Estimate genome size automatically

[tseemann]

In my assembler shovill i estimate the genome size from kmer frequencies and use that to subsample the reads to a fixed coverage (100x) much like rasusa does:

https://github.com/tseemann/shovill/blob/master/bin/shovill#L145-L175

Would you consider adding --genome-size auto to rasusa ?

For nanopore data one would want k<=15 and for illumina higher is better, say 24-32. For illumina, the number of kmers with freq > 2 is a good estimate normally. Not sure about nanopore.

[mbhall88]

This is a good suggestion.

One question I would have about this is how many extra parameters do you think this would be likely to add? i.e would I need a read technology-related flag, plus the ability for people to vary the kmer size?

[tseemann]

I would just try with k=15 and see how it goes. For illumina data i would just use R1 and ignore R2 as it is noisier and won't add any information.

# actual genome size = 6092523
# R1 of illumina 150 PE data

% mash sketch -m 3 -k {13 15 17 19 21 25 29 32}

Estimated genome size: 5.26142e+06
Estimated genome size: 5.87121e+06  k=15
Estimated genome size: 5.91182e+06
Estimated genome size: 6.31256e+06 k = 19
Estimated genome size: 5.99984e+06
Estimated genome size: 6.54007e+06 k = 25
Estimated genome size: 6.19869e+06
Estimated genome size: 6.04152e+06 k=32

With -m 2 you get larger estimates. But usually all within 10%

[mbhall88]

Neat. Thanks for this @tseemann . I will add it to the features planned for version 0.2.0

[mbhall88]

So I've done a lot of reading about different methods for estimating genome size and I now feel more confused than I was beforehand. I played around with a few things but couldn't get anything working. The main issue stems from ploidy. Estimating genome size for haploids is much simpler than for non-hapolid and I don't want to support one form and not the other.

I'm happy for someone to work on this if they like (I had the thought of using finch-rs for the k-mer work) so I will leave this open - but I am unlikely to come back to this unless it becomes crucial for my own research. Requiring the genome size from the user doesn't seem too big an ask for this particular task.

[tomazberisa]

One potential approach here could be to enable providing a faidx index of the reference genome (instead of --genome-size) and use it to calculate genome size.

[mbhall88]

You mean just summing the lengths of all sequences in the index?

[tomazberisa]

Yes, that is the approach.

[mbhall88]

@tomazberisa that should be easy enough to incorporate. See #31