Global versus local extinction in a network model of plant–pollinator communities

The loss of a species from an ecological community can trigger a cascade of additional extinctions; the complex interactions that comprise ecological communities make the dynamics and impacts of such a cascade challenging to predict. Previous studies have typically considered global extinctions, where a species cannot re-enter a community once it is lost. However, in some cases a species only becomes locally extinct, and may be able to reinvade from surrounding communities. Here, we use a dynamic, Boolean network model of plant–pollinator community assembly to analyze the differences between global and local extinction events in mutualistic communities. As expected, we find that compared to global extinctions, communities respond to local extinctions with lower biodiversity loss, and less variation in topological network properties. We demonstrate that in the face of global extinctions, larger communities suffer greater biodiversity loss than smaller communities when similar proportions of species are lost. Conversely, smaller communities suffer greater loss in the face of local extinctions. We show that targeting species with the most interacting partners causes more biodiversity loss than random extinctions in the case of global, but not local, extinctions. These results extend our understanding of how mutualistic communities respond to species loss, with implications for community management and conservation efforts.     

A "two-hit" model of cystogenesis in autosomal dominant polycystic kidney disease?

An intriguing feature of autosomal dominant polycystic kidney disease (ADPKD) is the focal and sporadic nature of individual cyst formation. Typically, only a few renal cysts are detectable in an affected individual during the first two decades of life. By the fifth decade, however, hundreds to thousands of renal cysts can be found in most patients. Additionally, significant intra-familial variability of ADPKD has been well documented. Taken together, these findings suggest that factor(s) in addition to the germline mutation of a polycystic kidney disease gene might be required for individual cyst formation. Indeed, recent studies have provided compelling evidence in support of a "two-hit" model of cystogenesis in ADPKD. In this model, inactivation of both copies of a polycystic kidney disease gene by germline and somatic mutations within an epithelial cell provides growth advantages for it to proliferate clonally into a cyst. This article highlights key findings of these recent studies and discusses the controversies and implications of the "two-hit" model in ADPKD.     

Convergence of a structured metapopulation model to Levins’s model

We consider a structured metapopulation model describing the dynamics of a single species, whose members are located in separate patches that are linked through migration according to a mean field rule. Our main aim is to find conditions under which its equilibrium distribution is reasonably approximated by that of the unstructured model of Levins (1969). We do this by showing that the (positive) equilibrium distribution converges, as the carrying capacity of each population goes to infinity together with appropriate scalings on the other parameters, to a bimodal distribution, consisting of a point mass at 0, together with a positive part which is closely approximated by a shifted Poisson centred near the carrying capacity. Under this limiting régime, we also give simpler approximate formulae for the equilibrium distribution. We conclude by showing how to compute persistence regions in parameter space for the exact model, and then illustrate all our results with numerical examples. Our proofs are based on Steins method.Supported in part by Schweizer Nationalfonds Projekt Nrs 20–61753.00 and 20–67909.02Supported in part by CNR of Italy under Grant n. 00.0142.ST74     

Recognition versus identity — the role of biometrics

The distinction between the recognition of an individual using an automated biometric system and the fundamental notion of their identity is subtle and therefore provokes much debate relating to privacy and other societal issues. This article seeks to clarify some of these issues relating to biometric recognition and identity by providing a framework, and by distinguishing between the technology and societal issues.     

Songs versus colours versus horns: what explains the diversity of sexually selected traits?

