In-dels Illumina vs. PacBio

[slumpy42]

Hello,

I have a PacBio-generated closed complete bacteria genome. I am aligning Illumina reads of the same strain to it using breseq, a pipeline which uses bowtie2 for read mapping.

The alignment finds 100s of insertions, all of them are in homo-polymer runs (AAA...) of nucleotides.

2 questions:

1) Is it known that Illumina produces extra nucleotides in repetitive regions, or that PacBio is too short? Or do they both have In-del problems?

2) How should I deal with this? Just ignore all of them?

Thanks,

[Brian Bushnell]

Illumina does not usually have problems with indels, homopolymer or not. PacBio has a high indel rate in the raw reads and homopolymers are harder to correct, particularly if you don't have enough coverage. You might run quiver to attempt to correct these errors. There are also other programs for correcting assemblies via alignment (ideally using Illumina reads as they generally lack indel errors), but I don't know which is best.

[dgscofield]

We've noticed single-base deletions in PacBio assemblies, via alignment of Illumina reads to otherwise very nice Pacbio assemblies. These occur about every 10^3 to 10^4 bases, and we've also found them to be connected with even quite short homopolymers.

I wrote a little two-stage script to correct these given the BAM of illumina alignments, you can find out more at its repository (below). As Brian writes I'm sure there are other solutions as well.

https://github.com/douglasgscofield/PacBio-utilities

[flxlex]

Try Pilon from the Broad: http://www.broadinstitute.org/software/pilon/

[slumpy42]

Thanks for all your replies and suggestions.

I am going through the PacBio training webinars and they mention that deletions are the most common form of error in the circular consensus reads, but don't give a rate.

I have a 3.2 Mb genome and roughly 5,000 single base insertions. All look like they are in homopolymers.

@Biran_Bushnell : I assembled with HGAP.3, includes quiver as the final step. Should I run it again with different settings?

@dgscofield : Thanks, I'll try your script and Pilon and send you the before and after stats, if that would be helpful for you.