Mechanically isolated cell cultures were chosen as a model system to examine wound-induced dedifferentiation at the molecular level as large quantities of physiologically and morphologically similar G1-arrested mesophyll cells could be obtained. Within 5 days of culture such non-dividing, photosynthetic cells become heterotrophic, and have completed a first nuclear division and cytokinesis. There are few changes in cell morphology during the first 2-3 days in culture. However, during this period there is a massive increase in respiration rate and total RNA synthesis. Following DNA synthesis there is a rapid cell expansion, mitosis and cytokinesis. Steady state transcript populations were monitored through the first 8 days of culture by analysis of the products of in vitro translations on 2-D gels. Large changes in gene expression were evident during the first 3 days in culture with several genes highly up-regulated and others down-regulated. Dedifferentiation can be separated into 3 different phases. Firstly, reactivation of the cell cycle during which there are few cytological or physiological changes but gross changes in the expression of genes possibly associated with wounding or stress. Secondly, DNA synthesis, first mitosis event and phragmoplast formation during which there are minor changes in transcript abundance. Finally a continuation of the cell cycle with little alteration in transcript abundance. Changes in plastid morphology are only apparent after 10-14 days resulting in the formation of proplastid like structures. However, mRNA for both large subunit ribulose bisphosphate carboxylase and small subunit ribulose bisphosphate carboxylase decrease to basal levels within a day of culture and photosynthetic capacity diminishes when the first cell division is evident. Plastid dedifferentiation can therefore be considered separately and proceeds slowly being more or less complete after 2-3 cell divisions. Dedifferentiation is therefore seen to be a complex process which involves the interaction of several factors i.e wounding and hormones and results in temporal changes in transcript abundance, changes in the mode of respiration, morphology and cell proliferation.

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A TEM study of the plastic deformation around indentations made in Ga(1-x)A1xAs of various compositions ranging from x=0 to 0.3 i.e. 0 to 30%A1 is described in this work. The GaAlAs examined was in the form of capping layers grown on the {001} face of GaAs substrates. The indentations were made with a load of 5g at room temperature using a Vickers diamond indenter with its diagonals parallel to the directions. Slip dislocations and microtwins were seen around the indentations in the TEM. The slip dislocations extend far into the crystal whereas the microtwinning is concentrated closer to the indentation. The occurrence of microtwinning has been found to be more dependent on the type of doping rather than the Al concentration. In n-doped crystals microtwinning occurs asymmetrically around the indentations where as when p-doped they occur more symmetrically. For n- and p-doping the leading twinning dislocations are As(g) and Ga(g) respectively. In n-doped GaAlAs microtwinning occurs predominantly on slip planes that converge under the indent. In p-doped GaAlAs microtwinning occurs on both converging and diverging slip planes. All the microtwins are formed from intrinsic overlapping stacking faults.

Avian species are known to have the capacity to respond to environmental changes through physiological adjustments. The process whereby organisms adjust their phenotype without genetic change is termed phenotypic plasticity and it is mostly observed to be a phenotypic improvement to ecological challenges. Metabolic rate (MR), which is the rate of energy expenditure in a species, is a highly flexible physiological parameter which results in a great diversity of avian standardised metabolic rates. Like birds from high latitudes, Afrotropical bird species are expected to have the capacity to adjust their energy expenditure to match the availability of resources. Previous studies on the flexibility of physiological parameters in birds have focused on the magnitude of change of physiological adjustments and the cues inducing these changes. Comparative research has furthermore investigated metabolic rates across aridity, altitude, latitude and temperature gradients. Recently, a clear dichotomy has become evident with elevated metabolic rates observed in high latitude birds in winter and a down-regulation of metabolic rates observed in birds exposed to low latitude mild winters. In this study, the shape of the reaction norm, the magnitude, the reversibility, the direction and the rate of change of two physiological parameters, basal metabolic rate (BMR) and summit metabolic rate (Msum), were investigated in a coastal and an inland population of Southern Red Bishops (Euplectes orix) through seasonal acclimatisation and laboratory acclimation. Summer and winter basal metabolic rates as well as body mass, were highly flexible traits in free-ranging coastal and inland Red Bishops. Birds acclimatised to a mild coastal climate in winter exhibited reduced basal and summit metabolic rates, whereas birds originating from a more variable inland climate increased basal metabolic rate in winter, but did not show increases of Msum in winter. Red Bishops responded to short term thermal acclimation under laboratory conditions by gradually changing body mass. Acclimation periods of 21 days revealed a negative relationship between body mass and acclimation air temperature. Peak responses of basal metabolic rate to ambient temperature change were observed in both coastal and inland birds between two and eight days after the change in acclimation air temperature. The influences of seasonal acclimatisation on energy expenditure differed between coastal and inland birds, however, during laboratory acclimation individuals from the two populations showed no difference in response. Within the individuals of the coastal and inland Southern Red Bishops, phenotypic flexibility is observed in body mass, basal metabolic rate and summit metabolic rate as a response to environmental changes. This flexibility is thought to increase thermoregulatory capacities of the Southern Red Bishop in different habitats and climates.

