Global asymptotic stability of density dependent integral population projection models

Many stage-structured density dependent populations with a continuum of stages can be naturally modeled using nonlinear integral projection models. In this paper, we study a trichotomy of global stability result for a class of density dependent systems which include a Platte thistle model. Specifically, we identify those systems parameters for which zero is globally asymptotically stable, parameters for which there is a positive asymptotically stable equilibrium, and parameters for which there is no asymptotically stable equilibrium.     

Approximate solution for capillary exchange models with and without axial mixing

Explicit expressions are obtained which give quite accurate values for the effluent blood concentration for the model with axial mixing in tissue for the cases in which the initial capillary concentration is either zero or one. These simple expressions are useful since the formal solutions of Gosselin and Gosselin (1987,Bull. math. Biol 49, 329–349) are cumbersome to use. Reasonably accurate expressions are also provided for the model without axial mixing since the formal solution for the case in which the initial capillary concentration is zero (Gosselin and Gosselin, 1987) and for the case in which the initial concentration equals that of the tissue, the solution for which is provided here, require numerical integration.     

Pervasive compensatory adaptation in Escherichia coli

To investigate compensatory adaptation (CA), we used genotypes of Escherichia coli which were identical except for one or two deleterious mutations. We compared CA for (i) deleterious mutations with large versus small effects, (ii) genotypes carrying one versus two mutations, and (iii) pairs of deleterious mutations which interact in a multiplicative versus synergistic fashion. In all, we studied 14 different genotypes, plus a control strain which was not mutated. Most genotypes showed CA during 200 generations of experimental evolution, where we define CA as a fitness increase which is disproportionately large relative to that in evolving control lines, coupled with retention of the original deleterious mutation(s). We observed greater CA for mutations of large effect than for those of small effect, which can be explained by the greater benefit to recovery in severely handicapped genotypes given the dynamics of selection. The rates of CA were similar for double and single mutants whose initial fitnesses were approximately equal. CA was faster for synergistic than for multiplicative pairs, presumably because the marginal gain which results from CA for one of the component mutations is greater in that case. The most surprising result in our view, is that compensation should be so readily achieved in an organism which is haploid and has little genetic redundancy This finding suggests a degree of versatility in the E. coil genome which demands further study from both genetic and physiological perspectives.     

Modelling nutrient responses in the field

Models of the yield responses of crops to applied nutrients are a recent addition to the methods available for making fertilizer recommendations. They have a place in integrating nutrient information with information on other factors which affect yield and its response to added nutrients. This review deals with nitrogen models classified into three groups: those which predict yield-response curves based on empirical factors; those which simulate the yield response from complex simulation models of many processes regulating crop growth and the soil environment; and those which aim to simulate yield and selected processes based on simplified functional relationships which apply to a target region or industry. Three case studies representing the three classes of model are drawn from research on dryland wheat in different parts of Australia. They show examples in which models provide information which is unobtainable from experimental procedures and which provide information useful to farmers in making decisions about fertilizers.Suggestions are made for future developments in crop-nutrient modelling including further comparisons of models, linkage of models with tissue tests, modelling co-limiting nutrients, deciding on the appropriate level of detail within a model and the need for methods for calibrating and testing models on attributes other than yield alone.     

Bumblebee visits to Impatiens spp.: pattern and efficiency

Summary Three Japanese species of Impatiens, which secrete nectar continuously in long spurs, were visited by Bombus diversus workers consecutively throughout the day. B. diversus workers showed characteristic patterns of behavior in flower use, flower choice, and patch departure. (1) Bumblebees stayed longer on a flower which had been unvisited for a while than on a flower which had been visited recently. (2) Bumblebees preferred visiting flowers which had been unvisited for a while to visiting those which had been visited recently, and to visiting those which had been unvisited for a long period. (3) Bumblebees had a higher probability of leaving a patch after they had stayed on a flower for a short period than after they had stayed for a longer period. The bumblebees appeared to perceive both remotely and proximately chemical cues deposited by other foraging individuals, which indicated nectar rewards in a flower, and thus obtained a higher nectar intake than the mean amount of nectar left in a flower.     

