Molecular based taxonomy:
For a couple of centuries, the diversity of life has been organized as taxonomic systems, based on nested clustering [SS73]. This relied mainly as an expertise on key dissimilarities between some phenotypic traits, and systematics was a cornerstone of natural history. More recently, systematics has shifted towards molecular systematics [HMM96], where distances between sequences have been organized along phylogenies, which model the history of speciation in a given group [Fel04]. Slightly later, so called barcoding has been an international effort to normalize sequencing (primers choice) to encompass as much as possible of the diversity of all organisms, using very large and shared databases [HCBd03, HFSa09, PAAe12]. Such an approach is perfectly suited for the geometric view of biodiversity as a replicable procedure, because there exists algorithms for computing distances between two sequences. In this work we focus on a molecular based construction which scales with massive data sets produced by NGS. Diversity characterization is obtained by visualization of the produced point clouds by projection in low dimension spaces, as finding a shape in a high dimensional space still remains an open question.
As soon as molecular diversity has been put in relation with taxonomic diversity, several studies have shown that most of the existing diversity had been hidden to our eyes. This is particularly true for microbial diversity in the oceans [LGRVPAM01, SMH06]. In parallel, metagenomics has emerged as a technique to sequence simultaneously all genomes in a given microbial community. Such a technique has been developed as metabarcoding [HSZ11, BPC12, TCHR12, KFR14, DBC14, JDA14, PLE14] by amplifying and sequencing all the sequences of a given marker of taxonomic interest in a given community. Here, we develop some data analysis on metabarcoding of environmental samples of freshwater diatoms. We have selected this clade as it is one of the clades of microbial organisms for which an expertise exists for optical based taxonomic identification [Man99]. Hence, consistency between morphological based and molecular based taxonomy can be evaluated.
2 Data analysis
As in machine learning, two main families of methods exist to assign taxonomically a set of sequences:
supervised learning, where an unknown sequence is assigned to a taxon by comparison with a reference database, where sequences have been taxonomically annotated
There exists classical and efficient tools for each of this task. BLAST [AGM90] is by far the most used tool for supervised learning. However, there is no equivalent golden standard for unsupervised clustering. It should be nested agglomerating clustering, which suffers from computation load as computing all distances between sequences and performing agglomerative clustering are each quadratic with , i.e. the complexity is in [Mü13]Edg10]. It can handle huge datasets, as it can work in streaming, but it suffers from some flaws, as the fact that the result depends on the order the sequences have been presented. Swarm [MRQ14] has solved some of these issues, but still relies on some heuristics. However, there is no better tool for huge data sets (millions of sequences) for clustering. Here, we compute exact distances between sequences, and run Multidimensional Scaling as a way to visualize the structure of the diversity as given by pairwise distances.
Historically, Multidimensional Scaling has been derived to find configurations of points in Euclidean space of small dimension the shape of which fits as much as possible a set of pairwise distances [Tor52]. However, the machine learning algorithms used for building a point cloud knowing pairwise distances, as well as just computing the distances between all pairs of reads, have a complexity respectively cubic and quadratic to the number of specimens. Such a complexity has been acknowledged as a difficulty for dealing with full matrices of pairwise distances, and several procedures have been designed to circumvent it (see [AK13] and references therein). In this work, we implement an algorithm to circumvent the complexity of the problem by computing pairwise distances on an hyperparallel supercomputer, and by calculating the desired eigenpairs by methods based on random projection [Vem04, HMT11].
2.1 A generic approach for large biological datasets
The pipeline we have designed and implemented is the sequence of following steps:
compute pairwise distances between all pairs of reads,
run Multidimensional Scaling on the distance matrix,
visualize the first components with .
The novelty lies in an implementation on large data sets, i.e., scaling up to data sets produced by NGS sequencing operations, currently reads. Computing pairwise distances between reads is quadratic in , and requires elementary operations, being a constant quadratic in the length of the reads. Running MDS requires computing eigenpairs of a matrix, which can be done in operations. We address this complexity issue by
parallelization of the computation of pairwise distances on an hyperparallel machine (an IBM Blue Gene Q)
using a MDS approach powered by random projection and efficient C++ implementation.
