L'uranium est un radioélément naturel, généralement retrouvé à l'état de traces, mais dont la concentration peut être significativement augmentée à proximité de certaines installations du cycle du combustible nucléaire ou de zones d'agriculture intensive. En raison de son association dans les minerais avec différents éléments traces métalliques tels que le cadmium, l'U est la plupart du temps présent en mélange avec d'autres contaminants dans l'environnement. L'étude de leurs effets combinés est indispensable afin de mieux appréhender le risque engendré par les contaminations métalliques multiples pour les écosystèmes. L'évaluation de la toxicité des mélanges et l'identification des interactions synergiques ou antagonistes sont généralement réalisées sur la base de modèles d'additivité de référence intégrés à des approches descriptives, purement statistiques et sans réelles considérations biologiques. Plus récemment, des modèles mécanistes ont été proposés en alternative afin de mieux rendre compte de la dynamique des processus biologiques et des mécanismes de toxicité des contaminants. Ceux-ci n'ont cependant été mis en pratique que dans un nombre limité de cas d'étude de la toxicité des mélanges. L'objectif de ce projet de thèse a été d'étudier les effets combinés chroniques de l'U et du Cd sur les traits de vie du nématode Caenorhabditis elegans, en utilisant en parallèle une approche descriptive (MixTox) et une approche mécaniste (DEBtox). Pour cela, une exposition des nématodes à différentes concentrations d'U et de Cd, seuls ou en mélange, a été réalisée durant onze jours. Une interaction antagoniste importante entre l'U et le Cd a été identifiée pour les critères d'effet de croissance et de reproduction, à partir des deux approches. L'étude du transfert de l'U et du Cd du milieu vers la nourriture ainsi que de leur bioaccumulation par C. elegans nous a permis de montrer que la coprésence d'U diminuait la fraction de Cd disponible pour l'exposition des nématodes. Afin de déceler la présence d'une éventuelle interaction au niveau des organismes, durant les phases toxicocinétique ou toxicodynamique, les données d'effet ont été réanalysées à partir des concentrations d'U et de Cd dans la nourriture, supposées plus proches des concentrations disponibles pour C. elegans. Des effets combinés globalement additifs, sans interaction notable, ont été mis en évidence pour l'U et le Cd même si des conclusions légèrement contrastées ont été obtenues à partir des approches descriptives et mécanistes. La présente étude permet d'illustrer la complexité de l'étude des effets des mélanges et de l'identification des interactions entre les substances. Malgré quelques difficultés d'application, l'approche mécaniste DEBtox semble particulièrement prometteuse pour décrire la toxicité de mélanges au cours du temps et tester d'éventuels mécanismes d'interaction. À terme, le perfectionnement des outils d'analyse des effets combinés des contaminants devrait permettre une meilleure prise en compte de la problématique des mélanges dans les démarches d'évaluation des risques. / Uranium is a natural radioactive trace element for which elevated concentrations can be found in the vicinity of some nuclear fuel cycle facilities or of intensive farming areas. Due its co-occurrence with different trace metals, such as cadmium, in geological ores, U is generally found associated with other contaminants in the environment. The study of their combined effects on ecosystems is of interest to better characterize such multi-metallic polluted sites. The mixture toxicity assessment and the identification of synergistic or antagonistic interaction are generally performed on the basis of additive reference models integrated to descriptive and purely statistical approaches with no real biological basis. Recently, mechanistic models were proposed to better account for the dynamics of biological and toxicological processes. However, such models have only been put into practice in a few number of mixture toxicity case studies. The aim of this PhD project was to assess the chronic U/Cd combined toxicity on the life history traits of the nematode Caenorhabditis elegans using both a descriptive (MixTox) and a mechanistic (DEBtox) approach. To do so, nematodes were exposed during eleven days to different U and Cd concentrations, alone or in mixture. A strong antagonistic interaction between U and Cd was identified for length increase and brood size endpoints on the basis of both approaches. From the study of the U and Cd media-to-food transfer and of the U and Cd bioaccumulation by C. elegans, we showed that the co-presence of U reduced the available Cd fraction for nematodes. To identify a possible interaction at organism level, occurring during the toxicokinetic or toxicodynamic steps, data were re-analyzed on the basis of U/Cd concentrations in food, assumed to be more closely related to available concentrations for C. elegans. Overall additive effects, without interaction, were identified between U and Cd, even if slightly contrasted conclusions were obtained on the basis of the descriptive and mechanistic approaches. The present study underlines the complexity of studying mixture toxicity and identifying chemical interactions. Despite some application problems, the mechanistic approach DEBtox is particularly promising to describe the toxicity of chemical mixtures over time and to test hypothetical interaction mechanisms. In the future, the improvement of tools to analyze the combined toxicity of contaminants would allow to better address the issue of mixtures in ecotoxicological risk assessment processes.