A Zn - 1 wt % Al alloy has been studied in an attempt to relate tensile data and metallographic observations to assumed modes of superplastic deformation. Tensile testing, was carried out at a constant crosshead speed to determine the stress versus strain rate relationships as a function of grain size and temperature. Further analysis included activation energy determinations, low temperature (-100°C) deformation behavior, deformation grain growth characteristics, and surface and internal metallography. Stage II deformation was consistent with a process combining boundary sliding and migration and boundary diffusion. Deformation in stage I was characterized by a low strain rate sensitivity and a variable activation energy. Normal slip processes were operative during stage III deformation. Deformation grain growth and anisotropic grain shape changes were pronounced and variable in stages I and II. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

A qualitative analysis of current creep theory has been made by studying the creep of polycrystalline lead. The behaviour of the low temperature stress-strain curve with prior creep history, and the strain response to decreases in creep stress were examined. The effect of temperature and creep strain on the low temperature stress-strain curve was investigated over the temperature range 0.5Tm to 0.8Tm. Specimens were quenched to 77°K. after creep and strained to determine the stress-strain curve. The 77°K. yield stress was found to increase during primary creep and remain constant in steady state. Increasing the creep temperature drastically lowered the low temperature yield stress. The reversible flow stress ratio was found to decrease with increasing temperature. These observations were in qualitative agreement with both a reaction rate theory and a rearrangement model. Stress decrease tests were carried out by reducing the creep stress after deforming the specimen varying amounts into primary and steady state in the temperature range 0.5Tm to 0.85Tm . The strain response to a stress decrease in steady state was in best agreement with the simple recovery theory. The variation In yield stress due to non-regular obstacle spacing was found to be extremely small at all temperatures and did not behave in accordance with the qualitative predictions of the rearrangement theory. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

No description available.

A key factor in studying evolutionary biology is an understanding of the mechanisms organisms utilise in the ongoing process of adaptation. When faced with a heterogeneous and unpredictable environment, we expect organisms to evolve either as specialists or generalists, yet a unifying theory as to which will evolve is still lacking due to conflicting hypotheses based on limited empirical evidence. Phenotypic plasticity allows a single genotype to express different phenotypes, and has been found as an adaptive response to changing environments in all major taxa. With the advent of genomics it has become possible to study the underlying genetics of this phenomenon. It is however becoming clear that there is no single principle governing the plastic response, but rather a complex set of interactions between what appears to be regulatory and structural genes. With empirical data only recently becoming more readily available, the modelling of plastic responses are often still founded on the theoretical predictions and assumptions for which there is little proof. To bridge the gap between theory and nature, the challenge facing scientists today is the construction of experimental systems where theoretical predictions can be scrutinised. Given that phenotypic plasticity is a widespread phenomenon, understanding the magnitude and constraints of this response is an important issue in the study of evolution. Models have predicted a correlation between genome size and phenotypic plasticity, with increased genome size (complexity) linked to higher levels of phenotypic plasticity. Experimental findings, however, increasingly point to plasticity being governed by complicated sets of interactions between various parts of the genome, the adaptive landscape, and environmental cues. In the work presented here, a study was designed to test for a correlation between genome size and the level of plasticity by, looking at the fitness response of phages exposed to varying temperature. Seven phages differing in genome size and genome composition were used. Genome sizes ranged from 5386 bp to 170 000 bp. Taking advantage of the short generation times of phages, fitness could be measured as the growth rate per hour, which was compared among the different phage groups. The growth of large populations within a constant, controlled environment minimized the complications of environmental heterogeneity, and allowed for quantitative measure of the response to different temperatures. This was used to gain insight into how genome size relates to the level of phenotypic plasticity. Limited generation numbers were allowed for, to ensure population growth could be directly related to the plasticity of the genome, since numerous generations would be required for the effects of selection to become apparent. Adsorption rates are influenced by temperature, and were therefore measured to determine if it had a significant effect on the resulting population density. Results showed a marginal interaction between genome size and phenotypic plasticity, with adsorption rate having no significant effect. More experimental work would be required to verify this finding. / Dissertation (MSc (Genetics))--University of Pretoria, 2006. / Genetics / unrestricted

