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See detailWill top-down interactions be affacted by climate change?
Boullis, Antoine ULg; Verheggen, François ULg; Francis, Frédéric ULg

Poster (2014, April 02)

Greenhouse gas emissions are rising since the beginning of the industrial era: atmospheric carbon dioxide now exceeds 400 ppm (30% increase since the mid-1800s) and tropospheric ozone has reached 40 ppb ... [more ▼]

Greenhouse gas emissions are rising since the beginning of the industrial era: atmospheric carbon dioxide now exceeds 400 ppm (30% increase since the mid-1800s) and tropospheric ozone has reached 40 ppb in industrialized areas. According to forecasting models, these concentrations are expected to double by the end of the 21st century. To date, impacts of elevated greenhouse gases have been extensively studied in plant-insect interactions, whereas interactions between herbivores and their natural enemies have been neglected. Because most interactions between herbivorous insects and their natural enemies are orchestrated by natural molecules produced as semiochemicals, we hypothesize that modification in atmospheric gas composition will alter chemical communication in insects, and affect the performance of natural enemies to locate their prey. In this project, we shall attempt to analyze the effect of increase in greenhouse gas concentrations on insect communication, particularly in top-down interactions. The study model will consist in the interactions between the pea aphids (Acyrthosiphon pisum (Harris)) and their natural enemies Harmonia axyridis (Pallas) and Episyrphus balteatus (DeGeer). Under elevated CO2 and/or O3 concentrations, we will quantify the production of several aphid semiochemicals that are currently used as kairomones by natural enemies, including the aphid alarm pheromone E-β-farnesene and honeydew volatile compounds. Olfactometry assays will then be set up to examine the ability of natural enemies to locate their prey in the predicted atmospheric conditions of the end of the 21st century. Finally, using an Ecotron structure to be built in Gembloux Agro-Bio Tech, we will simulate a future ecosystem with the possibility to control additional biotic and abiotic parameters. [less ▲]

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See detailBiological control of aphids in wheat and vegetable crops : a multi-approach case study in Shandong province (China)
Chevalier Mendes Lopes, Thomas ULg; Bosquée, Emilie ULg; Chen, Julian et al

Poster (2014, April 02)

This multi-approach study was conducted in the Shandong province (China) to assess the effectiveness of different biological control methods against aphids in wheat and vegetable crops. Three approaches ... [more ▼]

This multi-approach study was conducted in the Shandong province (China) to assess the effectiveness of different biological control methods against aphids in wheat and vegetable crops. Three approaches were tested: (1) wheat/oilseed rape and wheat/pea associations, (2) potatoes/peas association, and (3) E-β-farnesene (aphid alarm pheromone) releasers in squashes under plastic tunnels. Aphids and aphidophagous beneficials were monitored by observations on plants. Wheat associations and E-β-farnesene reduced aphid infestations compared with control treatments, while no effect was observed in the potatoes/peas association. Moreover, aphidophagous beneficials were significantly more abundant in the three kinds of associations compared with control treatments. Few individuals were observed on squashes. Additionally, adult aphidophagous beneficials were monitored using yellow pan traps. The parasitoid species Aphidius gifuensis (Ashmead) was prevalent, while Propylea japonica (Thunberg) and Harmonia axyridis (Pallas) were the most abundant species among predators. Globally, wheat associations were more efficient to reduce aphid infestations and attract aphidophagous beneficials to promote biological control. The trapping of natural enemies allowed to assess the diversity and abundance of aphidophagous species in this region. This study shows a beneficial effect of crop associations and E-β-farnesene on the control of aphid populations and contributes to assess the potentialities of these alternative methods to the use of insecticides. [less ▲]

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See detailConservatoire entomologique de Gembloux Agro-Bio Tech: les coccinelles de Wallonie et de Bruxelles de 2001 à 2009
Vandereycken, Axel ULg; Fassotte, Bérénice ULg; Durieux, Delphine ULg et al

Poster (2014, April 02)

Un inventaire des Coccinellidae présents au sein de la collection de l’unité d’Entomologie fonctionnelle et évolutive (ULg - Gembloux Agro-Bio Tech) a été réalisé sur la période s’étendant de 2001 à 2009 ... [more ▼]

