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Coming dissertations at TekNat

  • Demographic History and Adaptation in African Populations

    Author: Mário Vicente
    Publication date: 2020-04-03 13:28

    Africa is the continent where modern humans originated and yet, African demographic history remains largely unknown. Through analyzing the genetic composition of extant and extinct individuals, it is possible to reveal signals of past demographic history and adaptation. In this thesis, I applied population genetic methods to investigate both deep African history and demographic changes associated with the migrations of farmers in Africa. While Paper I and II assess the genomic landscape before the arrival farming groups, Paper III, IV and V focus on the demographic patterns associated with the emergence of various African agro-pastoral societies and how shifts in ways of subsistence resulted in different selective pressures on the genomic level. The genomes from Southern African San hunter-gatherers harbor the earliest diverging lineages and represent the first population divergence event within the modern human phylogeny. However, gene-flow from farming groups has complicated the investigation of genetic relationships between different San groups. In Paper I, I established that Southern African hunter-gatherer genetic diversity fitted an isolation-by-distance model when...

  • Genomic and transcriptomic investigation of reproductive incompatibility in Drosophila

    Author: Guilherme Costa Baião
    Publication date: 2020-04-02 11:28

    Both nuclear and cytoplasmic elements can contribute to the emergence of reproductive incompatibilities that influence evolution and speciation. In the projects that compose this thesis, we use genomics and transcriptomics to study some of those elements in Drosophila.

    In the first study, we show that Wolbachia, an endosymbiotic bacterium known to cause reproductive alterations in its hosts, influences gene expression in D. paulistorum. Affected genes were associated with biological functions such as metabolism, immunity, reproduction, and chemical communication. Our results indicate that Wolbachia accentuates the differences in expression profiles between semispecies and suggest that the symbiont influences host pre-and postmating isolation.

    In the second paper, we uncover widespread persistent heteroplasmy in D. paulistorum. We reveal that D. paulistorum mitochondria are polyphyletic, with two divergent mitotypes, and that the heteroplasmy likely originated through introgression. One of the mitotypes shows biparental inheritance, non-responsiveness to host energy demands and rapid titer...

  • Functional properties of silicon nitride based materials for joint applications

    Author: Luimar N. Corrêa Filho
    Publication date: 2020-04-01 13:50

    Total hip and total knee replacements are generally successful procedures for patients suffering with pain due to bone fracture or diseases affecting the joints. However, the materials that are commonly used still have limitations. In particular, corrosion products and wear debris may give rise to negative body reactions.

    In this thesis, silicon nitride based materials were investigated for use in joint implants, namely as a coating for e.g. femoral heads and the metallic modular taper junction in hip implants, as well as a bulk bioceramic for joint applications. One of the main advantages of these materials is the potential to dissolve slowly in aqueous solutions, releasing only biocompatible ions.

    To understand the mechanical and wear properties of these materials, thin film coatings were deposited using magnetron-based techniques onto Si wafers and a CoCrMo alloy, the latter frequently used in biomedical implants. Coatings up to 8.8 µm thick were deposited on 2D flat discs as well as full 3D implant heads, following a CrN interlayer for improved adhesion. The chemical composition, microstructure, surface roughness, adhesion, wear resistance, and dissolution...

  • Long bone growth and evolution revealed by three-dimensional imaging

    Author: Jordi Estefa
    Publication date: 2020-03-31 13:08

    Propagation phase-contrast synchrotron radiation microtomography is a non-destructive method used for studying histology in three dimensions (3D). Using it, the 3D organization of the diaphyseal cortical vascularization in the humerus of two seymouriamorphs was analyzed in this thesis. Their vascularization suggests a combination of active growth and a long pre-reproductive period, an intermediate condition between that of Devonian tetrapods and early amniotes, reflecting a gradual change in evolution. The focus of the thesis then shifts to the metaphysis of long bones. The latter possesses complex 3D structures difficult to capture in 2D images. Observations in extant tetrapods have shown that hematopoiesis in long-bones requires the presence of tubular marrow processes opening onto an open medullary cavity with a centralized vascular system. A network of tubular marrow processes was found in connection with interconnected small cavities in the metaphyses of seymouriamorphs which may have acted as open spaces containing a centralized vascular mesh. Based on this interpretation, the long-bone marrow cavity of the Permian stem-amniotes studied here could have been the oldest...

  • Microstructure and Mechanical Properties of Magnetron Sputtered Refractory Metal Thin Films

    Author: Stefan Fritze
    Publication date: 2020-03-26 12:20

    The design and development of new multifunctional materials that exhibit a combination of high hardness and ductility, as well as a high corrosion resistance and thermal stability, is one of the key challenges in the field of material science. The focus of this thesis is on the development of novel multifunctional magnetron sputtered CrNbTaTiW–C based thin films. Carbon was selected as an alloying element to investigate if it could modify the microstructure (via grain refinement) and improve the properties (e.g. the hardness and ductility).

    TaW-rich and near-equimolar high entropy alloys in the CrNbTaTiW system were selected as starting points for this study. The latter alloys were predicted, based on empirical design rules, to form a single-phase solid solution. In contrast, thermodynamic calculations showed that the films at equilibrium should be composed of a mixture of several phases at temperatures below 1100 °C.  Experimentally, however, a single-phase bcc structure was observed for the deposited films and it was concluded that the films were kinetically and not entropy stabilised. A hypothesis is that the kinetics during sputtering allow a ’direct’ phase selection...