Markovian processes in aneuploids: I. Diploid populations.

The case of diploid selfing organisms which present monosomy and nullisomy for a certain pair of homologous chromosomes is considered. Three cases of dominance for fitness (i) no dominance, (ii) complete dominance and, (iii) overdominance are considered. Expressions for the average, coefficient of variation and coefficient of asymmetry for distribution which represent the number of generations needed to reach an absorbing state for a process which begins in a transient state are obtained. Expressions for the parameters are from by the application of Markov's theory of chains which was developed by Kemeny & Snell (1960, Finite Markov Chains, Princeton, NJ: Van Nostrand) and Bosso et al. (1969, Biometrics 22, 17-26).     

慢性支气管炎患者下呼吸道感染病原菌分布及耐药性分析

目的:研究老年慢性支气管炎患者合并下呼吸道感染病原菌分布以及耐药性。方法:选取2009年1月到2013年12月我院收治的老年慢性支气管炎患者合并下呼吸道感染患者261例,采集所有患者的痰液,然后进行病原菌鉴定和药敏试验。结果:261例患者中,144例革兰阴性杆菌感染(55.2%),51例革兰阳性杆菌感染(19.5%),66例真菌感染(25.3%),其中混合感染者36例(13.8%)。革兰阴性杆菌以肺炎克雷伯菌最多(18.4%),革兰阳性杆菌以金黄色葡萄球菌最多(9.2%)。革兰阴性杆菌对亚胺培南的耐药性最低,其次是头孢哌酮和阿米卡星,对氨苄西林耐药率最高。金黄色葡萄球菌和表皮葡萄球菌对青霉素的耐药率均为100.0%,均对万古霉素敏感,其次是对环丙沙星敏感。结论:老年慢性支气管炎患者合并下呼吸道感染以革兰阴性杆菌感染为主,真菌和混合感染也占一定的比例,应该引起注意。     

Front-face fluorometry of liquid samples.

Quantitative analytical spectrofluorometry is usually performed with solutions which are optically dilute at the excitation wavelength and employs square cuvettes for observation at right angles to the excitation beam. In many biochemical applications, particularly when studying fluorophores in blood, front-face fluorometry of optically dense samples offers certain advantages which include fluorescence intensities which are an order of magnitude larger. The features which characterize quantitative fluorometry employing these two geometries are compared and a design for a front-face cell and cell holder, suitable for use in any standard spectrofluorometer, is presented.     

Dynamical Interactions between Multiple Cancers

Multiple tumors in a patient have the possibility to interact with each other, through the competition for new blood supply which is required for growth and progression (angiogenesis). The multiple tumors can be independent, multiple primary cancers. Alternatively, they can be metastases which originate from one primary tumor. This paper uses mathematical models to investigate such dynamical interactions between multiple cancers. We start with a model which describes the growth of a single angiogenic tumor, and then generalize this model to include multiple tumors which compete for circulating endothelial progenitor cells in order to build new blood vessels. We explore under which conditions multiple tumors can coexist, and when one tumor can exclude other tumors from growing. Based on this framework, we discuss the circumstances under which independent multiple primary tumors can arise. We further discuss the inefficiency of metastatic cells to grow successfully, and suggest an explanation for the occurrence of multiple metastases with an unknown primary cancer.     

A modeling framework describing the enzyme regulation of membrane lipids underlying gradient perception in Dictyostelium cells

Gradient perception describes the process by which information about the chemoattractant concentration field surrounding a cell is transformed into an internal signal which is responsible for directed cell motion. Recently, many important biochemical details in immobilized and mobile Dictyostelium cells have been uncovered regarding the roles of enzymes regulating phosphoinositide lipids on the cell membrane which are responsible for gradient perception. We report on a modeling framework that describes the relationship between the membrane concentration of the primary 3'phosphoinositide lipids and the enzymes which regulate them. The model takes the form of partial differential equations describing the membrane concentration of these lipids. Working within this framework, we describe mechanisms which can be responsible for spatial amplification of these lipids and which do not employ lipid-enzyme feedback. An analysis of a basic module underlying this process is also performed.     

