Section: New Results

A Geometric View of Diversity

Diversity may be understood as a set of dissimilarities between objects. The underlying mathematical construction is the notion of distance. Knowing a set of objects, on which pairwise distances can be measured, it is possible to build a Euclidean image of it as a point cloud in a space of relevant dimension. The objects under study are microbial communities, given as a set of reads produced by NGS technologies. Distances between specimen are computed as genetic distances between associated reads (so called amplicon approach). Then, the diversity of a community can be associated with the shape of the point cloud built from such distances. Such an embedding is classically implemented by MDS (Multidimensional Scaling). Such an approach triggers two methodological questions, addressed in 2016:

• the numerical solution is through finding the eigenvectors and eigenvalues of a large, full, symmetric matrix. Current algorithm, parallelized or not, are in complexity $𝒪\left(n^3\right)$ if $n$ is the number of specimen on which to study patterns of diversity, i.e. the size of the matrix. This is not feasible for dataset produced by NGS technologies, which can assemble ${10}^5$ to ${10}^6$ sequences. We have set up a collaboration with a PhD student in HIEPACS Team (Inria Bordeaux SO), Pierre Blanchard, to develop a connection between random projection methods and MDS. Random Projection Methods are methods relying on Johnson-Lindenstrauss Lemma, which states that the likelihood that the distances are very well conserved is very high when projecting a point cloud in a space of very large dimension (say $n$) to a random space of large dimension (say, proportional to $\mathrm{Log}n$) . This permits to compute eigenvectors and eigenvelaues in space of reasonable dimension. The method for MDS has been studied by Pierre Blanchard, under supervision by Olivier Coulaud and Alain Franc, and proved to be surprisingly efficient and precise. This work builds one chapter of the PhD thesis of P. B. to be defended by early 2017. This collaboration has lead to a joint poster at Platform for Advanced Scientific Computing (PASC), June 2016, Lausanne, Switzerland.

• The eigenvalues of the matrix under study can be positive or negative. Positive eigenvalues lead to Euclidean structure behind MDS. Classically, negative eigenvalues are ignored. We have begun a study on the role of negative eigenvalues in the discrepancy between Euclidean distances computed between points in MDS, and genetic distances between reads produced by NGS, which adds to the well understood discrepancy in MDS due to dimension reduction. This has lead to three seminars or presentations:

  • a seminar at MIAT research Unit, Toulouse, on February 19

  • a seminar at LABRI on April, 28

  • a presentation at the days of mathematics and computing sciences organized by MIA INRA division (INRA global meeting), on October, 5

  These three events have permitted to “polish” the analysis of the problem through several exchanges, and to orientate its study towards quadratic embedding, or isometry into pseudo-euclidean spaces.