Study of squirrelpox virus in red and grey squirrels and an investigation of possible routes of transmission

The red squirrel (Sciurus vulgaris) is native to Eurasia, but in the UK its survival is being threatened by the non-native grey squirrel (Sciurus carolinensis). Since its introduction to the UK from the USA the grey squirrel has increased its range at the expense of the red squirrel. Although competition for resources clearly plays a role in this replacement, an infectious viral disease, caused by squirrelpox virus (SQPV) and hosted apparently asymptomatically by the grey squirrels, has now been recognised as a major contributing factor. Little is known about the pathogenesis of infection in grey squirrels in comparison to red squirrels, but understanding this is essential to determining how the virus spreads within and between the red and grey squirrels. The aims of this thesis were to investigate the course of SQPV infection in red and grey squirrels and possible routes of virus transmission. Specifically, for the first time, a novel Real Time PCR (qPCR) assay and immunohistochemistry were used to investigate the presence of SQPV in various tissues from naturally infected red squirrels and experimentally infected grey squirrels. In diseased red squirrels SQPV DNA was found in several tissues with the highest amounts being found in skin samples. This reflects the multiple lesions that were easily visible on the red squirrel carcasses. There was no indication of systemic disease although the viral DNA was detected, at lower levels, in other internal organs. Grey squirrels were experimentally infected with SQPV isolated from naturally-infected red squirrels with fatal clinical disease. In contrast to SQPV-infected red squirrels no clinical lesions, other than mild scab formation at the site of inoculation, were found in the grey squirrels post-infection. No gross pathological changes indicative of systemic infection were observed and these findings were reflected in the qPCR and histopathology results. Viral DNA was only detected by qPCR in samples from the site of inoculation (scarified skin) and at lower concentrations in other skin tissues such as digital and eyelid skin. In addition, histopathology and immunohistochemistry examination revealed evidence of infection characterized by ballooning degeneration of keratinocytes, and acanthosis and spongiosis of the epidermis. These skin lesions were self limiting and minor compared to the infected red squirrel skin samples. The molecular variation in the virus isolated over time from different parts of the UK was also investigated. Seven SQPV isolates (4 from Scotland and 3 from England) were tested and results indicated that there are no significant changes in the amino acid sequence of any of the three genes examined apart from one amino acid change (one base change) in one gene. All Scottish isolates examined showed this change in comparison to English isolates. The results in this thesis show that there is a mild pathology associated with SQPV infection in grey squirrels. Scabs form at the site of infection but are less proliferative than in infected red squirrels, though they may still serve to contaminate the environment with virus leading to further outbreaks of disease. In contrast it seems likely that the proliferative lesions suffered by red squirrels and the greater amounts of virus that this leads to are likely to be more significant to the epidemiology of disease in localised outbreaks

Exploitative and Hierarchical Antagonism in a Cooperative Bacterium

Social organisms that cooperate with some members of their own species, such as close relatives, may fail to cooperate with other genotypes of the same species. Such noncooperation may take the form of outright antagonism or social exploitation. Myxococcus xanthus is a highly social prokaryote that cooperatively develops into spore-bearing, multicellular fruiting bodies in response to starvation. Here we have characterized the nature of social interactions among nine developmentally proficient strains of M. xanthus isolated from spatially distant locations. Strains were competed against one another in all possible pairwise combinations during starvation-induced development. In most pairings, at least one competitor exhibited strong antagonism toward its partner and a majority of mixes showed bidirectional antagonism that decreased total spore production, even to the point of driving whole populations to extinction. Differential response to mixing was the primary determinant of competitive superiority rather than the sporulation efficiencies of unmixed populations. In some competitive pairings, the dominant partner sporulated more efficiently in mixed populations than in clonal isolation. This finding represents a novel form of exploitation in bacteria carried out by socially competent genotypes and is the first documentation of social exploitation among natural bacterial isolates. Patterns of antagonistic superiority among these strains form a highly linear dominance hierarchy. At least some competition pairs construct chimeric, rather than segregated, fruiting bodies. The cooperative prokaryote M. xanthus has diverged into a large number of distinct social types that cooperate with clone-mates but exhibit intense antagonism toward distinct social types of the same species. Most lengthy migration events in nature may thus result in strong antagonism between migratory and resident populations, and this antagonism may have large effects on local population sizes and dynamics. Intense mutual antagonism appears to be more prevalent in this prokaryotic social species than has been observed in the eukaryotic social slime mold Dictyostelium discoideum, which also exhibits multicellular development. The finding of several cases of facultative social exploitation among these natural isolates suggests that such exploitation may occur frequently in nature in many prokaryotes with cooperative traits

Low-frequency noise measurements in silicon power MOSFETs as a tool to experimentally investigate the defectiveness of the gate oxide

Peer Reviewe

A Distributed Electrical Model for Interdigitated back Contact Silicon Solar Cells

AbstractIn this paper we introduce a quasi 3-D electrical model for a high efficiency interdigitated back contact (IBC) solar cell. This distributed electrical network is based on two-diodes circuit elementary units. It allows accounting for the resistive losses due to the transport through the emitter, the back surface field (BSF) and the fingers and busbars metallization. Moreover, it can model the electrical shading losses attributed to the BSF busbar. We calibrated the electrical components of the model according to experimental measurements on real devices. The validity of the model is demonstrated by the good agreement between simulation and experimental results for dark and illuminated IV measurements with and without masked busbars. The model can now easily be applied to simulate and optimize different metal grid layouts

Simulation Study of Light-induced, Current-induced Degradation and Recovery on PERC Solar Cells