The time needed for each of these steps is given in table 1.
|Sample||compute distances||transfer and convert||subsample||perform MDS|
|(Lake Geneva)||S-W on Turing||iRodsplafrim||reads||full SVD|
2.2 Scalable pairwise distances computation
There are several ways to compare sequences (see e. g. [Gus97, Yan06]). The dissimilarity used here is computed from a local alignment score [Gus97, p. 232] using Smith-Waterman algorithm [SW81]. If the length of the sequences is , the algorithm scales like . As it is quadratic in the number of sequences, computing the whole matrix has a complexity of . We have written a program in C, called disseq, which takes as inputs two fasta files, of length and each, and returns the matrix where is the distance between sequence of first file and of second file. This program has been parallelized with MPI (Message Passing Interface) as a map-reduce process, in a program called mpi-disseq, as all distances can be computed independently. mpi-disseq has been run on a BlueGene Q (IBM) located at IDRIS to compute the matrix of all pairwise distances ( values). Its architecture is made of 6 racks, of 1,024 nodes each, with 16 cores per node, hence 98,304 cores. We have used Turing with cores (one rack). Turing peak power is 1,258 Pflops/s. One advantage of such a choice, beside massive parallelization, is a low energy consumption. Such an architecture is particularly suitable for massive embarrassingly parallel jobs. The program has been tested on Avakas (Mésocentre de Calcul Intensif Aquitain, 264 computing nodes, 12 cores per node), and then ported to Turing. It scales perfectly.
3 A random projection-based spectral decomposition
In this section, denotes the pairwise distance matrix and the Gram matrix built from it (see section 6). Next step is to perform MDS on . The calculation is presented in section Material and Methods. It amounts to perform a SVD of a Gram matrix built from the distance matrix , and of same dimension. In this section we describe an efficient approach for computing the spectral decomposition of the real symmetric matrix at a quadratic cost in . We put the emphasize on the genericity of the approach and its competitive numerical performance (see [Bla17] for details).
3.1 Standard iterative approach
The standard methods used to compute the full eigenvalue decomposition (EVD) of an arbitrary-by- real matrix usually take operations. This complexity makes those methods computationally untractable to large data sets. However, there exists well-known iterative techniques that compute the first eigenpairs at a cost, e.g., Arnoldi or Lanczos algorithm [Sor97]. Despite presenting several flaws, these variants provide an exact spectral decomposition.
3.2 A random projection-based algorithm
Our novel contribution to MDS consists in using low-rank approximation techniques based on random projection, to compute an approximate rank- spectral decomposition of . Random projection, along with random sampling, belongs to a broader class of dimensionality reduction tools known as random sketching [Woo14]. Although most fast approaches to MDS rely on random sampling [Pla05], to our knowledge there are very few contributions to this field that involve random projection if any. Despite involving more intensive computations than random sampling, random projection presents many benefits such as better accuracy, robustness and low variability. Low-rank approximation techniques based on random projection were made popular by a SIAM review paper by Halko et al. [HMT11]. Among theses algorithms, the randomized Singular Value Decomposition (or randomized SVD) computes an approximate rank- SVD of a -by- matrix in operations. This algorithm can be used to compute the first eigenpairs of , by simply remembering that for a symmetric real matrix singular values can be related to the eigenvalues by .
3.3 Numerical performance
This approach represents a significant alternative to standard EVD algorithms, as it shares the same complexity, namely , but usually performs significantly better and parallelizes straightforwardly. However, the input matrix has to fulfill a few criteria for the method to work properly. should be low-rank, i.e., , and have a fast decreasing spectrum or a low stable-rank, i.e., , i.e. a low ratio between the squared Frobenius norm and the squared spectral norm . Finally, as stated in [HMT11]
, even though the algorithm has a non-zero chance to fail, tight Frobenius and spectral error bounds were shown to hold with high probability, ifverifies the aforementioned conditions. -
4 Visualization of a large diatoms sample
The main goal of our approach is to achieve an efficient and handy visualization technique that provides relevant information on the diversity of some real-life samples, currently of about sequences. Due to the longstanding coevolution between clustering and numerical taxonomy, a possible analysis of the shape of the point cloud consists in identifying clusters. Clustering is an immense domain, and we will not enter here into the discussion for the best methods knowing a pairwise distance matrix: this is ongoing work. We focus in this report on studying the concentration of the reads in some subsets in the projection on the first few dimensions of MDS.
