An active poroelastic model for mechanochemical patterns in protoplasmic droplets of Physarum polycephalum

Motivated by recent experimental studies, we derive and analyze a twodimensional model for the contraction patterns observed in protoplasmic droplets of Physarum polycephalum. The model couples a model of an active poroelastic two-phase medium with equations describing the spatiotemporal dynamics of the intracellular free calcium concentration. The poroelastic medium is assumed to consist of an active viscoelastic solid representing the cytoskeleton and a viscous fluid describing the cytosol. The model equations for the poroelastic medium are obtained from continuum force-balance equations that include the relevant mechanical fields and an incompressibility relation for the two-phase medium. The reaction-diffusion equations for the calcium dynamics in the protoplasm of Physarum are extended by advective transport due to the flow of the cytosol generated by mechanical stresses. Moreover, we assume that the active tension in the solid cytoskeleton is regulated by the calcium concentration in the fluid phase at the same location, which introduces a chemomechanical feedback. A linear stability analysis of the homogeneous state without deformation and cytosolic flows exhibits an oscillatory Turing instability for a large enough mechanochemical coupling strength. Numerical simulations of the model equations reproduce a large variety of wave patterns, including traveling and standing waves, turbulent patterns, rotating spirals and antiphase oscillations in line with experimental observations of contraction patterns in the protoplasmic droplets.Comment: Additional supplemental material is supplie

Intracellular mechanochemical waves in an active poroelastic model

Many processes in living cells are controlled by biochemical substances regulating active stresses. The cytoplasm is an active material with both viscoelastic and liquid properties. We incorporate the active stress into a two-phase model of the cytoplasm which accounts for the spatiotemporal dynamics of the cytoskeleton and the cytosol. The cytoskeleton is described as a solid matrix that together with the cytosol as an interstitial fluid constitutes a poroelastic material. We find different forms of mechanochemical waves including traveling, standing, and rotating waves by employing linear stability analysis and numerical simulations in one and two spatial dimensions.Peer ReviewedPostprint (published version

Factors determining the occurrence of Flat Bugs (Aradidae) in beech dominated forests

The habitat requirements of most Aradidae and the decisive factors that influence their occurrence are still poorly understood. To reduce this lack of knowledge a standardised survey of Aradidae in two large beech forest areas of Bavaria (northern Steigerwald and High Spessart) was conducted. The following hypotheses were tested: 1) With increasing habitat tradition (temporal continuity), population densities of Aradidae increase, 2) Increasing dead wood supply supports higher abundance of Aradidae, 3) There are key structures for particular species with respect to type of dead wood, diameter and exposure to sunlight, and 4) The occurrence of particular fungi species determines the occurrence of Aradidae. In our study, Aradidae were sampled in point sample plots by flight-interception traps and time-standardised hand collection. To analyse specific habitat requirements additional sampling was performed to increase the sample size. Four species of Aradidae were observed in the two forest areas: Aneurus avenius, Aradus betulae, A. conspicuus and A. depressus. The results demonstrate that all species have different and specific habitat requirements. Especially for A. betulae habitat tradition of standing dead wood of large dimensions infested by Fomes fomentarius seems to be crucial. Hence, A. betulae was only observed in the Spessart and only at the sites with habitat tradition.Die Habitatansprüche der meisten Aradidae sowie die entscheidenden Faktoren die ihr Auftreten bedingen werden immer noch wenig verstanden. Um diese Wissenslücke zu verringern wurde eine standardisierte Aufnahme der Aradidae in zwei großen Buchenwaldgebieten Bayerns (nördlicher Steigerwald, Hochspessart) durchgeführt. Dabei wurden folgende Hypothesen ge-testet: 1) Die Populationsdichten der Aradidae steigen mit zunehmender Totholztradition, 2) erhöhte Verfügbarkeit an Totholz fördert eine höhere Abundanz der Aradidae, 3) es gibt Schlüsselstrukturen für bestimmte Arten in Bezug auf Totholztyp, -durchmesser und Sonnen-Exposition und 4) das Auftreten bestimmter Pilzarten bestimmt das Vorkommen der Aradidae. Zu diesem Zweck wurden Aradidae im Probekreisen mit Hilfe von Lufteklektoren und zeitnormierte Handaufsammlungen erfasst. Zur Analyse von spezifischen Habitatansprüchen wurden zusätzliche Fänge durchgeführt um die Stichprobengröße zu erhöhen. Vier Arten der Aradidae wurden in den bei-den Gebieten festgestellt: Aneurus avenius, Aradus betulae, A. conspicuus und A. depressus. Die Ergebnisse zeigen, dass alle Arten unterschiedliche und spezifische Habitatansprüche haben. Besonders für A. betulae scheint die Totholztradition, von starkem, von Fomes fomentarius besiedeltem, stehendem Totholz, eine entscheidende Rolle zu spielen. Folglich wurde sie Art nur am Totholz-traditionsreicheren Waldstandort, dem Spessart, gefunden

