Le génome du bananier (Musa sp.) est envahi par un nombre important de séquences de Banana streak virus (BSV), virus à ADN double brin de la famille des Caulimoviridae qui n'a aucune étape d'intégration au génome hôte au cours de son cycle de multiplication. La majorité de ces intégrations eBSV (endogenous BSV) est défective mais certaines sont restées fonctionnelles et peuvent être à l'origine de particules virales suite à des stress. L'objectif de ce travail de thèse est de préciser si les eBSV sont maintenus ou non dans le génome de Musa balbisiana des bananiers et d'étudier les conséquences évolutives que cela engendre. Nous avons tout d'abord caractérisé les eBSV pour trois espèces de BSV (Banana streak goldfinger virus (BSGFV), Banana streak obino l'ewai virus (BSOLV), Banana streak imove virus (BSImV) présents dans le génome du bananier modèle M. balbisiana cv Pisang Klutuk Wulung (PKW). Nous avons montré que les intégrations eBSGFV et eBSOLV étaient di-alléliques avec un seul allèle fonctionnel à chaque fois, contrairement à eBSImV qui est mono-allélique et pour lequel nous n'avons pas pu identifier l'allèle à l'origine de l'infection. Leur contexte génomique d'intégration diffère avec une co-localisation d'eBSGFV et d'eBSOLV sur le chromosome 1 et d'eBSImV sur le chromosome 2. Ces résultats nous ont permis de développer les outils moléculaires nécessaires à la caractérisation de ces trois eBSV dans la diversité de M. balbisiana. Cette caractérisation a révélé la diversité de structures des eBSV et éclairé une partie encore inconnue de la phylogénie de l'espèce M. balbisiana. Dans un second temps nous avons étudié les mécanismes de régulation des eBSV. Ce travail a porté sur les mécanismes d'ARN interférent pouvant expliquer le maintien des eBSV dans le génome des bananiers. Cette analyse révèle que les eBSV sont effectivement sous contrôle d'un mécanisme de type ARNi et la forte production de petits ARNs de 24nt ciblant les eBSV suggère qu'il s'agit d'un silencing au niveau transcriptionnel (TGS). En parallèle, nous avons aussi recherché les mécanismes mis en place par les bananiers non-porteurs d'eBSV en cas d'infection afin de connaître les défenses constitutives des bananiers face à une attaque virale BSV. Nous avons, sur la base de ces résultats, proposé un modèle de régulation des eBSV et des BSV et discuté de l'impact que ces mécanismes auraient pu avoir sur l'évolution des eBSV. L'ensemble des données de ce travail ont permis de préciser les étapes évolutives qu'ont connues les eBSV dans le génome du bananier, expliquant le maintien que l'on observe aujourd'hui. / The nuclear genome of banana plants is invaded by numerous viral sequences of banana streak virus (BSV), a DNA virus belonging to the family Caulimoviridae which does not require integration for its replication. These endogenous BSV (eBSV) are mostly defective; however, some can release a functional viral genome following activating stresses. The objectives of this work were to identify if the eBSV are maintained or not in the M. balbisiana genome and to study the impacts of this on the evolution of banana plants. First, we characterized three functional eBSV sequences present within the Musa balbisiana cv PKW genome: (Banana streak goldfinger virus (BSGFV) ; Banana streak obino l'ewai virus (BSOLV) ; and, Banana streak imove virus (BSImV). We show that eBSOLV and eBSGFV are di-allelic with just one functional allele contrary to eBSImV which are mono-allelic and for which we cannot reveal the functional allele. Their genomic areas of integration are different and we also observe that eBSOLV and eBSVGFV are both on chromosome 2 whereas eBSImV is on chromosome 1. These results allowed us to develop the molecular tools required for the characterization of these 3 functional eBSVs within the diversity of M. balbisiana. This characterization has revealed the structural diversity of eBSV and has thus clarified previously unresolved details of M. balbisiana phylogeny. Secondly, we studied the regulatory mechanism of eBSV expression. This work investigated if RNA interference (RNAi) mechanisms could explain the maintenance of eBSV in the Musa genome. Our analyses have shown that, as expected, eBSV was under the control of RNAi mechanisms and the strong production of 24nt small RNAs that target eBSV suggests that Transcriptional Gene Silencing (TGS) was involved in this control. In parallel, we investigated the mechanisms implicated in the anti-viral defense during a BSV infection on a banana plant without eBSV in order to understand the constitutive defense of banana plants. On the basis of these results we have proposed a regulation model of eBSV and BSV and we discuss the impact of silencing regulation on eBSV evolution. Data accumulated during this work have clarified several steps in the co-evolutionary history of Musa sp. and eBSV and explain the maintenance of eBSVs in Musa genomes that we observe today.