Abstract The way to permanently recover the well-known Light-Induced Degradation (LID) which affects the p-type Cz-Si PERC solar cells represents one of the main challenges of photovoltaic research. In this work we have set up a numerical simulations flow which allows us to reproduce the experimental measured values of figures of merit (FOMs) of four different Cz-PERC solar cells lots subjected to a degradation and two regeneration processes. The recombination centres in bulk and the Boron-Oxygen complexes (B-O) are modeled by means of two trap levels tuned on the basis of experimental data. From simulations we confirm that the FOM degradation levels off after 16hours and the regeneration process characterized by relatively long time process is preferred in terms of performance recovery. In addition, further cells with different passivation films are analyzed by adopting the same methodology

Full Understanding of Hot Electrons and Hot/Cold Holes in the Degradation of p-channel Power LDMOS Transistors

Degradation induced by hot-carrier stress is a crucial issue for the reliability of power LDMOS transistors. This is even more true for the p-channel LDMOS in which, unlike the n-channel counterpart, both the majority and minority carriers play a fundamental role on the device reliability. An in-depth study of the microscopic mechanisms induced by hot-carrier stress in new generation BCD integrated p-channel LDMOS is presented in this paper. The effect of the competing electron and hole trapping mechanisms on the on-resistance drift has been thoroughly analyzed. To this purpose, TCAD simulations including the deterministic solution of Boltzmann transport equation and the microscopic degradation mechanisms have been used, to the best of our knowledge, for the first time. The insight gained into the degradation sources and dynamics will provide a relevant basis for future device optimization

Pathological and phylogenetic characterization of Amphibiothecum sp. infection in an isolated amphibian (Lissotriton helveticus) population on the island of Rum (Scotland)

Outbreaks of cutaneous infectious disease in amphibians are increasingly being attributed to an overlooked group of fungal-like pathogens, the Dermocystids. During the last 10 years on the Isle of Rum, Scotland, palmate newts (Lissotriton helveticus) have been reportedly afflicted by unusual skin lesions. Here we present pathological and molecular findings confirming that the pathogen associated with these lesions is a novel organism of the order Dermocystida, and represents the first formally reported, and potentially lethal, case of amphibian Dermocystid infection in the UK. Whilst the gross pathology and the parasite cyst morphology were synonymous to those described in a study from infected L. helveticus in France, we observed a more extreme clinical outcome on Rum involving severe subcutaneous oedema. Phylogenetic topologies supported synonymy between Dermocystid sequences from Rum and France and as well as their distinction from Amphibiocystidium spp. Phylogenetic analysis also suggested that the amphibian-infecting Dermocystids are not monophyletic. We conclude that the L. helveticusinfecting pathogen represents a single, novel species; Amphibiothecum meredithae

Numerical Simulation of Vertical Silicon Nanowires based Heterojunction Solar Cells

Abstract Nanowires (NWs) solar cells are expected to outperform the thin-film counterparts in terms of optical absorptance. In this theoretical study we optimize the geometry of vertical crystalline-amorphous silicon core-shell NW arrays on doped ZnO:Al (AZO)-Glass substrate by means of 3-D optical simulations in order to maximize the photon absorption. The optimized geometry is investigated by means of 3-D TCAD numerical simulation in order to calculate the ultimate efficiency and the main figures of merit by taking into account recombination losses. We show that optimized 10 μm-long crystalline – amorphous silicon core-shell (c-Si/a-Si/AZO/Glass) NWs can reach photo-generated current up to 22.94 mA/cm 2 (above 45% larger than that of the planar counterpart with the same amount of absorbing material) and conversion efficiency of 13.95%

Analysis of the impact of doping levels on performance of back contact - back junction solar cells

AbstractIn this work, by exploiting two-dimensional (2-D) TCAD numerical simulations, we performed a study of the impact of the doping levels on the main figures of merit in the different regions of a crystalline silicon Back-Contact Back-Junction (BC-BJ) solar cell: the emitter, the Back Surface Field (BSF) and the Front Surface Field (FSF). The study is supported by a dark loss analysis which can highlight the contribution of several recombination mechanisms to the total diode saturation current. The efficiency curve as a function of doping level exhibits a bell-shape with a clearly identifiable optimum value for the three regions. The decrease in efficiency observed at lower doping values is explained in terms of higher contact recombination for BSF and emitter, and in terms of higher surface recombination for FSF. The efficiency decrease observed at higher doping values is ascribed to the higher surface recombination for FSF and Auger recombination for all cases

Simulation Study of Multi-wire front Contact Grids for Silicon Solar Cells

Abstract Multi-wire (MW) front-contact schemes represent a promising alternative to standard H-pattern structure with ribbon busbar (BB) in silicon solar cells. In the case of MW schemes, busbar are replaced by copper wires. MW have been demonstrated to enhance the photo-generation with respect to a standard H-pattern structure with ribbon busbar when solar cells are encapsulated and assembled in modules. However, the influence of the geometrical and optical properties of the encapsulation layers as well as of wires on the optical effective shading is not exhaustively treated by the literature. In this work, we have performed electro-optical simulations of MW and BB based solar cells in order to calculate the effective optical shading factor, the enhancement of conversion efficiency and the saving of contact-paste, with respect to the BB design. Specifically, we have studied by means of a ray-tracing simulation tool the significant impact of the front contact grid geometry, of the encapsulation layer thickness and of the optical properties of the cell front interface on the effective optical shading. The calculated values of effective optical shading are used to determine the enhancement of the figures of merit and the paste saving with respect to the reference silver BB scheme. On the basis of our calculations the adoption of optimized MW designs may enhance the conversion efficiency up to 0.5% abs , allowing paste saving up to 50 mg per cell