arXiv subjects

Claude Loverdo

Publications and source records attributed to Claude Loverdo.

9 recordsLinked to original sources

arXiv2026

Energetics, shearing and pumping efficiency of propagating contractions over villi-patterned wall

Intestinal villi undergo pendular-wave motility -- an active, propagating tissue motion driven by underlying longitudinal muscles. This motility drives irreversible, counter-wave fluid pumping, akin to the antiplectic metachrony of ciliary carpets, and generates a viscous mixing boundary layer above the villi tips, whose height is controlled by flow inertia. Using a simplified 2D model of the rat duodenum, we quantify the system's viscous energy dissipation and axial pumping efficiency. In contrast to the classical Stokes' second problem, we show that the fluid volume dominating energy dissipation is dictated by the intervillous geometry, remaining insensitive to the dynamically varying viscous mixing boundary layer height. The computed pumping efficiency is orders of magnitude lower than that of canonical peristalsis for equivalent flux pumping. We thus infer that bulk fluid pumping is not the primary biophysical function of propagating pendular-wave motility; instead, we postulate that its main role is to shear the mucus barrier layer over the villi-lined mucosa. Comparing the strain rate in the barrier region with canonical peristaltic reference values for a villi-free wall strongly supports our hypothesis. Finally, for biomimetic microfluidic applications, geometric optimization reveals that pumping efficiency scales quadratically with the channel-to-villi height ratio in Stokes flow, whereas in the inertial regime, dynamic flux confinement renders this geometric optimization strategy redundant.

physics.flu-dyn↗

arXiv2025

Environment heterogeneity creates fast amplifiers of natural selection in graph-structured populations

Complex spatial structure, with partially isolated subpopulations, and environment heterogeneity, such as gradients in nutrients, oxygen, and drugs, both shape the evolution of natural populations. We investigate the impact of environment heterogeneity on mutant fixation in spatially structured populations with demes on the nodes of a graph. When migrations between demes are frequent, we find that environment heterogeneity can amplify natural selection and simultaneously accelerate mutant fixation and extinction, thereby fostering the quick fixation of beneficial mutants. We demonstrate this effect in the star graph, and more strongly in the line graph. We show that amplification requires mutants to have a stronger fitness advantage in demes with stronger migration outflow, and that this condition allows amplification in more general graphs. As a baseline, we consider circulation graphs, where migration inflow and outflow are equal in each deme. In this case, environment heterogeneity has no impact to first order, but increases the fixation probability of beneficial mutants to second order. Finally, when migrations between demes are rare, we show that environment heterogeneity can also foster amplification of selection, by allowing demes with sufficient mutant advantage to become refugia for mutants.

q-bio.PE↗

arXiv2025

Intestinal villi and crypts density maximizing nutrient absorption

The villi and crypts of the gastrointestinal tract increase the effective surface area of the intestinal mucosa, potentially enhancing nutrient absorption. It is commonly assumed that this is their primary function, and that a higher villi density necessarily leads to improved absorption. However, when villi are packed too closely together, diffusion can be hindered, potentially offsetting this benefit. In this work, we investigate the relationship between the density of these structures and the overall efficiency of absorption. In three different simplified geometries, approximating crypts, leaf-like villi, and finger-like villi we calculate analytically the concentration profile and the absorption flux, assuming that there is only diffusion between these structures while the lumen is well mixed. When plotting the absorption flux per unit of gut length as a function of the structures' density, we observe that there is a density maximizing absorption. We study numerically this optimum. It depends weakly on the absorption properties of the given nutrient, so that a geometry optimal for one nutrient is close to optimum for another nutrient. Physiological data from various animal species align with this predicted optimal range and potentially reflect evolutionary selection for efficient nutrient uptake, supporting the model's validity.

q-bio.TO↗

arXiv2025

Hydrodynamics in a villi-patterned channel due to pendular-wave activity

Inspired by small intestine motility, we investigate the flow induced by a propagating pendular-wave along the walls of a channel lined with rigid, villi-like microstructures. The villi undergo harmonic axial oscillations with a phase lag relative to their neighbours, generating travelling patterns of intervillous contraction. Using two-dimensional lattice Boltzmann simulations, we resolve the flow within the villi zone and the lumen, sampling small to moderate Womersley numbers. We uncover a mixing boundary layer (MBL) just above the villi, composed of semi-vortical structures that travel with the imposed wave. In the lumen, an axial steady flow emerges, surprisingly oriented opposite to the wave propagation direction, contrary to canonical peristaltic flows. We attribute this flow reversal to the non-reciprocal trajectories of fluid trapped between adjacent villi, and derive a geometric scaling law that captures its magnitude in the Stokes regime. The MBL thickness is found to depend solely on the wave kinematics given by intervillous phase lag in the low-inertia limit. Above a critical threshold, oscillatory inertia induces dynamic confinement, limiting the radial extent of the MBL and leading to non-monotonic behaviour of the axial steady flux. We further develop an effective boundary condition at the villus tips, incorporating both steady and oscillatory components across relevant spatial scales. This framework enables coarse-grained simulations of intestinal flows without resolving individual villi. Our results shed light on the interplay between active microstructure, pendular-wave and finite inertia in biological flows, and suggests new avenues for flow control in biomimetic and microfluidic systems.

