Continuing in the saga of "what do sequencing errors do to our de Bruijn graph density measure" (read the first post here), I have some new results.
The conclusion of the first post was that on random (non-real) genomes, both with and without repeats, we see that de Bruijn graph connectivity is decreased by random sequencing errors. Zam Iqbal and I had a reasonably robust discussion in the comments, and he suggested trying a real genome. (Yes, it was on my list. But he upped the ante by saying he didn't believe my results were relevant because they weren't real genomes. Fair 'nuff!)
So I applied the make-reads.py and make-reads-biased-random.py scripts to E. coli MG1655, and --
The results are in!
Basically, we see the same effect as with Fig 1 in the last post: when there are more errors in the second half of the read, the average local graph connectivity is lower. Also note that (comparing the Y axis levels in Fig 3 from the last post to Fig 1 above) E. coli isn't very repetitive at all, which we kind of knew.
So, what could be going on?
- E. coli isn't repetitive enough to give us a real test. But I think it does directly address Zam's concern that the polymorphisms in IS elements and other repeats would lead to inadvertent connectivity -- it appears it's not quite that simple.
- What we really need are metagenome-like abundances, which is to say multiple somewhat overlapping genomes with different abundances; this will then supply the necessary graph density increase in the face of random errors. I'll be testing that next.
- Aliens. Some explanation we haven't thought of.
- Our original explanation in the assembly artifacts paper: the sequencer is sticking gunk on the end.
Obviously it's going to be hard to rule out #3, but I think we lay out a pretty strong argument for #4 in the paper, at least once we can rule out #2 and previous.
--titus
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