Papers on sexual selection often highlight the incredible diversity of sexually selected traits across animals. Yet, few studies have tried to explain why this diversity evolved. Animals use many different types of traits to attract mates and outcompete rivals, including colours, songs, and horns, but it remains unclear why, for example, some taxa have songs, others have colours, and others horns. Here, we first conduct a systematic survey of the basic diversity and distribution of different types of sexually selected signals and weapons across the animal Tree of Life. Based on this survey, we describe seven major patterns in trait diversity and distributions. We then discuss 10 unanswered questions raised by these patterns, and how they might be addressed. One major pattern is that most types of sexually selected signals and weapons are apparently absent from most animal phyla (88%), in contrast to the conventional wisdom that a diversity of sexually selected traits is present across animals. Furthermore, most trait diversity is clustered in Arthropoda and Chordata, but only within certain clades. Within these clades, many different types of traits have evolved, and many types appear to have evolved repeatedly. By contrast, other major arthropod and chordate clades appear to lack all or most trait types, and similar patterns are repeated at smaller phylogenetic scales (e.g. within insects). Although most research on sexual selection focuses on female choice, we find similar numbers of traits (among sampled species) are involved in male contests (44%) and female choice (55%). Overall, these patterns are largely unexplained and unexplored, as are many other fundamental questions about the evolution of these traits. We suggest that understanding the diversity of sexually selected traits may require a shift towards macroevolutionary studies at relatively deep timescales (e.g. tens to hundreds of millions of years ago).     

A nonlinear kinetic model of γ-aminobutyric acid metabolism in a rabbit model of acute liver failure

To investigate the mechanisms by which serum levels of γ-aminobutyric acid (GABA) become elevated in experimental acute liver failure, a multicompartmental model of GABA metabolism has been constructed and used to simulate previously generated data on the kinetics of ³H-GABA uptake by isolated hepatocytes from normal rats and the kinetics of ³H-GABA in the plasma of normal rabbits, rabbits with galactosamine-induced acute liver failure, and rabbits with divascularized livers. Modeling analysis revealed that acute liver failure was associated with values for the mean fractional catabolic rate of GABA, plasma volume, and hepatic extraction of GABA that were 29%, 12%, and 49% less, respectively, than the corresponding control values. The defect in hepatic tissue extraction of GABA was sufficient to account for only 60% of the 10-fold increase in serum GABA levels that occurs in acute liver failure. Furthermore the 10-fold increase in serum GABA levels occured in acute liver failure before the onset of overt hepatic encephalopathy when hepatic extraction of GABA was not appreciably different from that found in normal rabbits. Thus the increase in serum GABA levels that occurs in acute liver failure cannot be attributed to a defect in hepatic extraction of GABA alone. Indeed, the modeling analysis indicated that in acute liver failure there is a 3—8-fold increase in the rate of delivery of GABA to the systemic circulation, but did not indicate its source.     

Reaction–diffusion model of atherosclerosis development

Atherosclerosis begins as an inflammation in blood vessel walls (intima). The inflammatory response of the organism leads to the recruitment of monocytes. Trapped in the intima, they differentiate into macrophages and foam cells leading to the production of inflammatory cytokines and further recruitment of white blood cells. This self-accelerating process, strongly influenced by low-density lipoproteins (cholesterol), results in a dramatic increase of the width of blood vessel walls, formation of an atherosclerotic plaque and, possibly, of its rupture. We suggest a 2D mathematical model of the initiation and development of atherosclerosis which takes into account the concentration of blood cells inside the intima and of pro- and anti-inflammatory cytokines. The model represents a reaction-diffusion system in a strip with nonlinear boundary conditions which describe the recruitment of monocytes as a function of the concentration of inflammatory cytokines. We prove the existence of travelling waves described by this system and confirm our previous results which suggest that atherosclerosis develops as a reaction-diffusion wave. The theoretical results are confirmed by the results of numerical simulations.     

The ‘division of labour’ model of eye evolution

The ‘division of labour’ model of eye evolution is elaborated here. We propose that the evolution of complex, multicellular animal eyes started from a single, multi-functional cell type that existed in metazoan ancestors. This ancient cell type had at least three functions: light detection via a photoreceptive organelle, light shading by means of pigment granules and steering through locomotor cilia. Located around the circumference of swimming ciliated zooplankton larvae, these ancient cells were able to mediate phototaxis in the absence of a nervous system. This precursor then diversified, by cell-type functional segregation, into sister cell types that specialized in different subfunctions, evolving into separate photoreceptor cells, shading pigment cells (SPCs) or ciliated locomotor cells. Photoreceptor sensory cells and ciliated locomotor cells remained interconnected by newly evolving axons, giving rise to an early axonal circuit. In some evolutionary lines, residual functions prevailed in the specialized cell types that mirror the ancient multi-functionality, for instance, SPCs expressing an opsin as well as possessing rhabdomer-like microvilli, vestigial cilia and an axon. Functional segregation of cell types in eye evolution also explains the emergence of more elaborate photosensory–motor axonal circuits, with interneurons relaying the visual information.     