Call number: LD2668 .T4 1959 B68

Elongation of the vertebrate rostrocaudal axis depends on localised populations of axial progenitors. Previous work has demonstrated the presence of Neuromesodermal (NM) progenitors that behave as multipotent stem cells, which contribute to the neurectoderm and mesoderm throughout axis elongation. They have been localised to the Node-­‐Streak Border (NSB) in the primitive streak region, and the Chordoneural Hinge (CNH) in its descendant, the tail bud. At primitive streak stages, the Caudal Lateral Ectoderm (CLE) on either side of the primitive streak itself is also fated for neurectoderm and mesoderm. However, fate mapping studies in mouse and chick have suggested that these progenitors are more transitory than those in the NSB and CNH, leading to the idea that two different types of progenitor cell exist in the primitive streak region; long-­‐term (stem cell-­‐like) and transient progenitors. In this thesis, I have examined the potency of the CLE cells by heterotopically grafting them into the NSB. Anterior CLE cells are exquisitely sensitive to their position and differentiate predominantly as neurectoderm, mesoderm, or both, depending on their exact location in the NSB. Most significantly, their descendants are retained in the CNH, indicating that CLE cells show equal potential to NSB progenitors on transplantation to the border environment. The relationship between fate and potency within the streak stage embryo suggest a mechanism by which stem cells are maintained not only by their intrinsic stem cell program, but are also influenced by their location. Furthermore, I have investigated the expression of candidate markers of NM-­‐progenitors, and have found that the combined expression of Sox2 and T genes in the progenitor area coincides with observed NM-­‐potency, and could serve as a marker for this stem cell population. Over time, axial elongation comes to a halt and NM-­‐progenitors are thereafter not longer active. It is still unclear how exactly this process occurs and specifically whether axial elongation ceases because NM progenitors are eliminated. I have investigated the events occurring immediately before the end of axial elongation. Morphological and gene expression analysis shows that apoptosis reaches a peak only after the complete axis has been laid down, and is not dramatically elevated in the progenitors themselves before that. In order to test signalling pathways that lead to progenitor maintenance, I have developed an in vitro tail growth assay that recapitulates in vivo development, as measured by several morphometric criteria. I show that, even though FGF signalling is critical for most cells in the tail bud including NM-­‐progenitors, it is not sufficient for NM-­‐ progenitor maintenance. In contrast, exposing tail buds to elevated Wnt/β-­‐catenin signalling does prolong the lifetime of NM-­‐progenitors in the ageing tail bud, as judged by the presence of Sox2-­‐T double-­‐positive cells. In this regard we have found that the time of cessation coincides with the disappearance of Sox2-­‐T double-­‐positive cells, the disappearance of Wnt3a and concomitant neuralisation of the progenitor region. This data suggest an important governing role for Wnt signalling in both maintenance and fate decision of NM progenitors. Thus the disappearance of Wnt signalling in the tail bud over time could well be the main reason for triggering the halt of caudal elongation.

No description available.