Un inventaire des Coccinellidae présents au sein de la collection de l’unité d’Entomologie fonctionnelle et évolutive (ULg - Gembloux Agro-Bio Tech) a été réalisé sur la période s’étendant de 2001 à 2009. Cette collection est essentiellement constituée par les récoltes des étudiants de première Bachelier de Gembloux Agro-Bio Tech. A travers ce recensement, nous avons étudié l’évolution des espèces récoltées au sein de cette famille en termes d’effectif relatif et de richesse spécifique. Notre étude s’est focalisée sur les données issues de Wallonie et de la Région de Bruxelles-Capitale, trop peu de collectes ayant été réalisées en Flandre. Parmi les individus identifiés, 27 espèces ont été observées, dont 21 appartiennent à la sous-famille des Coccinellinae, 2 à celle des Epilachninae et 4 à celle des Chilocorinae. La plupart des espèces sont communes à la Belgique. Cependant, Harmonia axyridis (Pallas 1773), espèce originaire du sud-est de l’Asie, a été introduite sur le territoire belge en 1997. Cette coccinelle exotique fait son apparition dans les récoltes de 2002 et son effectif ne cesse d’augmenter au fil des années prospectées. En parallèle à cette augmentation, une diminution de la richesse spécifique, ainsi que de l’effectif relatif d’Adalia bipunctata (L. 1758), de Propylea quatuordecimpunctata (L. 1758) et de Psyllobora vigintiduopunctata (L. 1758), est observée. [less ▲]

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See detailImages fixes, cadre mobile. Fantômes de cinéma dans la bande dessinée expérimentale contemporaine
Belloi, Livio ULg

Conference (2014, April 02)

La présente communication porte sur la bande dessinée expérimentale contemporaine et se focalise plus particulièrement sur "Travelling Square District" (2010) du jeune dessinateur belge Greg Shaw. Objet ... [more ▼]

La présente communication porte sur la bande dessinée expérimentale contemporaine et se focalise plus particulièrement sur "Travelling Square District" (2010) du jeune dessinateur belge Greg Shaw. Objet très singulier, cet album se présente comme une expérimentation systématique autour du cadre mobile, dans un art fondé sur des successions d'images fixes. En cela, cette œuvre peut être considérée comme relevant d'une para-histoire du cinéma. [less ▲]

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See detailApport des examens biologiques en pratique allergologique
GADISSEUR, Romy ULg

Conference (2014, April 02)

Apport des examens biologiques en pratique allergologique

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See detailInvolved PhD research – a case study between agronomy and social sciences
Klaedtke, Stephanie ULg; Chable, Véronique; Stassart, Pierre M ULg

Conference (2014, April 02)

In research projects based on questions which emerge from practice and from interactions with different stakeholders, researchers engage with research partners in a cooperative and interdisciplinary ... [more ▼]

In research projects based on questions which emerge from practice and from interactions with different stakeholders, researchers engage with research partners in a cooperative and interdisciplinary approach. In PhD projects, including a diverse network of partners and interdisciplinarity may be challenged by the short duration of the projects and by the positivist and reductionist approaches in which students are initially trained. Based on a concrete PhD research project on small-scale organic seed production, the objective of this work is to elucidate how interdisciplinarity and an action-base can nevertheless gradually be integrated by PhD students into an approach best termed as involved research. Reflection on the roles of partners, their involvement, motivation and points of view and on the evolution of questions posed during interactions between students and stakeholders contributes to recognizing the role of each partner, situating research postures, and identifying the types of knowledge and transformations produced. While PhD students and their research projects can evolve towards more holistic and constructivist approaches, it is crucial to maintain opportunities for reflection during their research in order to realize the potentials of involving all those involved in the projects and produce effective social learning processes. [less ▲]

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See detailDiversité et abondance entomologique en cultures maraîchères à Libreville et à Owendo (Gabon)
Bayendi-Loudit, Sandrine ULg; Poligui, René Noël ULg; Ndoutoume Ndong, Auguste et al

Conference (2014, April 02)

In order to improve knowledge of the diversity and abundances of the entomofauna occurring within market gardenings in Gabon, this study was conducted in three localities in Libreville and Owendo, from ... [more ▼]

