Tractable Model (tractable + model)

Distribution by Scientific Domains

Selected Abstracts

The stratification theory for plant coexistence promoted by one-sided competition

Takashi Kohyama
Summary 1It is an essential feature of plants that leaves at higher levels have better access to light than those at lower levels. Thus, larger plants generally enjoy greater success in competing for light than smaller ones. We analyse the effect of such size-asymmetry, or one-sided competition, on the successful coexistence of plant species, using an analytically tractable model for stratified populations, in which a plant in the same layer exhibits the same crowding effect as any other, irrespective of species. 2A two-layer population that is reproductive in upper layer and juvenile in lower layer has a uniquely stable (plant-size-weighted) equilibrium density, as long as its fecundity is sufficient to compensate for its mortality rate. We also calculate a unique threshold lower-layer density of this layered population when there is no upper-layer plant. This threshold lower-layer density is larger than the weighted equilibrium density with upper layer, except for the case of perfect two-sided competition. 3A two-layer species can stably coexist with a one-layer, understorey species as a result of one-sided, but not two-sided competition. The coexistence condition is that the equilibrium density of the one-layer species lies between the threshold lower-layer density and the equilibrium density of the two-layer species. For an understorey species to coexist successfully with a two-layer species, any advantage in demographic performance, most prominently in a sufficiently high fecundity per plant must offset the disadvantage of living in dark conditions. 4Results from a model of multi-layer populations suggest that several species differing in terms of maximum layer and fecundity can coexist under conditions of one-sided competition. We demonstrate an example of the stable coexistence of eight species. The inter-specific trade-offs predicted by the model correspond to patterns observed in a rain forest. 5Synthesis. We propose a stratification theory that explains the generation and maintenance of the successful coexistence of plant species. Under the condition of one-sided competition, a canopy population that takes advantage of escaping from understorey competition shows an ability to invade an understorey with a density higher than its own equilibrium density, and which offers opportunities for an understorey population with high fecundity and/or shade tolerance to coexist. The predicted coexistence of species that share maximum canopy height is most pronounced for trees of tropical rain forests. [source]

Compliant grasping with passive forces

Cai-Hua Xiong
Because friction is central to robotic grasp, developing an accurate and tractable model of contact compliance, particularly in the tangential direction, and predicting the passive force closure are crucial to robotic grasping and contact analysis. This paper analyzes the existence of the uncontrollable grasping forces (i.e., passive contact forces) in enveloping grasp or fixturing, and formulates a physical model of compliant enveloping grasp. First, we develop a locally elastic contact model to describe the nonlinear coupling between the contact force with friction and elastic deformation at the individual contact. Further, a set of "compatibility" equations is given so that the elastic deformations among all contacts in the grasping system result in a consistent set of displacements of the object. Then, combining the force equilibrium, the locally elastic contact model, and the "compatibility" conditions, we formulate the natural compliant model of the enveloping grasp system where the passive compliance in joints of fingers is considered, and investigate the stability of the compliant grasp system. The crux of judging passive force closure is to predict the passive contact forces in the grasping system, which is formulated into a nonlinear least square in this paper. Using the globally convergent Levenberg-Marquardt method, we predict contact forces and estimate the passive force closure in the enveloping grasps. Finally, a numerical example is given to verify the proposed compliant enveloping grasp model and the prediction method of passive force closure. 2005 Wiley Periodicals, Inc. [source]

The complex Bingham quartic distribution and shape analysis

J. T. Kent
Summary., The complex Bingham distribution was introduced by Kent as a tractable model for landmark-based shape analysis. It forms an exponential family with a sufficient statistic which is quadratic in the data. However, the distribution has too much symmetry to be widely useful. In particular, under high concentration it behaves asymptotically as a normal distribution, but where the covariance matrix is constrained to have complex symmetry. To overcome this limitation and to provide a full range of asymptotic normal behaviour, we introduce a new ,complex Bingham quartic distribution' by adding a selection of quartic terms to the log-density. In the simplest case this new distribution corresponds to Kent's FB5 -distribution. Asymptotic and saddlepoint methods are developed for the normalizing constant to facilitate maximum likelihood estimation. Examples are given to show the usefulness of this new distribution. [source]