4.1 Dimensionality reduction
When the dimension of a space is large, the so called curse of dimensionality enters into the game. In brief, it can be summarized by telling that there is much room in a high-dimensional space. For example, two opposite vertices of an hypercube are at distance (i.e. the distance between two points in a symmetric body of unit volume can be arbitrarily large), whereas the volume of the hypersphere of radius one shrinks to zero: the probability to have a neighbor at distance one or less is negligible [Ize08, Wan12]. This can impact any algorithm based on distances. Therefore, a first step is often to reduce the dimension of the problem, by building a mapping of the original point cloud on a Euclidean space of far lower dimension. This is called dimensionality reduction, and is a key step in machine learning [Ize08, LV07, Mur12]. MDS is the tool for visualizing the geometry of the point cloud associated to distances between reads in low dimension. Hence we present such a visualization by projection on first axis of MDS, and provide some hints for interpretation by a comparison with supervised clustering. We do not report here on unsupervised clustering, which is deferred to further work. We have worked with a full sample named (, see sample description in Materials and Methods section). We have built the associated point cloud by mean of a rank randomized SVD. Here, we show representations of the full sample in 3D for easy visualization of the point cloud. In such a representation, each point represents a read. We have used a supervised clustering technique (a program called diagno-syst, see [FRB16]) to assess a taxonomical name to each read for which it has been possible, to test whether the clusters were related to species. Then, we discuss the visualization of the cloud in higher dimensions.
4.2 Reads concentration in low dimension
Because of the number of points, several reads are projected on the same pixel in regions with high concentration of reads in first axis. We have quantified this by hexagonal binning: the plane formed by axis and (with ) is tessalated by hexagons. Then, the number of points per hexagon is counted, and these figures are displayed, in logarithmic scale. This is presented in figure 1. In particular, this figure shows that only very specific regions of the full domain contain most of the individuals and that they are surrounded by regions of far lower density. The pattern is an archipelago of islands of high density surrounded by an ocean of low density. It may be tempting to associate high density islands to clusters and clusters to species.
4.3 Interpretation with supervised classification
In order to validate our approach, and test whether high density islands correspond to reads belonging to a same species, with ideally a one to one correspondence between islands and species, we have colored the points according to an information provided by supervised clustering on all the reads. Indeed, we have at disposal a reference database for diatoms, which is fairly good for Geneva lake [RCK16]. We have computed all pairwise distances between the reads in our sample , and the taxonomically annotated reads in the reference database . This is intensive computing too, which has been executed on Turing, with MPI-disseq as well. Then, we have selected an homology gap (classically 97%), and, for each read in our sample, selected all reads in the reference database which were at distance lower than the homology gap. If they all belong to a same species, the read has been annotated with this species name. If not, it has been annotated as ambiguous status. This has been done with a program called diagno-syst, as a work available in [FRB16]. Then, we have colored the individuals w.r.t. their species as identified by supervised clustering in different colors for different species. (see figure 2). We have checked as well that the species identified this way in the community have been identified optically too. This supports a consistency between morphological based and molecular based taxonomy.
4.4 Representation in many dimensions
5 Conclusions and perspectives
We have implemented Multidimensional Scaling of the matrix which contained exact distances between all pairs or reads of an environmental sample of reads. This has permitted to visualize the points cloud associated to those distances in low dimension spaces. We have shown that points were clustered in islands of high density regions forming an archipelago within an ocean of low density regions. We have shown by supervised classification that some clusters could be associated to those species which were optically identified in the sample and in the reference database. This establishes consistency between supervised and unsupervised approaches, and paves the way for using unsupervised methods when taxonomic information for supervised approaches is lacking, which will greatly expand the scope of these methods.
It can be observed as well that some islands could not be assigned to species by supervised clustering. It may be tempting to hypothesize that those clusters correspond to species which are present in the sample, but not in the reference database. More precisely, some parts of the cloud contains unknown individuals but still exhibits a certain structure and a significant density, which suggests that a deeper analysis of the cloud is possible. Supervised method permit a dictionary between molecular based patterns as derived in metabarcoding and previous patterns in biodiversity studies as derived in Natural History. However, one of the most critical shortcoming of supervised methods is that reference databases will never be complete, because of the tremendous number of species, most of them still unknown. Hence, there is an urgent need for unsupervised methods, which are consistent with supervised methods when taxonomic information is available. We have proposed here such a method by an analysis of high density regions in projection in a low dimension space. Nest step may be the use of image analysis techniques to identify and characterize those high-density islands.
