one of four strategies:

1. identity: only reads with identical UMI sequences are grouped together. This strategy may be useful for evaluating data, but should generally be avoided as it will generate multiple UMI groups per original molecule in the presence of errors.
2. edit: reads are clustered into groups such that each read within a group has at least one other read in the group with <= edits differences and there are inter-group pairings with <= edits differences. Effective when there are small numbers of reads per UMI, but breaks down at very high coverage of UMIs.
3. adjacency: a version of the directed adjacency method described in umi_tools (http://dx.doi.org/10.1101/051755) that allows for errors between UMIs but only when there is a count gradient.
4. paired: similar to adjacency but for methods that produce templates such that a read with A-B is related to but not identical to a read with B-A. Expects the UMI sequences to be stored in a single SAM tag separated by a hyphen (e.g. ACGT-CCGG) and allows for one of the two UMIs to be absent (e.g. ACGT- or -ACGT). The molecular IDs produced have more structure than for single UMI strategies and are of the form {base}/{A|B}. E.g. two UMI pairs would be mapped as follows: AAAA-GGGG -> 1/A, GGGG-AAAA -> 1/B.

Strategies edit, adjacency, and paired make use of the –edits parameter to control the matching of non-identical UMIs.

By default, all UMIs must be the same length. If –min-umi-length=len is specified then reads that have a UMI shorter than len will be discarded, and when comparing UMIs of different lengths, the first len bases will be compared, where len is the length of the shortest UMI. The UMI length is the number of [ACGT] bases in the UMI (i.e. does not count dashes and other non-ACGT characters). This option is not implemented for reads with UMI pairs (i.e. using the paired assigner).

Note: the –min-map-q parameter defaults to 0 in duplicate marking mode and 1 otherwise, and is directly settable on the command line.

Cell Barcodes

If the input data contains cell barcodes (e.g. from single-cell sequencing), reads at the same genomic position are partitioned by cell barcode before UMI grouping. This ensures that reads from different cells are never grouped together, even if they share a UMI sequence and mapping position. The cell barcode is read from the standard CB tag. No correction or error-handling is performed on cell barcodes; they must be corrected upstream.

Multi-threaded operation is supported via –threads N which spawns exactly N threads. Threads are allocated based on the command’s workload profile to optimize performance.

Example: –threads 8 spawns exactly 8 threads (2 reader, 4 workers, 2 writer)

Arguments