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popTAP

Rationale
Most available tools developed to detect genome-wide copy number variations (CNV) make use of complex algorithms because they are challenged by two issues: the length and location of the CNV is unknown and also the type of CNV to detect is unknown. A review of the topic can be found here: https://doi.org/10.3389/fgene.2015.00138. Transposon absence polymorphisms, however, are a type of CNV in which location, length and variant signature are known beforehand and, therefore, more straightforward algorithms are sufficient to reliably detect this type of CNV.

Pipeline description
This pipeline evaluates transposon absence polymorphisms (TAPs) using pair-end short read illumina data in a pair-wise manner. It evaluates wheter a given individual, a sample, has any TAPs compared to anotherv individual, a control, given an assembly, reference. One can take advantage of short read linbraries used to polish the genome assembly as controls, but it will work to also compare TAPS of one individual relative to another. In this case however, TAPs common to both samples relative to the reference will be not detected.

This tool first calculates GC corrected read depth coverage (RD) per user-defined bin (recommended 10bp) across the whole genome of an array of samples and controls (you can use a common control for all samples). Then, for every TE present in the provided gff annotation file (recommended to exclude TEs smaller than 300 bp), a non-parametric approach is used to compare each sample versus the control using the binned RD information (with a minimum of 30 bins per population if TE < 300 bp were excluded). If the tested TE shows a significant difference and also its median RD shows at least a 10 fold reduction than the median sample genome-wide RD, that tested TE is considered an absence polymorphism for the sample.

Requeriments
To run this pipeline several packages and tools must be installed and be accessible through the $PATH :

  • bwa
  • mosdepth
  • samtools
  • bedtools
  • awk
  • bioawk
  • samblaster
  • vroom (R library)
  • dplyr (R library)

It also requeires a GFF file with only the TE annotation of the reference genome. The reference genome must have been indexed with bwa index.


Example Usage

First Step

Run get_cov_sample.sh for each one of the samples (including controls) as folllows:

bash ./main/get_cov_sample.sh
-g genome_file
-w 10
-f first_pair.fq.gz
-t 8

This will create a file with the extension "depthGCcorrected.per10_bp.bed.gz" with the base name of the provided read pair. Important: This script expects the pair end raw reads to be adaptor free and to have the same name just differing at the following extensions:

  • First mate of the pair: "_1.fastq.gz"
  • Second mate of the pair: "_2.fastq.gz"

Second Step

Run the main script TAP_main.sh for each sample to test (you can use the same control everytime):

bash ./main/TAP_main.sh
-i sample_depthGCcorrected.per10_bp.bed.gz
-r control_depthGCcorrected.per10_bp.bed.gz
-a TE_annotation_gff
-t 300
-S ./main/Wilcoxon_test.R

Note: Because TE_CNV.sh only uses one thread, one can use GNU parallel to speed up this step:

parallel --jobs 8 "bash ./main/TE_CNV.sh -i {} -r control_depthGCcorrected.per10_bp.bed.gz -a TE_annotation.gff
-t 300 -S ./main/Wilcoxon_test.R" ::: *.depthGCcorrected.per10_bp.bed.gz

In this example we run 8 samples at the same time.

Final output

For each sample file run with TE_CNV.sh. Two final tables must have beein created:

  • summarized_output.tsv file with the postion of all tested reference TE plus Pvalues and median calculated coverages.
  • TAPs.tsv same format as previous file but only the reference TEs considered absent in the tested sample are present.