In order to improve knowledge of the diversity and abundances of the entomofauna occurring within market gardenings in Gabon, this study was conducted in three localities in Libreville and Owendo, from July1st to August 31st, 2012. Insect collections from yellow traps and visual observations were performed weekly within plots in which grow Amaranth (Amaranthus sp., Amaranthaceae), red sorrel (Hibiscus sabdariffa L, Malavaceae), Tomato (Lycopersicon esculentum Mill, Solanaceae) and Cabbage (Brassica oleracea L, Brassicaceae). A total of 7119 insects were assessed and taxonomically classified in 8 distinct orders comprising 81 families. The study highlighted some entomologic families with agronomic interest. The main pests were represented by Aphididae (31,0%), Cecidomyiidae (5,1%), Chrysomelidae (4,0%) and Thripidae (2,0%), the useful insects (pollinators and predators) by Dolichopodidae (5,4%), Andrenidae (2,6%) Coccinellidae (1,7%), Syrphidae (0,6%), Braconidae (0,5%) and Hemerobiidae (0,1%) and the associated insects were Psychodidae (7,4%), Muscidae (5,9%), Formicidae (4,4%) and Anthomyiidae (4,2%). Entomological diversity is discussed according to the local conditions and potential for sustainable pest control encountered. [less ▲]

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See detailA pheromone-based formulation against phytophagous pests
Fassotte, Bérénice ULg; Vandereycken, Axel ULg; Heuskin, Stéphanie ULg et al

Poster (2014, April 02)

Innovative integrated pest management methods are needed to overcome market withdrawal of synthetic pesticides. Therefore, the identification of environment-friendly bio-products carrying direct or ... [more ▼]

Innovative integrated pest management methods are needed to overcome market withdrawal of synthetic pesticides. Therefore, the identification of environment-friendly bio-products carrying direct or indirect biocide activity is one promising alternative option. Our researches focus on the identification of appropriate formulations releasing volatile organic compounds that are attractive for natural enemies of insect pests. However, the elaboration of slow-release devices that ensure stable and controlled release of active volatile compounds is quite challenging. Here, we developed a formulation based on E-β-farnesene and (–)-β-caryophyllene, these two semiochemicals having strong attractive potential on aphid natural enemies including ladybeetles and hoverflies. Both compounds were encapsulated together in alginate gel beads. The blend efficiency is currently being evaluated through laboratory and field assays. [less ▲]

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See detailFirst evidence of a volatile sex pheromone in the invasive Harmonia axyridis (Pallas) (Coleoptera: Coccinellidae)
Fassotte, Bérénice ULg; Fischer, Christophe ULg; Durieux, Delphine ULg et al

Conference (2014, April 02)

Data about sex pheromones, or any semiochemicals that play a role in Coccinellid mating, remain limited. Since years, various studies and behavioral observations have hypothesized that such molecules are ... [more ▼]

Data about sex pheromones, or any semiochemicals that play a role in Coccinellid mating, remain limited. Since years, various studies and behavioral observations have hypothesized that such molecules are involved in sexual communication of ladybeetles. In this study, we collected volatile organic compounds released by virgin females of the multicolored Asian ladybeetle, Harmonia axyridis (Pallas), which were either allowed or not allowed to feed on aphids. In the presence of aphids, virgin females exhibited “calling behavior”, which has been associated with the emission of a sex pheromone in several Coleoptera species. Bioassays showed that these females released a blend of volatile compounds that is involved in the attraction of males. Gas Chromatography-Mass Spectrometry analyses highlighted specific volatile cues emanating from females, whereas males did not produce these compounds. Five components were identified: (–)-β-caryophyllene, β elemene, methyl-eugenol, α humulene, and α bulnesene. All compounds were produced after virgin females were fed aphids, and their quantity increased across the experimental period. The results confirm that female H. axyridis produce a volatile sex pheromone. Therefore, this study provides important biological information that could promote the development of efficient pest control management methods to manipulate the movements of this invasive ladybeetle, and to reduce its negative impacts on biodiversity. [less ▲]

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See detailStudy of sex ratio and morphotypes of the Multicoloured Asian Ladybird, Harmonia axyridis Pallas in Belgian corn
Vandereycken, Axel ULg; Fassotte, Bérénice ULg; Barsics, Fanny ULg et al

Poster (2014, April 02)

Several insect species are known to have different morphotypes, an adaptation to heterogeneities in changing environments. This phenotypic plasticity could be a factor used by the Multicoloured Asian ... [more ▼]