  • Polyporoid fungi of Tanzania : Taxonomy, transcriptomics and biochemical analyses of Kusaghiporia usambarensis and Piptoporellus baudonii

    Author: Juma Mahmud Hussein
    Publication date: 2020-03-26 08:38

    Polyporoid fungi refers to basidiomycetes with fruiting bodies with the hymenium located to the inner surfaces of pores or narrow tubes. The majority of polyporoids belongs to Polyporales. Most Polyporales are saprobes, but some are plant pathogens. The overall aim of this thesis was to study the taxonomy, systematics and chemistry of the two species Kusaghiporia usambarensis (saprobic) and Piptoporellus baudonii (a plant pathogen) collected from Tanzania, using morphological and molecular approaches, combined with transcriptomics and pharmacognostic investigations.

    The main contribution of this thesis includes the description a new genus with the new species K. usambarensis from the Usambara Mountains, Tanzania; investigation of the chemical composition of volatile compounds from this medicinal mushroom; isolation and structure determination of a novel and most abundant peptide in K. usambarensis, and further to elucidate the phylogenetic position of Piptoporellus baudonii (formerly known as Laetiporus baudonii) by using a four molecular markers dataset.

    Paper I was...

  • Reasoning with thermal cameras : Framing and meaning-making in naturalistic settings in higher education

    Author: Robin Samuelsson
    Publication date: 2020-03-26 05:45

    In this Licentiate thesis, framed by the Resources framework and Social semiotics, I explore how students and instructors, investigating thermal phenomena with IR cameras, come to conceptually and epistemologically frame the naturalistic settings they participate in. Additionally, I look at how they productively employ resources , what barriers they encounter while reasoning about the thermal phenomena and how the semiotic resources of the IR camera relate to the framing and resources employed in their investigations.

    The thesis is based on three groups of participants: two chemical engineering students and their two lab instructors (PhD students) in a calorimetry lab part of a unit on thermodynamics in a chemistry introduction course, and primary school teacher students in a physics unit on thermodynamics that is part of a course on science. The engineering students and their instructors were studied in a chemistry lab involving the Born-Haber cycle and enthalpy change of solution for some salts. The primary school teacher students were studied in a classroom where they had just had a class on heat...

  • Latest Miocene – Early Pliocene Paleoclimate and Phytoplankton Productivity

    Author: Boris-Theofanis Karatsolis
    Publication date: 2020-03-19 13:00

    Paper I

    Pre-Quaternary paleoclimate studies in Australia mainly focus on terrestrial records from the southeastern part of the continent. IODP Expedition 356 drilled on the northwestern Australian shelf, yielding Miocene-Pleistocene paleoclimate records in an area where climate archives are scarce. Post-expedition research revealed a dry-to–humid transition across the latest Miocene and early Pliocene (start of the “Humid Interval”). However, the complex tectonic history of the area makes these interpretations challenging. In this study, we investigate late Miocene to early Pliocene sediment cores from two sites that are only 100 km apart, but situated in two adjacent basins (Northern Carnarvon and Roebuck Basins). Combining lithofacies study, time-series analysis of potassium content (K wt%) and calcareous nannofossil abundance counts (N/g), this work disentangles the complex interplay between basin evolution and climate change between 6.1-4 million years ago (Ma). Overall, the investigated proxies show high correlation between both sites, except during 6.1-5.7 Ma. During this latest Miocene interval, Site U1463 records a gradual increase in K wt%,...

  • Highly Concentrated Electrolytes for Rechargeable Lithium Batteries

    Author: Viktor Nilsson
    Publication date: 2020-03-17 08:36

    The electrolyte is a crucial part of any lithium battery, strongly affecting longevity and safety. It has to survive rather severe conditions, not the least at the electrode/electrolyte interfaces. Current commercial electrolytes are almost all based on 1 M LiPF6 in a mixture of organic solvents and while these balance the many requirements of the cells, they are volatile and degrade at temperatures above ca. 70°C. The salt could potentially be replaced with e.g. LiTFSI, but dissolution of the Al current collector would be an issue. Replacing the graphite electrode by Li metal, for large gains in energy density, challenges the electrolyte further by exposing it to freshly deposited Li, leading to poor coulombic efficiency and consumption of both Li and electrolyte. Highly concentrated electrolytes (HCEs) have emerged as a possible remedy to all of the above, by a changed solvation structure where all solvent molecules are coordinated to cations – leading to a lowered volatility, a reduced Al dissolution, and higher electrochemical stability, at the expense of higher viscosity and lower ionic conductivity.

    In this thesis both the fundamentals and various approaches to...

  • Who ate whom? Paleoecology revealed through synchrotron microtomography of coprolites (fossil feces)

    Author: Martin Qvarnström
    Publication date: 2020-03-13 08:58

    Fossil droppings, known as coprolites, are being increasingly recognized as a valuable source of paleoecological information with special regard to diets, parasitism, and physiology of extinct taxa. Here, it is suggested that the excellent preservation and amount of inclusions in the coprolites (e.g. food residues and parasites) qualifies them as Lagerstätten – deposits with exceptional paleontological information. However, two interlinked problems commonly arise when they are studied. Firstly, it is often difficult to tie coprolites to producers and, secondly, it is challenging to recognize the fragmented and randomly distributed inclusions in their matrix. Here I use propagation phase-contrast synchrotron microtomography (PPC-SRμCT) in combination with other techniques to solve these problems. As a result, the oldest known example of archosaurian osteophagy is uncovered based on inter alia the occurrence of serrated teeth and many crushed bones in coprolites assigned to the Late Triassic theropod-like archosaur Smok wawelski. Osteophagy has previously been thought to be rare among extinct archosaurs with the exception of Late Cretaceous...

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