NetAcet: prediction of N-terminal acetylation sites

We present here a neural network based method for prediction of N-terminal acetylation-by far the most abundant post-translational modification in eukaryotes. The method was developed on a yeast dataset for N-acetyltransferase A (NatA) acetylation, which is the type of N-acetylation for which most examples are known and for which orthologs have been found in several eukaryotes. We obtain correlation coefficients close to 0.7 on yeast data and a sensitivity up to 74% on mammalian data, suggesting that the method is valid for eukaryotic NatA orthologs.     

Positional Information and Positional Signalling in Hydra

A model is proposed for regulation and regeneration of the headend of hydra in terms of positional information, which involvestwo gradients. One is a diffusible substance made at the headend, which may be regarded as a positional signal. The otheris a more stable cellular parameter which is the positionalvalue. The rule for head end formation is that the concentrationof the diffusible substance falls a threshold amount below thepositional value. This model, for which some computer simulationis provided, can account for head end formation in a wide varietyof grafts. Evidence for a diffusable signal is provided by experimentsin which the time/distance relationships for head inhibitionby a grafted head are determined. Changes in positional valueduring regulation have been assayed and are much slower awayfrom the boundary. Polarity is interpreted in terms of the interactionbetween the two gradients. The biochemical basis of the gradientsis not known, but an approach to the problem has been made bytreating hydra with a variety of chemical agents.     

Cloning and deletion analysis of a genomic segment of Bacillus subtilis coding for the sdhA, B, C (succinate dehydrogenase) and gerE (spore germination) loci

From a Bacillus subtilis gene bank constructed in Escherichia coli and based on a low copy number cloning vector we have isolated a hybrid plasmid, pSH1047, containing an 8.0 kb segment of B. subtilis DNA coding for the sdhA, B and C genes, which code for the component polypeptides of succinate dehydrogenase, and the gerE gene, which may code for or regulate a protease involved in producing spores which germinate normally. We report the restriction map of this segment and the analysis of deletion derivatives which allow us to correlate the physical and genetic maps of these chromosomal segments.     

A class-I intron in a cyanelle tRNA gene from Cyanophora paradoxa: phylogenetic relationship between cyanelles and plant chloroplasts

Cyanelles are photosynthetic organelles which are considered as intermediates between cyanobacteria and chloroplasts, and which have been found in unicellular eukaryotes such as Cyanophora paradoxa. The nucleotide sequence of a 667-bp region of the cyanelle genome from Cyanophora paradoxa containing genes coding for tRNA(UUCGlu) and tRNA(UAALeu) has been determined. The gene coding for tRNA(UAALeu) is split by a 232-bp intron which has a secondary structure typical for class-I structured introns and which is closely related to the intron located in the corresponding gene from liverwort and higher plant chloroplasts. It appears therefore that these tRNA(UAALeu) genes are all derived from one common ancestral gene which already contained a class-I intron.     

Cystic echinococcosis in Sardinia: the current role of sheep

Cystic echinococcosis is one of the most widespread parasitosis in the Mediterranean region. Unfortunately this is also true for Sardinia for various reasons, among which is the close relationship between sheep and dogs in the farms. This work first epidemiologically analyses hydatidosis in sheep in Sardinia and then examines the most important causes of the persistence of these metacestodosis in sheep. The work looks at the factors which are responsible for the enormous quantity of illegal slaughtering and the widespread habit of abandoning the sheep carcasses in the grazing areas, as well as possible initiatives which may eliminate these practices. Such initiatives must include reducing the financial costs and bureaucracy involved when sheep are slaughtered in abattoirs, and also increasing the value of sheep meat. In conclusion new checks and controls are hoped for at a political level which will increase the financial support for the farmers and encourage the use of recombinant vaccines which have already been tested in Sardinia. These are extremely useful for effectively curbing cystic echinococcosis.     