Several insect species are known to have different morphotypes, an adaptation to heterogeneities in changing environments. This phenotypic plasticity could be a factor used by the Multicoloured Asian Ladybird, which is considered as an invasive species in Europe since its introduction in the 80’s. Harmonia axyridis has four major morphotypes: 2 melanic forms, conspicua and spectabilis; and 2 non-melanic, succinea and axyridis. In Belgium, only the three first are observed. Literature reports variations in morphotype frequencies, across native and introduced strains. These variations seem to be linked to climate (geographical and seasonal variation), with non-melanic forms being the most abundant in hot and arid climate. This study focused on the variation of morphotype abundance in H. axyridis according to season, field and gender. Adult ladybirds were caught from mid-July to mid-October 2012 into 5 corn fields. The proportions of observed morphotypes were compared according to the sampling season. The abundance of morphotypes was not significantly different through the fields; the observed ratios were 81.6%, 15% and 3.4% for succinea, spectabilis and conspicua respectively. Succinea was the most observed morphotype (χ2=755.8; P<0.001). The total number of collected males and females was the same with ratios of 48.2% and 51.8%: (χ2=0.93; P=0.33). The sex ratio was also not significantly different for each morphotype. There was no evolution of melanic and non-melanic form ratio according to the season, with 25.6% of melanic form in summer and 20.6% in autumn (χ2=1.22; P=0.27). Our results seem to show that morphotype adaptation according to the climate is not observed for Belgian ladybirds. We discuss these results according to all environmental data available. [less ▲]

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See detailExploiting SNP Correlations within Random Forest for Genome-Wide Association Studies
Botta, Vincent ULg; Louppe, Gilles ULg; Geurts, Pierre ULg et al

in PLoS ONE (2014)

The primary goal of genome-wide association studies (GWAS) is to discover variants that could lead, in isolation or in combination, to a particular trait or disease. Standard approaches to GWAS, however ... [more ▼]

The primary goal of genome-wide association studies (GWAS) is to discover variants that could lead, in isolation or in combination, to a particular trait or disease. Standard approaches to GWAS, however, are usually based on univariate hypothesis tests and therefore can account neither for correlations due to linkage disequilibrium nor for combinations of several markers. To discover and leverage such potential multivariate interactions, we propose in this work an extension of the Random Forest algorithm tailored for structured GWAS data. In terms of risk prediction, we show empirically on several GWAS datasets that the proposed T-Trees method significantly outperforms both the original Random Forest algorithm and standard linear models, thereby suggesting the actual existence of multivariate non-linear effects due to the combinations of several SNPs. We also demonstrate that variable importances as derived from our method can help identify relevant loci. Finally, we highlight the strong impact that quality control procedures may have, both in terms of predictive power and loci identification. [less ▲]

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See detailModélisation 2D par éléments finis du contact : effet de l’utilisation de méthodes de représentation des surfaces présentant un ordre de continuité élevé et méthodes permettant de passer le patch test
Nguyen, Duc Tue ULg

Doctoral thesis (2014)

Ce travail de thèse concerne la mécanique numérique du contact entre les solides déformables. La simulation numérique des problèmes de contact comporte de nombreuses difficultés notamment dans un contexte ... [more ▼]

Ce travail de thèse concerne la mécanique numérique du contact entre les solides déformables. La simulation numérique des problèmes de contact comporte de nombreuses difficultés notamment dans un contexte de grands déplacements et de grandes déformations. Le but de ce travail est d’analyser l’influence de l’ordre de continuité élevé de la représentation de surface et de développer une méthode pour passer le patch test, à savoir qu’un élément passe le patch test si il transmet correctement des pressions de contact entre deux surfaces. Pour la discrétisation des éléments finis, l'interface de contact est représentée par une série de lignes et de courbes à continuité C0. Par conséquent, lors du passage du nœud esclave entre deux segments maîtres adjacents, il peut arriver avoir une discontinuité de la direction du vecteur normal. Cette situation peut mener à des problèmes de convergence de l'algorithme de résolution. Pour éviter ce problème, l’approche proposée consiste à développer les représentations de surface maître à ordre de continuité C1 ou C2 pour les éléments nœud-surface linéaires et quadratiques. Les éléments nœud-surface ne passent pas le patch test. Plus récemment, un élément linéaire utilisant la méthode des nœuds esclaves virtuels pour Passer le Patch Test (PPT) a été proposée dans la littérature. Cependant, cet élément est à ordre de continuité C0 et peut donner des grandes oscillations de la pression de contact à cause des pénétrations initiales des nœuds esclaves virtuels qui sont les pénétrations des nœuds esclaves virtuels dans la surface maître à l’état initial. Pour tenter de surmonter cette difficulté, une nouvelle approche repose sur un élément linéaire utilisant la méthode PPT et la représentation de surface à ordre de continuité C1 pour la surface maître et pour la surface esclave. L’élément quadratique nœud-surface donne des oscillations de pression de contact à cause de la distribution non-uniforme des forces nodales. Afin de résoudre ce problème, la méthode PPT est développée pour l’élément quadratique. [less ▲]

