Moving to the Dark Side

Leaving the Pipette for a Keyboard.

Custom chromosome sizes for pybedtools

I use pybetools a lot in my little script. One issue that I have encountered recentely was when using those scripts with a custom genome, in this case mapping to the transcriptome. One of my scripts calculates using genome_coverage(bg=True, genome=genome) and the argument genome, is an input from the command line.

And what is this genome I speak of? It is a table that informs pybedtools of the sizes of the chromosomes in the reference. Useful among other things to calculate coverage and to avoid extending genomic regions more that the size of the reference chromosome with slop.

What is the problem?

Having to create custom tables for each genome might be annoying. Also, I would like to reduce the number of arguments in my scripts. So while this is not an issue for most application, at some point one will run into annoyances that are part and parcel of custom genomes/annotations/applications.

The solution

There is this thing called samtools idxstats which outputs the chromosome lengths (and read counts per chromosome) for bam files. It just happens that the main input of my scripts tend to be bam files, and that idxstats has been implemented in pysam. This means that with a simple function the chromosome lengths could be derived from the bam without extra hassle (said he before the bugs crept in).

def get_chrom_lengths(path_to_bam):
   Uses pysam to retrieve chromosome sizes form bam.
   Useful helper to use with some pybedtools functions (e.g. coverage), when a bam was mapped with custom genome not available in UCSC.
   Input: path to bam file (should be indexed)
   Output: dictionary.
   Example ouput:
   {'chr4': (0, 1351857), 'chr3L': (0, 24543557), 'chr2L': (0, 23011544), '*': (0, 0), 'chrX': (0, 22422827), 'chr2R': (0, 21146708), 'chr3R': (0, 27905053)}
   import pysam
   idx = pysam.idxstats(path_to_bam).splitlines()
   chromsizes = {}
   for element in idx:
      stats = element.split("\t")
      chromsizes[stats[0]] = (0, int(stats[1]))
   return chromsizes

and a little test will show that the chromosome sizes obtained with this function are equivalent to those retrieved by pybedtools from UCSC (or their database):

import pybedtools
bam = pybedtools.example_filename('x.bam')
pysam.index(bam) # with indexing it will not work

#my function
chromsizes = get_chrom_lengths(bam)

# pybedtools in-built function
dm3 = pybedtools.genome_registry.dm3.euchromatic
print dm3

# test
a = pybedtools.example_bedtool('x.bed')
cov_dm3 = a.genome_coverage(bg=True, genome='dm3')
cov_chrsizes = a.genome_coverage(bg=True, genome=chromsizes)

print cov_dm3.head()
print cov_chrsizes.head()

cov_dm3 == cov_chrsizes

Note: it is working with pysam 0.9.0 but might untested for other versions where the parsing might be different.