physics.flu-dyn↗

arXiv2024

Spatial structure facilitates evolutionary rescue by drug resistance

Bacterial populations often have complex spatial structures, which can impact their evolution. Here, we study how spatial structure affects the evolution of antibiotic resistance in a bacterial population. We consider a minimal model of spatially structured populations where all demes (i.e., subpopulations) are identical and connected to each other by identical migration rates. We show that spatial structure can facilitate the survival of a bacterial population to antibiotic treatment, starting from a sensitive inoculum. Specifically, the bacterial population can be rescued if antibiotic resistant mutants appear and are present when drug is added, and spatial structure can impact the fate of these mutants and the probability that they are present. Indeed, the probability of fixation of neutral or deleterious mutations providing drug resistance is increased in smaller populations. This promotes local fixation of resistant mutants in the structured population, which facilitates evolutionary rescue by drug resistance in the rare mutation regime. Once the population is rescued by resistance, migrations allow resistant mutants to spread in all demes. Our main result that spatial structure facilitates evolutionary rescue by antibiotic resistance extends to more complex spatial structures, and to the case where there are resistant mutants in the inoculum.

q-bio.PE↗

arXiv2024

Bridging Wright-Fisher and Moran models

The Wright-Fisher model and the Moran model are both widely used in population genetics. They describe the time evolution of the frequency of an allele in a well-mixed population with fixed size. We propose a simple and tractable model which bridges the Wright-Fisher and the Moran descriptions. We assume that a fixed fraction of the population is updated at each discrete time step. In this model, we determine the fixation probability of a mutant and its average fixation and extinction times, under the diffusion approximation. We further study the associated coalescent process, which converges to Kingman's coalescent, and we calculate effective population sizes. We generalize our model, first by taking into account fluctuating updated fractions or individual lifetimes, and then by incorporating selection on the lifetime as well as on the reproductive fitness.

q-bio.PE↗

arXiv2021

Hydrodynamic flow and concentration gradients in the gut enhance neutral bacterial diversity

The gut microbiota features important genetic diversity, and the specific spatial features of the gut may shape evolution within this environment. We investigate the fixation probability of neutral bacterial mutants within a minimal model of the gut that includes hydrodynamic flow and resulting gradients of food and bacterial concentrations. We find that this fixation probability is substantially increased compared to an equivalent well-mixed system, in the regime where the profiles of food and bacterial concentration are strongly spatially-dependent. Fixation probability then becomes independent of total population size. We show that our results can be rationalized by introducing an active population, which consists of those bacteria that are actively consuming food and dividing. The active population size yields an effective population size for neutral mutant fixation probability in the gut.

physics.bio-ph↗

arXiv2019

Antibody-mediated cross-linking of gut bacteria hinders the spread of antibiotic resistance

The body is home to a diverse microbiota, mainly in the gut. Resistant bacteria are selected for by antibiotic treatments, and once resistance becomes widespread in a population of hosts, antibiotics become useless. Here, we develop a multiscale model of the interaction between antibiotic use and resistance spread in a host population, focusing on an important aspect of within-host immunity. Antibodies secreted in the gut enchain bacteria upon division, yielding clonal clusters of bacteria. We demonstrate that immunity-driven bacteria clustering can hinder the spread of a novel resistant bacterial strain in a host population. We quantify this effect both in the case where resistance pre-exists and in the case where acquiring a new resistance mutation is necessary for the bacteria to spread. We further show that the reduction of spread by clustering can be countered when immune hosts are silent carriers, and are less likely to get treated, and/or have more contacts. We demonstrate the robustness of our findings to including stochastic within-host bacterial growth, a fitness cost of resistance, and its compensation. Our results highlight the importance of interactions between immunity and the spread of antibiotic resistance, and argue in the favor of vaccine-based strategies to combat antibiotic resistance.

q-bio.PE↗

arXiv2005

First evidence of anisotropic quenched disorder effects on a smectic liquid crystal confined in porous silicon

We present a neutron scattering analysis of the structure of the smectic liquid crystal octylcyanobiphenyl (8CB) confined in one-dimensional nanopores of porous silicon films (PS). The smectic transition is completely suppressed, leading to the extension of a short-range ordered smectic phase aligned along the pore axis. It evolves reversibly over an extended temperature range, down to 50 K below the \textit{N-SmA} transition in pure 8CB. This behavior strongly differs from previous observations of smectics in different one-dimensional porous materials. A coherent picture of this striking behavior requires that quenched disorder effects are invoked. The strongly disordered nature of the inner surface of PS acts as random fields coupling to the smectic order. The one-dimensionality of PS nano-channels offers new perspectives on quenched disorder effects, which observation has been restricted to homogeneous random porous materials so far.

cond-mat.soft↗