Classifying signatures: search, gather, and lca methods.

sourmash provides several different techniques for doing classification and breakdown of signatures.

Breaking down metagenomic samples with gather and lca

Neither search option (similarity or containment) is effective when comparing or searching with metagenomes, which typically have a mixture of many different genomes. While you might use containment to see if a query genome is present in one or more metagenomes, a common question to ask is the reverse: what genomes are in my metagenome?

We have implemented two approaches in sourmash to do this.

One approach uses taxonomic information from e.g. GenBank to classify individual k-mers, and then infers taxonomic distributions of metagenome contents from the presence of these individual k-mers. (This is the approach pioneered by Kraken and used by many other tools.) sourmash lca can be used to classify individual genome bins with classify, or summarize metagenome taxonomy with summarize. The sourmash lca tutorial shows how to use the lca classify and lca summarize commands, and also provides guidance on building your own database.

The other approach, gather, breaks a metagenome down into individual genomes based on greedy partitioning. Essentially, it takes a query metagenome and searches the database for the most highly contained genome; it then subtracts that match from the metagenome, and repeats. At the end it reports how much of the metagenome remains unknown. The basic sourmash tutorial has some sample output from using gather with GenBank. See the appendix at the bottom of this page for more technical details.

Some benchmarking on CAMI suggests that gather is a very accurate method for doing strain-level resolution of genomes. More on that as we move forward!

To do taxonomy, or not to do taxonomy?

By default, there is no structured taxonomic information available in sourmash signatures or SBT databases of signatures. Generally what this means is that you will have to provide your own mapping from a match to some taxonomic hierarchy. This is generally the case when you are working with lots of genomes that have no taxonomic information.

The lca subcommands, however, work with LCA databases, which contain taxonomic information by construction. This is one of the main differences between the sourmash lca subcommands and the basic sourmash search functionality. So the lca subcommands will generally output structured taxonomic information, and these are what you should look to if you are interested in doing classification.

The command lca gather applies the gather algorithm to search an LCA database; it reports taxonomy.

It’s important to note that taxonomy based on k-mers is very, very specific and if you get a match, it’s pretty reliable. On the converse, however, k-mer identification is very brittle with respect to evolutionary divergence, so if you don’t get a match it may only mean that the particular species isn’t known.

Abundance weighting

By default, sourmash tracks k-mer presence, not their abundance. The proportions and fractions reported also ignore abundance. So, if sourmash gather reports that a genome is 5% of a metagenome, it is reporting Jaccard containment of that genome in the metagenome, and it is ignoring information like the number of reads in the metagenome that come from that genome. Similarly, when sourmash compare compares genome or metagenome signatures, it’s reporting Jaccard similarity without abundance.

However, it is possible to take into account abundance information by computing signatures with --track-abundance. The abundance information will be used if it’s present in the signature, and it can be ignored with --ignore-abundance in any signature comparison.

There are two ways that abundance weighting can be used. One is in containment queries for metagenomes, e.g. with sourmash gather, and the other is in comparisons of abundance-weighted signatures, e.g. with sourmash search and sourmash compare. Below, we refer to the first as “abundance projection” and the second as “angular similarity”.

Projecting abundances in sourmash gather:

sourmash gather can report approximate abundance information for containment queries against genome databases. This will give you numbers that (approximately) match what you get from counting mapped reads.

If you compute your input signatures with --track-abundance, both sourmash gather and sourmash lca gather will use that information to calculate an abundance-weighted result. This will weight each match to a hash value by the multiplicity of the hash value in the query signature. You can turn off this behavior with --ignore-abundance.

For example, if you have a metagenome composed of two equal sized genomes A and B, with A present at 10 times the abundance of B, gather on abundance-weighted signatures will report that approximately 91% of the metagenome is A and approximately 9% is B. (If you use --ignore-abundance, then gather will report approximately 50:50, since the genomes are equal sized.)

You can also get count-like information from the CSV output of sourmash gather; the column median_abund contains the median abundance of the k-mers in the match to the given genome.

Buyer beware: There are substantial challenges in doing this kind of analysis on real metagenomic samples, relating to genome representation and strain overlap; see this issue for a discussion.

Computing signature similarity with angular similarity.

If signatures that have abundance information are compared with sourmash search or sourmash compare, the default comparison is done with angular similarity. This is a distance metric based on cosine similarity, and it is suitable for use in clustering.

For more information on the value of this kind of comparison for metagenomics, please see the simka paper, Multiple comparative metagenomics using multiset k-mer counting, Benoit et al., 2016. Initial comparisons of metagenome similarity approximations computed with sourmash to the output of simka suggest a significant correlation.

Implementation note: Angular similarity searches cannot be done on SBT or LCA databases currently; you have to provide lists of signature files to sourmash search and sourmash compare. sourmash will provide a warning if you run sourmash search on an LCA or SBT with an abundance-weighted query, and automatically apply --ignore-abundance.

What commands should I use?

It’s not always easy to figure that out, we know! We’re thinking about better tutorials and documentation constantly.

We suggest the following approach:

  • build some signatures and do some searches, to get some basic familiarity with sourmash;

  • explore the available databases;

  • then ask questions via the issue tracker and we will do our best to help you out!

This helps us figure out what people are actually interested in doing, and any help we provide via the issue tracker will eventually be added into the documentation.

Appendix: how sourmash gather works.

The sourmash gather algorithm works as follows:

  • find the best match in the database, based on containment;

  • subtract that match from the query;

  • repeat.

The output below is the CSV output for a fictional metagenome.

The first column, f_unique_to_query, is the fraction of the database match that is unique with respect to the original query. It will always decrease as you get more matches.

The second column, f_match_orig, is how much of the match is in the original query. For this fictional metagenome, each match is entirely contained in the original query. This is the number you would get by running sourmash search --containment <match> <metagenome>.

The third column, f_match, is how much of the match is in the remaining query metagenome, after all of the previous matches have been removed.

The fourth column, f_orig_query, is how much of the original query belongs to the match. This is the number you’d get by running sourmash search --containment <metagenome> <match>.

f_unique_to_query      f_match_orig  f_match                f_orig_query
0.3321964529331514     1.0           1.0                    0.3321964529331514
0.13096862210095497    1.0           1.0                    0.13096862210095497
0.11527967257844475    1.0           0.898936170212766      0.12824010914051842
0.10709413369713507    1.0           1.0                    0.10709413369713507
0.10368349249658936    1.0           0.3134020618556701     0.33083219645293316

A few quick notes for the algorithmic folk out there –

  • the key innovation for gather is that it looks for groups of k-mers in the databases, and picks the best group (based on containment). It does not treat k-mers individually.

  • because of this, gather does not saturate as databases grow in size, and in fact should only become more sensitive and specific as we increase database size. (Although of course it may get a lot slower…)