Please read the abstract in the section 00front of this document / Dissertation (MSc (Agric))--University of Pretoria, 2005. / Food Science / unrestricted

The importance of resource supply and herbivory in driving competitive interactions among species has been an important but contentious issue within ecology. These variables exhibit different effects on species competition when manipulated in isolation but interact when manipulated together. I tested the direct and interactive effects of nutrient addition and simulated grazing (clipping) on the competitive performance of primary producers and community structure of a seagrass bed in South Florida. One square meter experimental plots were established in a mixed seagrass meadow from August 2007 to July 2009. The experiment was a 3 x 3 factorial experiment: 3 fertility treatments: control, medium (2.4 mg N d-1 and 80 µg P day-1) and high (4.8 mg N d-1 and 160 µg P day-1) x 3 clipping intensities (0, 25% and 50 % biomass removal (G)) x 5 replicates for each treatment = 45 plots). Nutrient additions and simulated grazing were done every two months. Fertilization and simulated grazing decreased sexual reproduction in S. filiforme. Fertilization increased competitive dominance within the primary producers while simulated grazing counteracted this effect by removal of the dominant species. Fertilization ameliorated the negative impacts of simulated grazing while simulated grazing prevented competitive exclusion in the fertilized plots. Nutrient addition and simulated grazing both exerted strong control on plant performance and community structure. Neither bottom up nor top down influences was eliminated in treatments where both factors where present. The effects of fertilization on plant performance were marked under all clipping intensities indicating that the system is regulated by nutrient availability both in the presence or absence of grazers. Clipping effects were strong under both fertilized and unfertilized conditions indicating that the seagrass bed can be simultaneously under top-down control by grazers.

Les O-glycoconjugués et les dérivés glycosidiques sont des composés naturels impliqués dans de nombreux processus biologiques. Cependant, leurs propriétés sont grévées par la médiocre stabilité in vivo de la liaison osidique. Il est donc intéressant de développer des mimes non hydrolysables. Nous nous sommes intéressés au remplacement de l'oxygène anomérique par un groupement gem-difluorométhylène.La synthèse d'analogues difluorés d'α-galactosylcéramides avec diverses longueurs de chaînes grasses est décrite dans ce travail. La partie C-glycosidique de ces molécules est construite par l'application d'une séquence d'addition radicalaire et de réduction diastérosélective. La chaîne phytosphingosine est introduite par le synthon α-CF₂-galactose via un transfert de l'O-2 au carbone pseudo-anomérique, puis un couplage peptidique permet de former la partie céramide. Quatre analogues possédant diverses chaînes phytosphingosines (C₅H₁₁, C₂₄H₄₉) et divers acides gras (C₇H₁₅, C₁₅H₃₁ et C₂₅H₅₁) ont été synthétisés et évalués biologiquement. / O-glycoconjugates and carbohydrate-based molecules are natural compounds implied in many biological processes. However, their properties are burdened by the low in vivo stability of the osidic bond. It is thus interesting to develop non hydrolizable mimetics. We were interested in the replacement of the anomeric oxygen by a gem-difluoromethylene group.The synthesis of difluorinated α-galactosylceramides featuring various lipidic chain lenghts is described herein. The C-glycosidic part of these molecules is construted by a sequence of radical addition and diastereoselective reduction. The phytosphingosine chain is introduced on this intermediate by an O-2 to C-1' migration and the ceramide synthesis is completed by a petidic coupling. Four analogues featuring various phytosphingosines chains (C₅H₁₁, C₂₄H₄₉) and fatty acids (C₇H₁₅, C₁₅H₃₁ et C₂₅H₅₁) were synthetized and biologically assessed.

Deux types de revêtements constitués d'environ 80% d'huile végétale ont été élaborés par chimie thiol-X sous UV. Le premier a été obtenu sous UV à partir d'huiles de lin native et standolisées en présence de 0,5 équivalent (SH/ène) de divers thiols, sous air, sans photoamorceur ni solvant. L'effet bénéfique de l'oxydation des acides gras sur la réaction d'addition thiol-ène et les propriétés finales des revêtements a été démontré. Ces matériaux présentent des propriétés équivalentes à celles des films obtenus par oxydation de l'huile de lin, faisant de ce procédé une alternative intéressante à la siccativation des huiles. Le deuxième type de revêtement a été obtenu sous UV à partir d'une huile de coton époxydée en présence de 0,25 équivalent (SHlépoxy) de 7- mercapto-4-méthyl coumarine et d'un photogénérateur de base permettant d'amorcer, sous air, la réaction d'addition thiol-époxy et l'homopolymérisation des fonctions époxy. Ces revêtements sont autoréparables sous UV. / Two types of coatings constituted about 80 wt% of vegetable oil were prepared by photoinitiated thiol-X chemistry. The first one has been prepared by crosslinking of native and stand linseed oil triglycerides in the presence of 0.5 equivalent of various thiols (SH/ene) under air and UV radiation, without photoinitiator or solvent. The oxidation of fatty acids has shown a beneficial effect on the thiol-ene addition as well as on the final properties of coatings. This process is thus an interesting alternative for the drying of oils. The second type of coating has been obtained by photocrosslinking triglycerides of epoxidized cottonseed oil in the presence of 0.25 equivalent (SH/epoxy) of 7-mercapto-4-methyl coumarine and a photobase generator under air. This latter initiated both thiol-epoxy addition and homopolymerization of epoxy functions. This coating could be auto-healable under UV exposure.