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See detailThe BANANA Project. V. Misaligned and Precessing Stellar Rotation Axes in CV Velorum
Albrecht, Simon; Winn, Joshua N.; Torres, Guillermo et al

in Astrophysical Journal (2014), 785

As part of the Binaries Are Not Always Neatly Aligned project (BANANA), we have found that the eclipsing binary CV Velorum has misaligned rotation axes. Based on our analysis of the Rossiter-McLaughlin ... [more ▼]

As part of the Binaries Are Not Always Neatly Aligned project (BANANA), we have found that the eclipsing binary CV Velorum has misaligned rotation axes. Based on our analysis of the Rossiter-McLaughlin effect, we find sky-projected spin-orbit angles of β[SUB]p[/SUB] = -52° ± 6° and β[SUB]s[/SUB] = 3° ± 7° for the primary and secondary stars (B2.5V + B2.5V, P = 6.9 days). We combine this information with several measurements of changing projected stellar rotation speeds (vsin i [SUB]sstarf[/SUB]) over the last 30 yr, leading to a model in which the primary star's obliquity is ≈65°, and its spin axis precesses around the total angular momentum vector with a period of about 140 yr. The geometry of the secondary star is less clear, although a significant obliquity is also implicated by the observed time variations in the vsin i [SUB]sstarf[/SUB]. By integrating the secular tidal evolution equations backward in time, we find that the system could have evolved from a state of even stronger misalignment similar to DI Herculis, a younger but otherwise comparable binary. Based on observations made with ESOs 2.2 m Telescopes at the La Silla Paranal Observatory under programme ID 084.C-1008 and under MPIA guaranteed time. [less ▲]

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See detailUtilisation de biocapteurs microbiens GFP pour la caractérisation des performances des bioréacteurs
Brognaux, Alison ULg

Doctoral thesis (2014)

The scale-up of bioprocesses is a critical step during bioproducts development. Actually, the mixing operation’s efficiency drops when the reactor volume increases: gradients of glucose and oxygen appear ... [more ▼]

The scale-up of bioprocesses is a critical step during bioproducts development. Actually, the mixing operation’s efficiency drops when the reactor volume increases: gradients of glucose and oxygen appear when operating in fed-batch mode, causing losses of production. The aim of this project is the scale-up and sizing of bioreactors based on the direct physiological parameters to consider this heterogeneity. Concretely, it consists in obtaining an on-line signal of the physiological status of micro-organisms. The coding sequence of the Green Fluorescent Protein (GFP) has been inserted after gene promoters of interest in Escherichia coli to built biosensors. A particular focus has been paid on promoters responding to general stress or specifically to lack of glucose and on those responding to cell growth rate. The GFP biosensors of interest have been tested in scale-down bioreactors, allowing to reproduce industrial hydrodynamic conditions at a laboratory scale. A mini-bioreactor platform has also been developed to enable a high throughput screening of biosensors. The intracellular accumulation of GFP has been measured by flow cytometry and GFP release has been monitored by western blot analyses. For biosensors sensitive to stress general response or glucose limitation, GFP has been induced during a glucose limitation and repressed by comparison when glucose heterogeneities appear. The use of a destabilized GFP has been considered in this project for ribosomal biosensors to approach more instantaneous physiological responses of microorganisms. For these ones, the response is proportional to growth rate during the batch phase, but more complex mechanisms take part during a prolonged glucose limitation. Membrane permeability has also been studied and has been noticed more important in homogeneous fed-batch bioreactors than in scale-down reactors. As GFP leakage has been noted in the extracellular medium, a study has also been carry out about proteins released in the extracellular medium (leakage), and correlated with the cell permeability. Finally, an on-line flow cytometer has been developed for the characterization of physiological status of micro-organisms during the bioprocess, and a 3D-ORM probe allowed to measure their viability on-line [less ▲]

