However, in some situations it can be useful to merge the overlapping pair into one sequence read instead. The benefit is that you get longer reads, and that the quality improves (normally the quality drops towards the end of a read, and by overlapping the ends of two reads, the consensus read now reflects two read ends instead of just one).
In the CLC Genomics Workbench, there is a tool for merging overlapping reads:
Toolbox | NGS Core Tools () | Merge Overlapping Pairs ()
Select one or more sequence lists with paired end sequencing reads as input. Clicking Next allows you to set parameters as displayed in figure 23.24.
In order to understand how these parameters should be set, an explanation of the merging algorithm is needed: Because the fragment size is not an exact number of base pairs and is different from fragment to fragment, an alignment of the two reads has to be performed. If the alignment is good and long enough, the reads will be merged. Good enough in this context means that the alignment has to satisfy some user-specified score criteria (details below). Because of sequencing errors that typically are more abundant towards the end of the read, the alignment is not expected always to be perfect, and the user can decide how many errors are acceptable. Long enough in this context means that the overlap between the reads has to be non-coincidental. Merging two reads that do not really overlap, leads to errors in the downstream analysis, thus it is very important to make sure that the overlap is big enough. If only a few bases overlap was required, some read pairs will match by chance, so this has to be avoided.
The following parameters are used to define what is good enough and long enough
- Mismatch cost The alignment awards one point for a match, and the mismatch cost is set by this parameter. The default value is 2.
- Gap cost This is the cost for introducing an insertion or deletion in the alignment. The default value is 3.
- Max unaligned The alignment is local, which means that a number of bases can be left unaligned. If the quality of the reads is dropping to be very poor towards the end of the read, and the expected overlap is long enough, it makes sense to allow some unaligned bases at the end. However, this should be used with great care which is why the default value is 0. As explained above, a wrong decision to merge the reads leads to errors in the downstream analysis, so it is better to be conservative and accept fewer merged reads in the result.
- Minimum score This is the minimum score of an alignment to be accepted for merging. The default value is 10. As an example: with default settings, this means that an overlap of 13 bases with one mismatch will be accepted (12 matches minus 2 for a mismatch).
Click Next allows you to select a report to be produced as part of the output. The main result will be two sequence lists for each list in the input: one containing the merged reads (marked as single end reads), and one containing the reads that could not be merged (still marked as paired data). Since the CLC Genomics Workbench handles a mix of paired and unpaired data, both these sequence lists can be used in the further analysis. However, please note that one of the reasons for a pair not to be merged is low quality, so the list of reads that could not be paired is likely to contain more reads with errors than the one with the merged reads.