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Old 09-15-2017, 04:10 AM   #5
tomlodz
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Location: Poland

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Thank you for your answer.

Quote:
Originally Posted by nettybetty View Post
I think a lot of the run difference can be explained by as you state: issues with quantification. You've included the MiSeq run stats, but I think there's a more important question - what did the library QC looks like (BioAnalyser/Gel)?Ancient DNA is massively variable in terms of quality and quantity. Were you able to visualise the DNA in extracts to determine mean size length prior to library prep? Often there is high molecular weight DNA in aDNA samples as well. A large variation in size of the library will impact qPCR and clustering. You should be able to easily see an ancient DNA library on a BioAnalyser chip post-PCR.
I realize that the quality of aDNA and its length may be a problem here. I visualized the library after indexing on PAGE gel electrophoresis. The maximal length of inserts seemed to be about 500-600 bps depending on the age of the material. I'm talking here about the ones that were visible on the gel. There was a strong smear between 140 - 600 bp. Certainly there were longer fragments of modern DNA contamination present. Is there a way to get rid of them without losing a large part of the library. The second question is whether the larger DNA fragments have a very strong impact on the sequencing reaction itself.

Quote:
Originally Posted by nettybetty View Post
Endogenous DNA in truly ancient samples is always very low unless you enrich for it. The first Neanderthal genome sequence runs were around 90% microbial.

As a general rule - read 2 (second seq read) also is nearly always rubbish in ancient DNA for me on a HiSeq. I maybe get 20-30 bases before it turns to rubbish. I do know of a few possible reasons for this but won't go into detail to save text.
My pre-enrichment library had about 1000x more copies than after this operation. The obtained readings was mapped in most cases to the mitochondrial genome rather than the nuclear genome. After this I conclude the enrichment reaction was OK.

Quote:
Originally Posted by nettybetty View Post
What is the concentration of adapters are you using? If there's not much DNA then the advised adapter concentration should be dropped to prevent dimers and artefact. Are you sure your aDNA is still double stranded? Meyer et al found a lot of single stranded DNA in their really old material and actually designed a new protocol to be able to sequence this. If your aDNA is really damaged it won't work very with the t4 ligation.
I adjust the adapter concentration to the aDNA concentration. In this experiment I used material from several historical periods whose degree of preservation should be sufficient to create a library. Oldest sample was about 6000 years old.

Quote:
Originally Posted by nettybetty View Post
This protocol you are using is 7 years old and was designed for modern material (and derived from Meyer's et al. work on the 454). Are you changing anything e.g. this protocol shears the samples which is not advisable for most ancient DNA> A lot of this is no longer standard, even for modern material. Meyer and others have updated protocols to work much better for ancient DNA. You can also create ancient DNA libraries that don't require custom sequencing primer thus immediately removing a huge component of variability. And on that note - was your sequencing primer HPLC purified?
I searched the literature for what methods other teams use, and most use the ones mentioned here with minor modifications. There are more sophisticated methods to the aDNA material whose preservation state is very bad. Material used here I was able to investigate with the standard Sanger method and PCR amplification (186 bp). All used oliginucleotides were HPLC purified.

Quote:
Originally Posted by nettybetty View Post
A 2x250bp run will result in most ancient DNA sequencing reads being full of adapter. Again - look at the library distribution in QC. Adapter sequence will mess up quality due to being low diversity etc. You're also paying for sequencing you're just going to throw out. On a Hiseq people will wear this when it's only a lane or two, but a MiSeq enables you to choose read lengths and customise a lot better and cut down work on the bioinformatics end.
In the first run I actually had a lot of adapter dimmers because of failed blunting reaction of aDNA. In the second run adapter-dimers were present but not in such large numbers. After the reaction I removed the adapters present in the readings.

P.S.
In the second run the intensity was very low. What factors have the greatest influence on this parameter ?
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