The aim of this PhD thesis is to incorporate deformation twinning in a fullield viscoplastic crystal plasticity model based on fast Fourier transform in an effort to gain insights into its role on strain localization. This work is motivated by current experimental evidences on the important role that dislocation reactions at the twin interface play on damage initiation in materials during plastic deformation. We began first by investigating the role of slip on stress localization. To this end, we simulated the effect of macroscopic deformation path, which dictates a macroscopic stress state, as well as pre-existing microstructure in typical ferritic steel, where plastic deformation is accommodated by slip mechanism. The results show that the width of localized strain rate regions near grain boundaries is a function of the deformation path, and there is a positive correlation between local Taylor factor and local stress field, which slightly depends on deformation path. For the incorporation of mechanical twinning in twinning-induced plasticity (TWIP) steel, we implemented predominant reorientation scheme (PTR) in vpFFT, which was implemented previously in the mean field VPSC. The comparison between experimental and simulation results indicates that twin volume fraction, final texture, and stress-strain curve were satisfactorily predicted. Despite that predominant twin reorientation scheme was not suitable to capture lamellar shape of twins in the microstructure, twin domains were predicted to form and grow at or close to grain boundary regions. Finally, we surveyed current literature, which aimed at capturing the characteristic lamellar morphology of twins. Literature review shows several unsuccessful crystal plasticity simulations in capturing twin nucleation and twin lamellar shape at measocale. These inabilities can be attributed to i) twin nucleation that is controlled by local atomistic configurations and stress fluctuations at the grain boundaries, and ii) the random or stochastic nature of twin nucleation, which has been proved by EBSD observation. Based on the EBSD observations, twin nucleation depends on both microstructural (e.g, grain size, dislocation density) and loading conditions ( e.g, stress, strain). Furthermore, the propensity, frequency, and morphology of deformation twins are different among grain with the same orientation and applied boundary conditions.

Ca2+/calmodulin-dependent protein kinase II (CaMKII) is involved in Ca2+signaling throughout the body. CaMKII is enriched in the hippocampus and required for learning and memory formation. Four highly conserved genes encode CaMKII in vertebrates: A, B, G, and D. All CaMKII variants are constituted of a kinase domain, regulatory segment, variable linker, and hub domain. These domains comprise an individual subunit which oligomerize together via the hub domain to form multimeric holoenzymes. These four genes are most variable in the linker domain due to extensive alternative splicing. The variable linker significantly impacts the activation of CaMKIIA. Herein, I attempt to develop an in vitro assay which resembles physiological activation of CaMKII via Ca2+ oscillations. I provide preliminary data which indicate that alternative splicing of the variable linker in CaMKIIA modulates the Ca2+ frequency dependent autonomy of these variants. Additionally, neuronal CaMKII variants of CaMKIIA and CaMKIIB decode Ca2+ oscillations into different levels of autonomous activity. Lastly, I assess the impacts of three de novo mutations (Q274P, R275H, and F294S) on Ca2+/CaM sensitivity in CaMKIID by providing data that these 3 mutants increase the sensitivity of CaMKIId to Ca2+/CaM and that Q274P and F294S mutants display Ca2+/CaM independent activity.