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See detail« Penser avec et contre Fanon : sur Achille Mbembe, ‘’Le fouet de Dieu’’ (in De la postcolonie) »
Melice, Anne ULg

Conference (2014, April 01)

Achille Mbembe remarque, pour le regretter, que la réflexion qu’il a intitulée « Le fouet de Dieu » n’a guère fait l’objet de lectures critiques. C’est précisément à dégager les enjeux de ce chapitre de ... [more ▼]

Achille Mbembe remarque, pour le regretter, que la réflexion qu’il a intitulée « Le fouet de Dieu » n’a guère fait l’objet de lectures critiques. C’est précisément à dégager les enjeux de ce chapitre de son livre De la postcolonie que vise mon intervention. J’avais été très impressionnée, lors de mes recherches doctorales sur le kimbanguisme, par les analyses relatives au christianisme figurant dans Afriques indociles. Ces thèses sont tout à la fois creusées et déplacées dans « Le fouet de Dieu ». L’avant-propos de la deuxième édition du livre suggère brièvement une mise en perspective stimulante de ces pages. Prenant acte de la nécessité de désormais « penser avec et contre Fanon », Mbembe y souligne qu’elles ouvrent sur une conception du rapport éthique à autrui, qui prend chez lui la forme d’une prescription inconditionnelle : « donner la mort à la mort ». Mon interrogation se resserre autour de la thématique de l’altérité. En rassemblant en gerbe les problématiques du désir, de la domination et de la liberté, elle met au jour l’articulation profonde du religieux et du politique. [less ▲]

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See detailL’antonymie. Définition de l’antonymie en langue et description des fonctions sémantico-référentielles de la co-présence antonymique en discours
Steffens, Marie ULg

Doctoral thesis (2014)

Grâce à un examen critique des travaux consacrés à l’antonymie, nous avons pu définir l’antonymie comme une relation multiforme qui touche des unités très différentes sur les plans sémantique et morpho ... [more ▼]

Grâce à un examen critique des travaux consacrés à l’antonymie, nous avons pu définir l’antonymie comme une relation multiforme qui touche des unités très différentes sur les plans sémantique et morpho-syntaxique mais dont l’unicité réside dans une opposition binaire fondamentale enracinée lexicalement. Nous avons ainsi dépassé la simple distinction entre contraires et contradictoires pour entrer dans la fascinante diversité de l’opposition et montrer comme les notions de graduation, de graduabilité, de scalarité, de borne et de seuil se combinent pour forger l’identité sémantique de chaque paire d’antonymes, la rapprocher ou la distinguer des autres. Grâce à l’analyse empirique d’énoncés extraits du journal Le Monde, nous avons montré que la co-présence antonymique en contexte s’appuie pour faire sens sur les relations antonymiques inscrites dans la mémoire linguistique des locuteurs, où sont fortement associées la forme et le sens des antonymes. Nous avons également montré comment la plongée de deux antonymes dans un contexte d’emploi peut moduler leur opposition et la définition du lexème neutre ou intermédiaire, lorsqu’il existe. Les antonymes co-présents, grâce aux rôles sémantico-syntaxiques qu’ils peuvent jouer, contribuent à la structuration des énoncés dans lesquels ils sont employés parce qu’ils organisent les unités qui les entourent sur le plan sémantico-référentiel. [less ▲]

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See detailTheoretical studies of gravitational lensing phenomena: the case of multiply imaged quasars
Wertz, Olivier ULg

Doctoral thesis (2014)

In the early 1960s, gravitational lensing has received a special attention when S. Liebes and S. Refsdal have derived in detail some of the basic equations of the theory. While Liebes (1964) discussed the ... [more ▼]

