Tagged with #polyploid
1 documentation article | 1 announcement | 0 forum discussions


Comments (4)

Objective

Call variants on a haploid genome with the UnifiedGenotyper, producing a raw (unfiltered) VCF.

Prerequisites

  • TBD

Steps

  1. Determine the basic parameters of the analysis
  2. Call variants in your sequence data

1. Determine the basic parameters of the analysis

If you do not specify these parameters yourself, the program will use default values. However we recommend that you set them explicitly because it will help you understand how the results are bounded and how you can modify the program's behavior.

  • Ploidy (-ploidy)

In its basic use, this is the ploidy (number of chromosomes) per sample. By default it is set to 2, to process diploid organisms' genomes, but it can be set to any other desired value, starting at 1 for haploid organisms, and up for polyploids. This argument can also be used to handle pooled data. For that purpose, you'll need to set -ploidy to Number of samples in each pool * Sample Ploidy. There is no fixed upper limit, but keep in mind that high-level ploidy will increase processing times since the calculations involved are more complex. For full details on how to process pooled data, see Eran et al. (in preparation).

  • Genotype likelihood model (-glm)

This is the model that the program will use to calculate the genotype likelihoods. By default, it is set to SNP, but it can also be set to INDEL or BOTH. If set to BOTH, both SNPs and Indels will be called in the same run and be output to the same variants file.

  • Emission confidence threshold (–stand_emit_conf)

This is the minimum confidence threshold (phred-scaled) at which the program should emit sites that appear to be possibly variant.

  • Calling confidence threshold (–stand_call_conf)

This is the minimum confidence threshold (phred-scaled) at which the program should emit variant sites as called. If a site's associated genotype has a confidence score lower than the calling threshold, the program will emit the site as filtered and will annotate it as LowQual. This threshold separates high confidence calls from low confidence calls.

The terms called and filtered are tricky because they can mean different things depending on context. In ordinary language, people often say a site was called if it was emitted as variant. But in the GATK's technical language, saying a site was called means that that site passed the confidence threshold test. For filtered, it's even more confusing, because in ordinary language, when people say that sites were filtered, they usually mean that those sites successfully passed a filtering test. However, in the GATK's technical language, the same phrase (saying that sites were filtered) means that those sites failed the filtering test. In effect, it means that those would be filtered out if the filter was used to actually remove low-confidence calls from the callset, instead of just tagging them. In both cases, both usages are valid depending on the point of view of the person who is reporting the results. So it's always important to check what is the context when interpreting results that include these terms.


2. Call variants in your sequence data

Run the following GATK command:

java -jar GenomeAnalysisTK.jar \ 
    -T UnifiedGenotyper \ 
    -R haploid_reference.fa \ 
    -I haploid_reads.bam \ 
    -L 20 \ 
    -ploidy 1 
    --glm BOTH \ 
    --stand\_call\_conf 30 \ 
    --stand\_emit\_conf 10 \ 
    -o raw_haploid_variants.vcf 

This creates a VCF file called raw_haploid_variants.vcf, containing all the sites that the UnifiedGenotyper evaluated to be potentially variant.

Although you now have a nice fresh set of variant calls, the variant discovery stage is not over. The distinctions made by the caller itself between low-confidence calls and the rest is very primitive, and should not be taken as a definitive guide for filtering. The GATK callers are designed to be very lenient in calling variants, so it is extremely important to apply one of the recommended filtering methods (variant recalibration or hard-filtering), in order to move on to downstream analyses with the highest-quality call set possible.

Comments (5)

Until now, HaplotypeCaller was only capable of calling variants in diploid organisms due to some assumptions made in the underlying algorithms. I'm happy to announce that we now have a generalized version that is capable of handling any ploidy you specify at the command line!

This new feature, which we're calling "omniploidy", is technically still under development, but we think it's mature enough for the more adventurous to try out as a beta test ahead of the next official release. We'd especially love to get some feedback from people who work with non-diploids on a regular basis, so we're hoping that some of you microbiologists and assorted plant scientists will take it out for a spin and let us know how it behaves in your hands.

It's available in the latest nightly builds; just use the -ploidy argument to give it a whirl. If you have any questions or feedback, please post a comment on this article in the forum.

Caveat: the downstream tools involved in the new GVCF-based workflow (GenotypeGVCFs and CombineGVCFs) are not yet capable of handling non-diploid calls correctly -- but we're working on it.

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