A SNP scavenger hunt: finding SNPs in evolved Campylobacter jejuni

Brief overview: JP Jerome in the Mansfield lab performed an experimental evolution experiment with the bacterium Campylobacter jejuni, an enteric pathogen: he took a freezer strain (11168; see the genbank record) and passaged it through mice three times. During these passages it dramatically increased in virulence. JP then sequenced the pre-passage population and the post-passage population using Illumina short-read sequencing. We would like to provide JP with a set of loci that have changed during passage and could underlie the increase in virulence.

Note that Campylobacter is haploid. It also has an extremely high mutation rate in which “contingency loci”, poly-G tracts, expand and contract. Because of this there is no such thing as a clonal population of C. jejuni, but rather only a population “cloud”.

Your goal is to identify genomic locations that have changed during this passage, i.e. find locations where there is a real difference between pre-passage and post-passage genomes. Note that no structural variation (large-scale rearrangements, copy number variation, etc.) was detected by gel electrophoresis, so you’re only looking for single nucleotide changes (SNPs) and insertion-deletion characters (indels).

Be sure to consider how you might detect systematic bias, and whether or not a particular SNP is evolutionarily plausible. You should think about how you might systematize the process, too.

Working with the data

We’ve provided a bunch of mappings with a wide range of parameters under ‘/mnt’ on the two computers ‘ec2-75-101-241-129.compute-1.amazonaws.com’ and ‘ec2-184-72-194-33.compute-1.amazonaws.com’, username ‘root’, password ‘891’. Each directory, ‘mapping-*‘, contains a set of ‘pre’ and ‘post’ mapping files; you can use e.g. ‘tview’ (as in the Visualizing mappings with Samtools) to view them:

cd /mnt/mapping-n1
samtools tview pre.map.sorted.bam ../data/campy.fa


samtools tview post.map.sorted.bam ../data/campy.fa

At the bottom of this page, I’ve provided a list of locations for you to examine. There may be other “interesting” locations that you should feel free to explore, but these encompass the types of variation in the genome.

Looking at the data

The primary way to look at the data is through the use of ‘samtools tview’. For example, to look at the results of a default parameter mapping of the pre-passage reads to the Campy genome, use:

cd /mnt/default
samtools tview pre.map.sorted.bam ../data/campy.fa

To look at the mapping of the post-passage reads, look at post.map.sorted.bam:

samtools tview post.map.sorted.bam ../data/campy.fa

You can get a list of different mappings by doing:

ls -1d /mnt/mapping-*

and the individual mapping parameters are in the file ‘parameters’.

A list of locations

Our C. jejuni strain is approximately 1.5 mb in size, with a single chromosome named ‘campy_genome’.

Please try to classify some or all of the following locations as real SNP difference pre/post, real indel, sequencing error/bias, no variation.



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