Cassava brown streak disease (CBSD) threatens food and economic security for smallholder farmers throughout East and Central Africa, and poses a threat to cassava production in West Africa. CBSD is caused by two whitefly-transmitted virus species: Cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV) (Genus: Ipomovirus, Family Potyviridae). Although varying levels of tolerance have been achieved through conventional breeding, to date, effective resistance to CBSD within East African cassava germplasm has not been identified. RNAi technology was utilized to integrate CBSD resistance into the Ugandan farmer-preferred cassava cultivar TME 204. Transgenic plant lines were generated expressing an inverted repeat construct (p5001) derived from coat-protein (CP) sequences of CBSV and UCBSV fused in tandem. Northern blots using probes specific for each CP sequence were performed to characterize 169 independent transgenic lines for accumulation of CP-derived siRNAs. Transgenic plant lines accumulating low, medium and high levels of siRNAs were bud graft challenged with the virulent CBSV Naliendele isolate alone or in combination with UCBSV. Resistance to CBSD in the greenhouse directly correlated to levels of CP-derived siRNAs as determined by visual assessment of leaf and storage root symptoms, and RT-PCR diagnosis for presence of the pathogens. Low expressing lines were found to be susceptible to CBSV and UCBSV, while medium to high accumulating plant lines were resistant to both virus species. Absence of detectable virus in the best performing p5001 transgenic lines was further confirmed by back-inoculation via sap or graft challenge to CBSD susceptible Nicotiana benthamiana and cassava cultivar 60444, respectively. Data presented shows robust resistance of transgenic p5001 TME 204 lines to both CBSV and UCBSV under greenhouse conditions. Levels of resistance correlated directly with levels of transgene derived siRNA expression such that the latter can be used as predictor of resistance to CBSD.

Gas turbines are important for power generation and aircraft engines. Over thepast century, there has been improvements in components of the gas turbine such ascompressors, turbines and nozzles, but very little progress has been made in combustor technology. The combustion still occurs at constant pressure and the only changes made are in terms of its design and mixing of fuel and air streams. These design changes have only allowed minimal improvements in gas turbine efficiency. To achievea substantative improvement in efficiency, it is required to make a technology change such as the introduction of constant volume combustion.<div><br></div><div>In this work, one such constant volume combustion device in the form of wave rotor combustion is studied and further developed for use in gas turbines. Wave rotors are periodic-flow devices that provide dynamic pressure exchange and efficient energy transfer through internal pressure waves generated due to fast opening and closing of ports. In addition, there is also confined high speed turbulent deflagration. If the blades are curved, then the flow undergoes angular momentum change from inlet to outlet, generating shaft work. This will allow maximum extraction of work potential from the wave rotor. In addition, an attempt is made to check the applicability of plasma assisted ignition for wave rotors. </div><div><br></div><div>A computational tool is developed to understand physics of non-axial channelwave rotors. The governing equations are formulated in one dimension through a passage average approach. Shaft work is estimated using conservation of angular momentum and enrgy to verify the working of numerical model. The model shows increase in shaft work with increase in blade curvature, but as the angle is increased, the possibility of ignititing the reacting mixture becomes difficult since it is hard tomove the mixture towards the ignition port. </div><div><br></div><div>An alternate ignition source using plasma discharges is investigated through experiments. Two experiments are developed, one to make ultrafast measurements of plasma properties such as gas heating and lifetime of electronically excited molecules, and a second experiment to understand ignition characteristics of a pin to ring electrode configuration. The experiments show that excited nitrogen which reacts with molecular oxygen to form atomic oxygen is short lived and forms oxygen atoms extremely rapidly. This rapid formation of oxygen atoms assists in fast ignition. The ignition experiment with pin to pin electrode showed that even though there is fast ignition, the propagation speed does not change significantly with pulse repetition frequency. Ignition with pin to ring electrode showed fast ignition and increase inflame speed with pulse repetition frequency. Results show that plasma discharge can be used as an ignition source for wave rotors but will need further investigation.</div><div><br></div><div>The development of computational tool and plasma discharge experiments has provided a solid base for future efforts in wave rotor combustion and design of full scale non-axial wave rotor combustor experiment.</div>