The effects of tree crown plasticity on the structure and dynamics of near-natural beech forests: an individual-based modeling approach

A new individual-based forest model for the species beech (Fagus sylvatica L.) was developed and implemented. The model called BEEch Plasticity (BEEP) describes tree crown plasticity phenomenologically and is able to model aboveground competition for PAR on a process basis. The current debate about the tree interactions in near-natural beech forests and their role in emergent forest structures and dynamics led to the research questions if (1) observed patterns can be modeled and reproduced by only describing the aboveground tree interactions, (2) what effects tree crown plasticity has on the structure and dynamics of near-natural beech forests, and (3) what effects selective thinning has on the structure and dynamics of near-natural beech forests. The BEEP model was developed, parameterized, calibrated, and validated according to data from the unmanaged forest `Schattiner Zuschlag' near Lübeck, North-Germany, while additional data from the sites Langula (Thuringia) and Fabrikschleichach (Bavaria) was used for model parameterization and calibration. Three simulation experiments were conducted. In the first experiment, the BEEP model was run 10 times for 2000 time steps with plastic tree crowns and the emergent forest structure was analyzed using structural indices. In the second experiment, the BEEP model was run again 10 times for 2000 time steps but with a modified crown model that only uses rotation-symmetric tree crowns. In the third experiment, the BEEP model was enhanced with a selective thinning procedure that uses target trees with specific diameter and heights as thinning objects. Forest structure was analyzed through the application of structural indices that capture different aspects of forest structure and by means of characterization of forest development phases. Analysis was accomplished only for the time steps 1000-2000 in order to allow transient oscillation in forest dynamics to develop. The results showed that the focus on aboveground competition and tree interactions sufficed to model beech forests and reproduced a wide range of patterns observed in near-natural and old-growth beech forest. In particular, the BEEP model was able to simulate a multi-layered forest structure with a mosaic structure of several developmental stages on a relatively small area of 0.5 ha. The simulated forest had wide diameter and age distributions. The diameter distribution was reversed-J-shaped. The age range of canopy trees exceeded 200 years. The comparison between simulations with plastic and rotation-symmetric tree crowns revealed that crown plasticity reduced tree competition for crown space and PAR and enhanced the forest structure and heterogeneity in the long term by allowing more tree cohorts of dfferent developmental stages to coexist. This supports the notion that crown plasticity drives beech forest dynamics in near-natural forests. The comparison between simulations with plastic tree crowns and with additional selective thinning showed that thinning does not affect the forest structural heterogeneity and reduces tree crown competition, while spatial patterns of tree positions remained unaltered. However, crown centroids were more regularly distributed. Model assumptions in the submodel routines, especially in the radiation and mortality submodel, question the reliability of the model results, because of the high sensitivity that these routines evoke on model outcomes. Therefore, revised versions of the submodels and a thoroughly validated crown growth model, may produce different results. Thus, the results presented in this study should be treated with care and cannot be used for generalizations about tree interactions in near-natural beech forests

Oscillatory motion of a droplet in an active poroelastic two-phase model

We investigate flow-driven amoeboid motility as exhibited by microplasmodia of Physarum polycephalum. A poroelastic two-phase model with rigid boundaries is extended to the case of free boundaries and substrate friction. The cytoskeleton is modeled as an active viscoelastic solid permeated by a fluid phase describing the cytosol. A feedback loop between a chemical regulator, active mechanical deformations, and induced flows gives rise to oscillatory and irregular motion accompanied by spatio-temporal contraction patterns. We cover extended parameter regimes of active tension and substrate friction by numerical simulations in one spatial dimension and reproduce experimentally observed oscillation periods and amplitudes. In line with experiments, the model predicts alternating forward and backward ectoplasmatic flow at the boundaries with reversed flow in the center. However, for all cases of periodic and irregular motion, we observe practically no net motion. A simple theoretical argument shows that directed motion is not possible with a spatially independent substrate friction

At the Mercy of the Prisoner Next Door. Using an Experimental Measure of Selfishness as a Criminological Tool

Do criminals maximise money? Are criminals more or less selfish than the average subject? Can prisons apply measures that reduce the degree of selfishness of their inmates? Using a tried and tested tool from experimental economics, we cast new light on these old criminological questions. In a standard dictator game, prisoners give a substantial amount, which calls for more refined versions of utility in rational choice theories of crime. Prisoners do not give less than average subjects, not even than subjects from other closely knit communities. This speaks against the idea that people commit crimes because they are excessively selfish. Finally those who receive better marks at prison school give more, as do those who improve their marks over time. This suggests that this correctional intervention also reduces selfishness.experiment, Crime, Prison, Dictator Game, Hurdle Model