Engineering human MEK-1 for structural studies: A case study of combinatorial domain hunting

Structural biology studies typically require large quantities of pure, soluble protein. Currently the most widely-used method for obtaining such protein involves the use of bioinformatics and experimental methods to design constructs of the target, which are cloned and expressed. Recently an alternative approach has emerged, which involves random fragmentation of the gene of interest and screening for well-expressing fragments. Here we describe the application of one such fragmentation method, combinatorial domain hunting (CDH), to a target which historically was difficult to express, human MEK-1. We show how CDH was used to identify a fragment which covers the kinase domain of MEK-1 and which expresses and crystallizes significantly better than designed expression constructs, and we report the crystal structure of this fragment which explains some of its superior properties. Gene fragmentation methods, such as CDH, thus hold great promise for tackling difficult-to-express target proteins.     

Crystal structure of a thermostable lipase from Bacillus stearothermophilus P1

We describe the first lipase structure from a thermophilic organism. It shares less than 20% amino acid sequence identity with other lipases for which there are crystal structures, and shows significant insertions compared with the typical alpha/beta hydrolase canonical fold. The structure contains a zinc-binding site which is unique among all lipases with known structures, and which may play a role in enhancing thermal stability. Zinc binding is mediated by two histidine and two aspartic acid residues. These residues are present in comparable positions in the sequences of certain lipases for which there is as yet no crystal structural information, such as those from Staphylococcal species and Arabidopsis thaliana. The structure of Bacillus stearothermophilus P1 lipase provides a template for other thermostable lipases, and offers insight into mechanisms used to enhance thermal stability which may be of commercial value in engineering lipases for industrial uses.     

Mechanism of ultraviolet-induced mutagenesis: the coding properties of ultraviolet-irradiated poly(dC) replicated by E. coli DNA polymerase I.

We have identified three lesions rather than cyclobutane dimers which alter the properties of UV-irradiated poly(dC) as a template for E.coli DNA polymerase I, and have characterised these lesions with respect to their coding properties, rates of formation and decay, and their sensitivity to uracil DNA glycosylase. Our results lead us to conclude that these lesions are (1) cytosine hydrates, which code for cytosine and to a lesser extent thymine, (2) uracil hydrates, which code for adenine and are not sensitive to uracil DNA glycosylase, and (3) uracils, which code for adenine and are removed by uracil DNA glycosylase.     

DNA repair after gamma irradiation in lymphocytes exposed to low-frequency pulsed electromagnetic fields

The effect of exposure to extremely low-frequency pulsed electromagnetic fields (EMFs) on DNA repair capability and on cell survival in human lymphocytes damaged in vitro with gamma rays was studied by two different micromethods. In the first assay, which measures DNA repair synthesis (unscheduled DNA synthesis, UDS), lymphocyte cultures were stimulated with phytohemagglutinin (PHA) for 66 h and then treated with hydroxyurea (which blocks DNA replication), irradiated with 100 Gy of 60Co, pulsed with [3H]thymidine ([3H]TdR), and then exposed to pulsed EMFs for 6 h (the period in which cells repaired DNA damage). In the second assay, which measures cell survival after radiation or chemical damage, lymphocytes were first irradiated with graded doses of gamma rays or treated with diverse antiproliferative agents, and then stimulated with PHA, cultured for 72 h, and pulsed with [3H]TdR for the last 6 h of culture. In this case, immediately after the damage induced by either the radiation or chemicals, cultures were exposed to pulsed EMFs for 72 h, during which cell proliferation took place. Exposure to pulsed EMFs did not affect either UDS or cell survival, suggesting that this type of nonionizing radiation--to which humans may be exposed in the environment, and which is used for both diagnostic and therapeutic purposes--does not affect DNA repair mechanisms.