Construction of rat spinal cord injury model based on Allen’s animal model

The aim of this study is to explore the construction of rat spinal cord injury model guided by Allen's model. Methods: Male rats aged 4–5 weeks and weighing about 250 g are selected as subjects in the Animal Laboratory Center of XX Hospital. Rats are divided into two groups, which are experimental group 1 and experimental group 2, respectively, so as to construct spinal cord injury model in rats. The first group is given 300 g.cm hitting force of T10 spinal cord, and the second group is given 500 g.cm hitting force of T10 spinal cord. Within 25 days after spinal cord injury in Allen's rats, the survival, neurological function, diet, motor ability, tactile ability and auditory ability of the two groups are monitored and evaluated daily. Results: In terms of survival, the survival rate of rats in group 1 is 85%, while that of rats in group 2 is 21%, and there is a concentrated death phenomenon in group 2. In terms of neurological function recovery, experimental group 1 is stable and gets 7 points and experimental group 2 is stable and gets 3 points. In terms of diet, the experimental group 1 is stable and gets 5 points and the experimental group 2 is stable and gets 2 points. In terms of motor ability, the experimental group 1 is stable and gets 5 points and the experimental group 2 is stable and gets 2 points. In tactile sense, experimental group 1 is stable and gets 17 points and experimental group 2 is stable and gets 12 points. It can be seen that the post-operative recovery ability of the experimental group 1 is better than that of the experimental group 2. Conclusion: Under the guidance of Allen's model, compared with the group 2, the experimental group 1 of the rat spinal cord injury model has better recovery in each index. It can be seen that the smaller impact strength is more beneficial to the recovery of rats after spinal cord injury surgery.     

White-noise stimulation of the Hodgkin–Huxley model

 We studied the combined influence of noise and constant current stimulations on the Hodgkin–Huxley neuron model through time and frequency analysis of the membrane-potential dynamics. We observed that, in agreement with experimental data (Guttman et al. 1974), at low noise and low constant current stimulation the behavior of the model is well approximated by that of the linearized Hodgkin–Huxley system. Conversely, nonlinearities due to firing dominate at large noise or current stimulations. The transition between the two regimes is abrupt, and takes place in the same range of noise and current intensities as the noise-induced transition characterized by the qualitative change in the stationary distribution of the membrane potential (Tanabe and Pakdaman 2001a). The implications of these results are discussed. Received: 27 July 2001 / Accepted in revised form: 18 December 2001     

A texture-processing model of the ‘visual sense of number’

It has been suggested that numerosity is an elementary quality of perception, similar to colour. If so (and despite considerable investigation), its mechanism remains unknown. Here, we show that observers require on average a massive difference of approximately 40% to detect a change in the number of objects that vary irrelevantly in blur, contrast and spatial separation, and that some naive observers require even more than this. We suggest that relative numerosity is a type of texture discrimination and that a simple model computing the contrast energy at fine spatial scales in the image can perform at least as well as human observers. Like some human observers, this mechanism finds it harder to discriminate relative numerosity in two patterns with different degrees of blur, but it still outpaces the human. We propose energy discrimination as a benchmark model against which more complex models and new data can be tested.     

FSH versus estrogen: who's guilty of breaking bones?

Bone loss after menopause or gonadectomy has been attributed to the drop in estrogen levels. A recent paper (Sun et al, 2006) challenges this view by showing that the pituitary hormone FSH, previously thought to target only the gonads, also acts on osteoclasts to activate bone resorption. In conjunction with genetic studies, these data raise the possibility that FSH, independent of estrogen, causes hypogonadal bone loss.     

