Speaker
Francesc Font Clos

Description We will see how the setup introduced by J. Von Neumann to model growth in production economies can be used in the metabolic networks framework. In a probabilistic approach, we will derive self-consistency equations for the marginal distributions of the network's reaction fluxes, and discuss several ways of solving them numerically.

Speaker
Luca Giuggioli

Description To ensure exclusive access to resources, many mammals broadcast to neighbours their presence by scent-marking the regions of space they visit. Since scent remains active for a finite amount of time, an animal ensures that its presence is made aware to neighbours even during its temporary absence. As animals move, the terrain gets covered with scent from different individuals. The collection of scent marks belonging to one individual defines what is called a territory. As an animal encounters a foreign scent, it retreats from it to avoid costly confrontation making different territories exclude one another. By representing animals as random walkers that deposit scent wherever they go, the so-called territorial random walkers, it is possible to show that territorial patterns at the population level emerge dynamically from the movement and scent-mediated interaction at the level of the individual animals. The key parameters controlling territoriality are two: the average territory size, i.e. the inverse of the population density, and the time span during which animal scent marks remain active. Moreover, since territories undergo exclusion, it is possible to reduce the many-body problem of territorial emergence to a one-body problem of a single random walker roaming within subdiffusing territorial boundaries.

Speaker
Simone Pigolotti

Description The spread of a mutation in a spatially extended population presents strong analogies with reaction-diffusion problems, such as the Fisher equation, and models in non-equilibrium statistical mechanics, such as the voter model. With the aim of describing microbial populations in the ocean, I will present a new stochastic model where individuals are advected, reproduce, and die in the presence of competition. The introduction of a flow leads to a radically different phenomenology, where the fate of mutations depends on how interfaces among species collapse into the sinks of the velocity field. Biological evidences and possible experimental tests will be discussed.

Speaker
Jan Brugués

Description Spindles are arrays of microtubules that segregate chromosomes during cell division. It has been difficult to validate models of spindle assembly due to a lack of information on the organization of microtubules in these structures. Here I present two approaches to quantitatively study spindles at two length scales: (i) At the microtubule scale, we developed a novel method based on femtosecond laser ablation, which is capable of measuring the detailed architecture of spindles. We used this method to study spindles in Xenopus laevis egg extracts and find that microtubules are shortest near poles and become progressively longer towards the center of the spindle. These data, in combination with mathematical modeling, imaging, and biochemical perturbations, are sufficient to reject previously proposed mechanisms of spindle assembly. Our results support a new model of spindle assembly in which microtubule polymerization dynamics are not spatially regulated, and the proper organization of microtubules in the spindle is determined by non-uniform microtubule nucleation and the local sorting of microtubules by transport. (ii) At larger length scales compared to individual microtubules, I will show how we are using spatio-temporal correlations of microtubule density and orientation to formulate a continuum coarse grained theory for the spindle. Our results suggest that microtubule turnover plays a crucial role in stabilizing the giant density fluctuations predicted by traditional active hydrodynamic theories.

Description

The APS March Meeting is the largest physics meeting in the world, focusing on physics research from industry, universities, and major labs. Scientific sessions and supplemental events for 2012 took place at the Boston Convention & Exhibition Center (BCEC) and adjacent Westin Boston Waterfront Hotel.

Organizers
American Physical Society