Classically, unsupervised clustering of reads for metabarcoding is done by greedy methods, mentioned in the introduction of section 2 (a couple of them are included as choices in Mothur [SWR09] or Qiime [CKS10]). Once again, those methods rely on heuristics, and their accuracy are not assessed for very large datasets. Many different exact methods can be implemented provided one accepts to invest in computation load for a better accuracy. One can mention a few: nested aggregative clustering [Mü13], selecting a threshold and build a graph with being the set of reads, and , and build the connected components of (this is linear in time with
), or more elaborate methods on the same graph like spectral clustering to derive communities[vL07, GN02].
Next steps are not only to compare, but to associate all these methods and tools, in order to build Operational taxonomic Unit with best accuracy on very large data sets, as consistent as possible with our knowledge of the diversity of life as we have inherited it from centuries of studies in Natural History, and as automatized and sound as possible for exploring the unknown diversity.
6 Material and methods
6.1 A sample from Lake Geneva
We have considered 10 environmental samples, denoted , that were collected from Lake Geneva at about monthly intervals at times between April 2012 and March 2013 in order to investigate a seasonal dynamics. Amplicons of chloroplastic marker rbcL have been produced for each sample by DNA extraction, amplification, and sequencing on a Ion Torrent PGM (see [KFR13] for protocols). The various samples contain from about to reads. The diversity of each sample has been assessed both optically, and by supervised clustering of reads by mapping on a dedicated reference database (see [RCK16] for the reference database, and [FRB16] for the algorithm for mapping reads, and its implementation.) A fraction only of diversity can be assessed with these classical tools.
6.2 Multidimensional Scaling
The method implemented here is classical multidimensional scaling111There is another procedure bearing the same name, called nonmetric MDS due to Kruskal [Kru64], based on an optimization scheme, which currently cannot be implemented for more than a few thousands items. (see [BG05, Ize08] for a recent survey, and [CC01] for a seminal monograph) which follows Torgerson’s work [Tor52]. Let us have a set of items with , and a distance between items and . Then, is a finite metric space. For a given dimension , classical MDS is finding a map such that is as close as possible to . If the distances are such that the corresponding Gram matrix is definite positive, the solution is well known [CC01], and is implemented in three steps
The scalar product can be computed from the distances only, as
The scalar products are the elements of the Gram matrix with
compute the eigenvalues and eigenvectors of Gram matrix
(As is a Gram matrix, all its eigenvalues are non negative, as there is a matrix such that )
Let us denote where is the diagonal matrix with on its diagonal. Then compute
Then, the best representaton of as distances within a point cloud in embedded with standard inner product is , which is the extraction of the first columns of . is the th row of . The best rank approximation of is .
Two observations can be made:
In general, not all eigenvalues of are non negative. If all eigenvalues are non negative, then
can be embedded isometrically in a Euclidean space, and a best low dimensional approximation can be computed as the Principal Component Analysis of the point cloud[MKB79]. The condition can be seen directly on the distance matrix, and involves signs of Cayley-Menger determinents (see [LLMM14, thrm 2.1, p. 15]).
As far as calculation is concerned, step 2 is the most demanding: computing eigenpairs of the Gram matrix. We have implemented here a procedure relying on random projection (see [Vem04] for a survey).
Computation at IDRIS has been supported by DARI project Biodiversiton i2015037360. Experiments presented in this paper were carried out using the PlaFRIM experimental testbed, being developed under the Inria PlaFRIM development action with support from LABRI and IMB and other entities: Conseil Régional d’Aquitaine, FeDER, Université de Bordeaux and CNRS (see https://plafrim.bordeaux.inria.fr/). Sequencing has been performed at the Genome Transcriptome Facility of Bordeaux (grants from the Conseil Régional d’Aquitaine n°20030304002FA and 20040305003FA, from the European Union FEDER n°2003227 and from Investissements d’Avenir ANR-10-EQPX-16-01) and with support of ONEMA project on Mayotte. Humid lab (DNA extraction, PCR) have been done at UMR Carrtel, under guidance of Agnès Bouchez, with support of ONEMA project Mayotte. We acknowledge an Investissement d’Avenir grant of the Agence Nationale de la Recherche (CEBA: ANR-10-LABX-25-01). We acknowledge the nice atmosphere of network R-Syst, and support by INRA divisions EFPA and SPE, for methodological developments on metabarcoding, as well as its database infrastructure.
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