A numerical model of cellular blebbing: A volume-conserving,fluid–structure interaction model of the entire cell

In animal cells, blebs are smooth, quasi-hemispherical protrusions of the plasma membrane that form when a section of the membrane detaches from the underlying actin cytoskeleton and is inflated by flowing cytosol. The mechanics behind this common cellular activity are not yet clear. As a first step in the development of a full computational framework, we present a numerical model of overall cell behavior based upon the interaction between a background Newtonian-fluid cytosol and elastic structures modeling the membrane and filaments. The detailed micromechanics of the cytoskeletal network are the subject of future work. Here, the myosin-driven contraction of the actin network is modeled through stressed elastic filaments. Quantitative models of cytoskeletal micromechanics and biochemistry require accurate estimates of local stress and flow conditions. The main contribution of this paper is the development of a computationally efficient fluid–structure interaction model based on operator splitting, to furnish this data. Cytosol volume conservation (as supported by experimental evidence) is enforced through an intermediate energy minimization step. Realistic bleb formation and retraction is observed from this model, offering an alternative formulation to positing complex continuum behavior of the cytoplasm (e.g. poroelastic model of Charras et al., 2008).     

Dermatophyte–host relationship of a murine model of experimental invasive dermatophytosis

Recognizing the invasive potential of the dermatophytes and understanding the mechanisms involved in this process will help with disease diagnosis and with developing an appropriate treatment plan. In this report, we present the histopathological, microbiological and immunological features of a model of invasive dermatophytosis that is induced by subcutaneous infection of Trichophyton mentagrophytes in healthy adult Swiss mice. Using this model, we observed that the fungus rapidly spreads to the popliteal lymph nodes, spleen, liver and kidneys. Similar to the human disease, the lymph nodes were the most severely affected sites. The fungal infection evoked acute inflammation followed by a granulomatous reaction in the mice, which is similar to what is observed in patients. The mice were able to mount a Th1-polarized immune response and displayed IL-10-mediated immune regulation. We believe that the model described here will provide valuable information regarding the dermatophyte–host relationship and will yield new perspective for a better understanding of the immunological and pathological aspects of invasive dermatophytosis.     

Thermodynamic and structural analysis of homodimeric proteins: model of β-lactoglobulin

The energetics of protein homo-oligomerization was analyzed in detail with the application of a general thermodynamic model. We have studied the thermodynamic aspects of protein-protein interaction employing β-lactoglobulin A from bovine milk at pH=6.7 where the protein is mainly in its dimeric form. We performed differential calorimetric scans at different total protein concentration and the resulting thermograms were analyzed with the thermodynamic model for oligomeric proteins previously developed. The thermodynamic model employed, allowed the prediction of the sign of the enthalpy of dimerization, the analysis of complex calorimetric profiles without transitions baselines subtraction and the obtainment of the thermodynamic parameters from the unfolding and the association processes and the compared with association parameters obtained with Isothermal Titration Calorimetry performed at different temperatures. The dissociation and unfolding reactions were also monitored by Fourier-transform infrared spectroscopy and the results indicated that the dimer of β-lactoglobulin (N(2)) reversibly dissociates into monomeric units (N) which are structurally distinguishable by changes in their infrared absorbance spectra upon heating. Hence, it is proposed that β-lactoglobulin follows the conformational path induced by temperature:N(2)?2N?2D. The general model was validated with these results indicating that it can be employed in the study of the thermodynamics of other homo-oligomeric protein systems.     

Dynamics of viscoelastic spherical membranes—the balloon model of the alveolus

Double-valued pressure-volume relationships in dynamic conditions for spherical membranes, modelling the lung alveoli, were obtained at small deformations. This hysteretic behavior was considered to be produced by at least three independent mechanisms: (1) the lung parenchyma exhibits viscoelastic properties; (2) the lung surface film, independent of the tissue, exhibits viscoelastic properties and (3) the pressure acting on the inner membrane surface depends on the rate of the alveolus volume change, due to the air viscous resistance in the bronchial tree. In each case, the maximum volume change, the hysteresis loop area, the tilt angle of the hysteresis loop and the relaxation time of the system were calculated. The results show pronounced hysteresis at normal breathing due to the air viscous resistance and smaller one due to the tissue and surface viscoelastic properties. In quasistatic conditions the values of the surface viscoelasticity and the tissue viscoelasticity effects are comparable or different, depending on the concrete external conditions. Comparison with the available experimental data is discussed in detail.