Human-in-mouse modeling of primary head and neck squamous cell carcinoma,

Jonathan H. Law MD
Abstract Objectives/Hypothesis: To develop a reliable modeling system for head and neck squamous cell carcinoma (HNSCC). Study Design: Laboratory-based translational study. Methods: HNSCC tissue was obtained from patients at biopsy/resection, cultured, and implanted into mice. In vivo, tumor growth, and survival was monitored by bioluminescence imaging. Histology and immunohistochemistry (IHC) were used to confirm HNSCC and human origin. Results: Short-term culture techniques were optimized allowing survival of primary HNSCC cells more than 7 days in 76% of tumors. The size of the tumor biopsy collected did not correlate with the success of short-term culture or xenograft establishment. Xenograft modeling was attempted in primary HNSCCs from 12 patients with a success rate of 92%. Immunostaining confirmed human origin of epithelial tumor cells within the modeled tumor. Bioluminescence and Ki67 IHC suggested tumor proliferation within the model. Luciferase expression was maintained for as long as 100 days in modeled tumors. Conclusions: The techniques developed for short-term primary tumor culture followed by xenograft modeling provide a low-cost and tractable model for evaluation of HNSCC response to standard and novel therapies. The high success rate of human-in-mouse tumor formation from primary HNSCC suggests that selection pressures for tumor growth in this model may be less than those observed for establishment of cell lines. Bioluminescent imaging provides a useful tool for evaluating tumor growth and could be expanded to measure response of the modeled tumor to therapy. This model could be adapted for xenograft modeled growth of other primary tumor types. Laryngoscope, 2009 [source]

Human prion strain selection in transgenic mice,

Kurt Giles DPhil
Objective: Transgenic (Tg) mice expressing chimeras of mouse and human prion proteins (PrPs) have shorter incubation periods for Creutzfeldt-Jakob disease (CJD) prions than mice expressing full-length human PrP. Increasing the sequence similarity of the chimeric PrP to mouse PrP, by reverting human residues to mouse, resulted in a Tg line, denoted Tg22372, which was susceptible to sporadic (s) CJD prions in ,110 days. Methods: Mice expressing chimeric mouse/human PrP transgenes were produced. The mice were inoculated intracerebrally with extracts prepared from the brains of patients who died of CJD. Onset of neurological dysfunction marked the end of the incubation time. After sacrifice of the Tg mice, their brains were analyzed for PrPSc and neuropathological changes. Results: Reversion of 1 additional residue (M111V) resulted in a new Tg line, termed Tg1014, susceptible to sCJD prions in ,75 days. Tg1014 mice also have shorter incubation periods for variant (v) CJD prions, providing a more tractable model for studying this prion strain. Transmission of vCJD prions to Tg1014 mice resulted in 2 different strains, determined by neuropathology and biochemical analysis, which correlated with the length of the incubation time. One strain had the biochemical, neuropathological, and transmission characteristics, including longer incubation times, of the inoculated vCJD strain; the second strain produced a phenotype resembling that of sCJD prions including relatively shorter incubation periods. Mice with intermediate incubation periods for vCJD prions had a mixture of the 2 strains. Both strains were serially transmitted in Tg1014 mice, which led to further reduction in incubation periods. Conversion of vCJD-like to sCJD-like strains was favored in Tg1014 mice more than in the Tg22372 line. The single amino acid difference therefore appears to offer selective pressure for propagation of the sCJD-like strain. Interpretation: These 2 Tg mouse lines provide relatively rapid models to study human prion diseases as well as the evolution of human prion strains. ANN NEUROL 2010;68:151,161 [source]

Crystallization and preliminary X-ray diffraction studies of hyperthermophilic archaeal Rieske-type ferredoxin (ARF) from Sulfolobus solfataricus P1

Asako Kounosu
The hyperthermophilic archaeal Rieske-type [2Fe,2S] ferredoxin (ARF) from Sulfolobus solfataricus P1 contains a low-potential Rieske-type [2Fe,2S] cluster that has served as a tractable model for ligand-substitution studies on this protein family. Recombinant ARF harbouring a pET30a vector-derived N-terminal extension region plus a hexahistidine tag has been heterologously overproduced in Escherichia coli, purified and crystallized by the hanging-drop vapour-diffusion method using 0.05,M sodium acetate, 0.05,M HEPES, 2,M ammonium sulfate pH 5.5. The crystals diffracted to 1.85, resolution and belonged to the tetragonal space group P43212, with unit-cell parameters a = 60.72, c = 83.31,. The asymmetric unit contains one protein molecule. [source]