Noise and Robustness in Phyllotaxis

A striking feature of vascular plants is the regular arrangement of lateral organs on the stem, known as phyllotaxis. The most common phyllotactic patterns can be described using spirals, numbers from the Fibonacci sequence and the golden angle. This rich mathematical structure, along with the experimental reproduction of phyllotactic spirals in physical systems, has led to a view of phyllotaxis focusing on regularity. However all organisms are affected by natural stochastic variability, raising questions about the effect of this variability on phyllotaxis and the achievement of such regular patterns. Here we address these questions theoretically using a dynamical system of interacting sources of inhibitory field. Previous work has shown that phyllotaxis can emerge deterministically from the self-organization of such sources and that inhibition is primarily mediated by the depletion of the plant hormone auxin through polarized transport. We incorporated stochasticity in the model and found three main classes of defects in spiral phyllotaxis – the reversal of the handedness of spirals, the concomitant initiation of organs and the occurrence of distichous angles – and we investigated whether a secondary inhibitory field filters out defects. Our results are consistent with available experimental data and yield a prediction of the main source of stochasticity during organogenesis. Our model can be related to cellular parameters and thus provides a framework for the analysis of phyllotactic mutants at both cellular and tissular levels. We propose that secondary fields associated with organogenesis, such as other biochemical signals or mechanical forces, are important for the robustness of phyllotaxis. More generally, our work sheds light on how a target pattern can be achieved within a noisy background

Establishing human leukemia xenograft mouse models by implanting human bone marrow-like scaffold-based niches

To begin to understand the mechanisms that regulate self-renewal, differentiation, and transformation of human hematopoietic stem cells or to evaluate the efficacy of novel treatment modalities, stem cells need to be studied in their own species-specific microenvironment. By implanting ceramic scaffolds coated with human mesenchymal stromal cells into immune-deficient mice, we were able to mimic the human bone marrow niche. Thus, we have established a human leukemia xenograft mouse model in which a large cohort of patient samples successfully engrafted, which covered all of the important genetic and risk subgroups. We found that by providing a humanized environment, stem cell self-renewal properties were better maintained as determined by serial transplantation assays and genome-wide transcriptome studies, and less clonal drift was observed as determined by exome sequencing. The human leukemia xenograft mouse models that we have established here will serve as an excellent resource for future studies aimed at exploring novel therapeutic approaches

Pollination ecology and reproductive success in Jack-in-the Pulpit (Arisaema triphyllum) in Quebec (Canada)

International audienc

Pollination ecology and reproductive success in Jack-in-the Pulpit (Arisaema triphyllum) in Quebec (Canada)

International audienc

Sedimentation of large particles in a suspension of colloidal rods

The sedimentation at low Reynolds numbers of large, non-interacting spherical inclusions in networks of model monodisperse, slender colloidal rods is investigated. The influence of rod concentration, rod length, and inclusion stress on the inclusion’s creeping motion is investigated. The decrease in sedimentation speeds as a function of rod concentration is compared to the Stokes law, using the zero-shear viscosity from the Doi–Edwards theory for semi-dilute colloidal rod solutions. The experimental speeds display the same concentration dependence as the zero-shear viscosity and are, thus, strongly dependent on the rod length. The speed is, however, a fraction of 2 and 4 lower than expected for rods of 0.88 μm and 2.1 μm, respectively. The results for both rod lengths superimpose when plotted against the overlap concentration, hinting at an extra dependence on the entanglement