Sam's Notebook

Data Analysis - C.virginica BSseq Unmapped Reads Using MEGAN6

by Sam White 2 min read

After performing DIAMOND BLASTx and DAA “meganization” on 20220302, the next step was to import the DAA files into MEGAN6 for analyzing the resulting taxonomic assignments of the Crassostrea virginica (Eastern oyster) unmapped BSseq reads that Steven generated.

“Meganized” DAA files were imported into MEGAN6 (v.6.21.5), via the “Import from BLAST” dialog menu. All paired read DAA files were imported together (but not using the “paired reads” option, since I didn’t provide the corresponding FastQ files for this - no real reason; just didn’t think it necessary for this particulary analysis) using the megan-map-Jan2021.db for Taxonomy. This importation generated a single corresponding RMA6 (i.e one RMA6 file for each pair of DAA files). In retrospect, I should have just converted the DAA files to RMA6 during the DIAMOND BLASTx and DAA “meganization” on 20220302. It would’ve saved a lot of memory-related issues when trying to import many DAA files in a single MEGAN6 session…

After conversion to RMA6, a new “Comparison” was created, which allows visualization of taxonomic assignments across all samples, simultaneously.

RESULTS

Output folder:

It looked like the bulk of reads in all samples (i.e. > 75%) get assigned to the genus Crassostrea (see images at bottom of post).

Wordcloud of taxonomic assignments of reads for each sample (Genus level):

Wordcloud of taxonomic assignments for all unmapped BSseq reads in each sample at the "Genus" level generated using MEGAN6 software, with the Genus "Crassostrea" as the most prominent of all the words present. In fact, all other words so small as to be illegible.

Phylogenetic tree of taxonomic assignments of reads for each sample (Genus level):

Phylogenetic tree of unmapped BSseq reads generated by MEGN6 software showing that most reads in all samples examined are assigned to the Genus "Crassostrea"

And, here’s the tree at the species level to help confirm that there’s not a bunch of C.gigas contamination and that it’s primarily Crassostrea virginica (Eastern oyster):

Phylogenetic tree of unmapped BSseq reads generated by MEGN6 software showing that most reads in all samples examined are assigned to the Species"Crassostrea virginica"

So, not sure why they’re not getting mapped originally.

I’m assuming these came from Bismark? If yes, the manual provides a bit of insight into capturing unmapped reads. Additionally, a comparison of unmapped reads and ambiguous reads might allow a better grasp of what’s happening:

--un

Write all reads that could not be aligned to the file _unmapped_reads.fq.gz in the output directory. Written reads will appear as they did in the input, without any translation of quality values that may have taken place within Bowtie or Bismark. Paired-end reads will be written to two parallel files with _1 and _2 inserted in their filenames, i.e. unmapped_reads_1.fq.gz and unmapped_reads_2.fq.gz. Reads with more than one valid alignment with the same number of lowest mismatches (ambiguous mapping) are also written to unmapped_reads.fq.gz unless --ambiguous is also specified.

--ambiguous

Write all reads which produce more than one valid alignment with the same number of lowest mismatches or other reads that fail to align uniquely to _ambiguous_reads.fq. Written reads will appear as they did in the input, without any of the translation of quality values that may have taken place within Bowtie or Bismark. Paired-end reads will be written to two parallel files with _1 and _2 inserted in their filenames, i.e. _ambiguous_reads_1.fq and _ambiguous_reads_2.fq. These reads are not written to the file specified with --un.