In the early 1960s, gravitational lensing has received a special attention when S. Liebes and S. Refsdal have derived in detail some of the basic equations of the theory. While Liebes (1964) discussed the probability of detecting these effects and considered several astrophysical applications, Refsdal (1964) derived, inter alia, his famous relation which links the Hubble parameter (H0) to the expected time delays between pairs of lensed images. From that moment, the scientific community fully realized that gravitational lensing effects offer a new way of probing cosmology. However, an important fact has been to accept that the determination of H0 seems to be model dependent, not only on the universe model, but also on the mass distribution of the deflector. The main topic of the present thesis constitutes a straight continuation of this inquiry. We have been sounding parts of the mathematical lensing framework on two fronts. First, considering to first order a very small misalignment between the source, the lens and the observer, we have derived the expressions of the lensed image positions along with their amplification ratios, for the case of power-law axially symmetric mass distributions, the so-called ε-γ family of models (Wertz, Pelgrims & Surdej, 2012). After combining these results, it has allowed us to derive an expression for H0 independently of the model parameters. We have extended this study to the ε-γ family of models with external shear, as well as to the singular isothermal ellipsoid (SIE) models. For both these types of models, we have obtained an expression of H0 which is once again independent, to first order, of the model parameters. Furthermore, even though these families of models remain rigorously distinct, except for the singular isothermal sphere (SIS) and the perfect alignment, the expression of H0 in terms of observable quantities and of the source position takes surprisingly the same simple form. In addition, we have demonstrated the feasibility of analytically constraining to first order the model parameters by only using the astrometric positions of the lensed images. Therefore, for the case of a small misalignment between the source, the deflector and the observer, it is straightforward to determine whether the ε-γ or SIE family of models constitutes a judicious representation of the mass distribution of the deflector. It is conceivable that similar results can be deduced for other families of models. Secondly, we have developed a new analytical approach in order to determine the expression of the deflection angle, hereafter α. Since the latter depends on the deflector mass distribution, there exists no global explicit expression but only an implicit definition of α. Therefore, the analytical methods used to obtain the explicit expression differ for different types of mass distribution. However, using the Fourier transform theory, one may basically express α in terms of the Fourier transform of the surface mass density. Such a method allows us to approach any mass distribution in a unique way. As a first application, we have separately derived the expression of the two components of α for the case of homoeoidal symmetric lenses (Wertz & Surdej, 2013). This original result constitutes a first proof that the Fourier approach constitutes a promising alternative to the complex formalism introduced by Bourassa & Kantowski (1975, corrected by Bray 1984). A particular case of homoeoidal symmetric lenses lies in the non-singular isothermal ellipsoid (NSIE) family of models for which the analytical treatment has been somewhat limited (Kormann & al. 1994). The use of the Fourier approach has made possible to derive a complete analytical treatment of the NSIE, i.e. the expressions of the deflection angle, the deflection potential, and the critical and caustic curves even off the axis (Wertz & Surdej, submitted to MNRAS in february 2014). This original result has allowed us to investigate and better understand this family of models. Furthermore, it is of great interest for mass distribution modeling and to rigorously determine the expected time delays between pairs of lensed images. The previous analytical treatments mainly consisted of parametrical models for the deflector. An alternative way to grasp lenses consists in modeling their mass distribution using non-parametric models. With this aim in mind, we have proceeded as follows: we tessellate the lens plane with squared pixels, and associate to each of them a constant surface mass density. Making use of the Fourier approach, we have derived the expression of the deflection angle for the whole grid. This result contains the main advantage of the non-parametric models, i.e. to model any type of mass distribution without any preconception, and the usefulness of handling quantities which can be described with analytical functions. [less ▲]

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See detailStudy of the Bacillus subtilis PBP4* and RacX, YlmE racemases in relation with disassembly of biofilms
Vanden Broeck, Arnaud ULg

Conference (2014, April 01)

Bacillus subtilis is a PGPR (Plant Growth Promoting Rhizobacterium) Gram positive bacterium and a model for studying the in vitro formation or disruption of biofilms. At the liquid/air interface of ... [more ▼]

Bacillus subtilis is a PGPR (Plant Growth Promoting Rhizobacterium) Gram positive bacterium and a model for studying the in vitro formation or disruption of biofilms. At the liquid/air interface of standing cultures, B. subtilis forms thick pellicles of limited lifetimes. Some D-amino acids have been reported among the factors playing a role in the disassembly of B. subtilis biofilms and ylmE or racX mutants (in which the racemases YlmE or RacX are absent) show a delay in pellicle disruption. The racX encoding gene is part of a bicistronic operon in which the second gene (pbpE) codes for a Penicillin-Binding Protein, the PBP4* whose function is not characterized in Bacillus. Our studies aim to produce, purify and characterize the role of the PBP4* and RacX, YlmE racemases in relation with the disassembly of biofilms. Biochemical and structural data have been obtained and the role of the three proteins has been partly solved. X-ray structures will permit us to create inhibitor molecules against Bacillus subtilis biofilm formation or disassembly. [less ▲]

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