Rewards can be used to navigate a person’s behaviour through stimulation. Knowledge about how humans experience and react to rewards has been of use in many areas, one such example is how retailers use point systems to retain customers. This paper aimed to investigate how users of language learning applications feel that they were motivated by their reward systems. To analyse this earlier research in motivational perspectives was used as well as a model of user technology acceptance. This was researched through a case study, for which a pre-study was done on five chosen applications to identify what rewards systems had been implemented in them. Two applications were picked because of the document analysis, which were then used by the test subjects as part of the study. Before they started using the applications, each test subject was interviewed to ascertain their levels of interest and language skills. They were then asked to use the applications however much they’d like over a two to three-week period. The result showed that all our subjects had noticed in one way or another. What they perceived as most motivating differed between all the subjects, but all of them were affected by at least some of the reward system we asked about. We found that all our subjects agreed that the reward systems compromising of streaks were the most motivating. Our subjects were split in what sort of streak they preferred. Some preferred the sort of streak you collect from using the application daily while others preferred the streak you gain when you answer right to several questions in a row. / Belöningar kan stimulera en person och styra dennes beteende. Kunskaper om hur människan reagerar på och upplever belöningar har använts i olika områden, till exempel återförsäljare som använder poängsystem för att hålla kvar sina kunder. Syftet med den här uppsatsen var att undersöka hur användare av språkinlärningsapplikationer upplever att belöningssystemen motiverade dem. Teorin som användes var tidigare forskning om olika motiverande perspektiv samt en modell över hur användare accepterar ny teknologi. Undersökningen var strukturerad som en fallstudie där det först utfördes en förstudie av utvalda applikationer för att identifiera de olika belöningssystemen som fanns i språkinlärningsapplikationerna. Genom förstudien bestämdes även de två språkinlärningsapplikationer försökspersonerna fick använda.  Försökspersonerna intervjuades kort för att skapa en profil om vilka de var och vilken tidigare erfarenhet de hade av språk och språkinlärningsapplikationer. Därefter fick de använda båda applikationerna under 2 till 3 veckor. Slutligen utfördes längre intervjuer med varje person för att ta reda på hur de hade upplevt belöningssystemen. Resultatet visade på att alla försökspersoner hade märkt av belöningssystemen på något sätt. Det skilde sig från person till person vad de ansåg var mest motiverande, men alla blev motiverade av någon av de belöningssystemen som det frågades om. Det som försökspersonerna var mest överens om var att Streaks (vilket är när systemet håller koll på när användaren utfört handlingar flera gånger i rad) var motiverande. Författarna kunde också se att vissa av försökspersoner upplevde att den formen av streak som innebar att försökspersonen använde språkinlärningsapplikationen flera dagar i rad var mer motiverande, medan andra ansåg att den formen av streak som innebar att försökspersonen fick flera rätt svar i rad i språkinlärningsapplikationen var mer motiverande.

The satellite RNA of Lucerne Transient Streak Virus (LTSV) is a 322-nucleotide, single-stranded circular RNA that has a rod-like structure very similar to that of viroids. As it does not encode any translation products and cannot replicate independently of a helper virus, the satellite RNA is proposed to rely on viral-encoded proteins for the replication and/or cell-to-cell movement that facilitate its systemic infection in a host. To investigate the requirements for replication of the LTSV satellite RNA, transgenic plant systems were generated to express the viral RNA-dependent RNA polymerase and predicted viral transport protein independently as well as in combination. Results of infectivity assays of these transgenic lines demonstrated for the first time that the viral-encoded RNA-dependent RNA polymerase is necessary and sufficient for the replication of LTSV satellite RNA, and that no additional viral proteins are required for its cell-to-cell or systemic transport.

The satellite RNA of Lucerne Transient Streak Virus (LTSV) is a 322-nucleotide, single-stranded circular RNA that has a rod-like structure very similar to that of viroids. As it does not encode any translation products and cannot replicate independently of a helper virus, the satellite RNA is proposed to rely on viral-encoded proteins for the replication and/or cell-to-cell movement that facilitate its systemic infection in a host. To investigate the requirements for replication of the LTSV satellite RNA, transgenic plant systems were generated to express the viral RNA-dependent RNA polymerase and predicted viral transport protein independently as well as in combination. Results of infectivity assays of these transgenic lines demonstrated for the first time that the viral-encoded RNA-dependent RNA polymerase is necessary and sufficient for the replication of LTSV satellite RNA, and that no additional viral proteins are required for its cell-to-cell or systemic transport.

Thesis (Ph.D.)--University of Nebraska-Lincoln, 2006. / Title from title screen (site viewed May 23, 2007). PDF text: iv, 165 p. : ill. (some col.) ; 1.95Mb UMI publication number: AAT 3237062. Includes bibliographical references. Also available in microfilm and microfiche formats.