Syftet med denna studie är att ge en bild av undervisning som synliggör sambandet mellan addition och subtraktion. Elever har visat svårigheter med att använda sambandet mellan addition och subtraktion för att lösa olika aritmetiska problem. Samtidigt är det en viktig förutsättning för att utveckla effektiva lösningsstrategier. Studien är genomförd med inspiration från fenomenografi där kvalitativ intervju använts som metod. Fem lärare i årskurs 1–3 har intervjuats. Lärarna som har intervjuats har medverkat i två olika utvecklingsprojekt där de tillsammans med forskare fördjupat sig i addition och subtraktion. Resultatet visar på vikten av att tidigt synliggöra sambandet mellan addition och subtraktion samt uppdelningen av tal för att elever effektivt ska kunna lösa olika aritmetiska problem. Vidare framgår i resultatet att sambandet mellan addition och subtraktion kan synliggöras genom att arbeta med addition och subtraktion samtidigt, att undervisa om tals del-helhetsrelationer och genom medvetna val av konkret material och representationsformer av tal. / The purpose of this study is to provide a comprehensive understanding and insights regarding teaching that show the relation between addition and subtraction. Students have shown difficulty in using the relation between addition and subtraction to solve arithmetic problems. At the same time, it is an important prerequisite for developing effective strategies. The study is inspired by phenomenography where qualitative interview was used as a method. Five teachers have been interviewed, which is operative in the first to third grade. The participating teachers have been participating in development projects where they were accompanied by researchers who deepened their knowledge within addition and subtraction. The study shows the importance to visually illustrate the connection between addition and subtraction in an early phase, as well as the separation of numbers so that students are able to solve arithmetic problems more efficiently. Furthermore, the result shows that the relation between addition and subtraction can be shown by working with addition and subtraction simultaneously, working with part-whole relations, and through smart choice of concrete material and representation of numbers.

A set S of vertices in a graph G is a connected dominating set of G if S dominates G and the subgraph induced by S is connected. We study the graphs for which adding any edge does not change the connected domination number.

A set S of vertices in a graph G is a total dominating set if every vertex of G is adjacent to some vertex in S. The minimum cardinality of a total dominating set of G is the total domination number of G. A graph is total domination edge addition stable if the addition of an arbitrary edge has no effect on the total domination number. In this paper, we characterize total domination edge addition stable graphs. We determine a sharp upper bound on the total domination number of total domination edge addition stable graphs, and we determine which combinations of order and total domination number are attainable. We finish this work with an investigation of claw-free total domination edge addition stable graphs.

Addition reactions of pentafluorosulfur bromide (SF5Br) with fluoroolefins were studied. Three new adducts were prepared, SF5CHFCHC1Br, SF5CH2CH(CF3)Br, SF5CHFCFC1Br. SF5Br and CFC1=CHC1 yielded a small amount of product but attempts to add SF5Br to CF2=FCOF were unsuccessful. The mechanism for these addition reactions seems to involve a free radical addition pathway. Steric factors seem to be important in determining whether SF5Br will add to fluoroolefins. The new compounds, SF5CF(CF3)COF and SF5CF(CF3 )CONH2 have been produced from SF5CF=CF2. Analytical data, infrared, Raman, nmr and mass spectra are presented supporting the proposed structures for these new compounds.

The purpose of the study was to evaluate the efficacy of the procedures described in the PEAK-E curriculum in teaching addition skills to children with developmental disabilities. In the present study, three participants were taught to match sample addition problems (A) to a number of pictures (B) corresponding to the sum of A (A-B), and to match sample pictures (B) to textual numbers (C) (B-C). They were then tested to see if they could match sample addition problems (A) to with the textual numbers (C) that were the solution of the addition problems A (A-C). Following mastery of the A-B and B-C relations, none of the participants were able to demonstrate the derived transitive A-C relation. An additional training phase was conducted across all participants whereby two of the five stimulus classes were provided “equivalence” C-A training (matching the textual number C to the equation A), after which all of the participants were able to demonstrate the derived transitive A-C relations across all stimulus classes without direct training of any of the stimulus classes. The results expand on previous research evaluating behavioral approaches to teaching math skills by showing how the development of equivalence class can result in the untrained emergence of novel math skills. Keywords: Stimulus Equivalence, PEAK, Addition, Autism, Developmental Disabilities