Adult stem cells are vital for tissue maintenance. Stem cell over proliferation results in tumour formation, whilst loss of stem cells causes tissue degeneration and a variety of diseases. Stem cell maintenance and proliferation is regulated through somatic structures called niches. The germline stem cell niche in Drosophila ovary has been well defined and it is useful to better understand the interactions between niche and stem cells. Notch signalling is needed for germline stem cell niche creation and maintenance. The aim of this thesis is to better understand both the regulation of Notch signalling during development and its requirement in the adult niche. The first paper, "Reversible regulation of stem cell niche size through dietary control of Notch signalling", revolves around the dynamicity of the niche. The niche is found to respond to diet stimuli and has the ability to be restored. Notch was previously found to be involved in the maintenance of the niche. We found that Notch signalling is altered by diet, and we dissect its different maintenance and recovery roles in the ovary. In the second paper, "ZO-1 controls stem cell niche assembly by acting as an upstream regulator of Deltex-dependent Notch signalling", we show how Notch signalling is finely regulated during niche formation through interplay with the proteins Polychaetoid and Deltex. This paper leads to a better understanding of how the niche is assembled and how Notch signalling is regulated in a context-dependent way. The obtained results from both papers will help understand the dynamics of the model germline stem cell niche, and how Notch signalling is found at the convergence between internal and external stimuli regulating the ovary's response to a changing environment.

Current theoretical insights into firm internationalisation have mainly focused on large American multinationals or on smaller early internationalising firms. Global niche players, often competing alongside or even complementary, have been less researched. They, like larger players, often strive to go global and dominate their market segments simply to survive and endure processes of global consolidation and often it is necessary to achieve top global positions. One such successful global niche strategy variant is ‘Global Hidden Champions’ (GHCs), which are low profile, global top three under US$ 5 billion firms, mainly found in Germany (Simon, 2012). The question becomes pressing: how do GHCs reach world market leadership? This study seeks to identify through which internationalisation paths and associated market entry modes 30 GHCs investigated from Germany and Britain reached global dominance and the specific competitive advantages without which such strategies would not have succeeded. This thesis takes an evolutionary historic perspective by distinguishing enduring with less-enduring GHCs, established at various points in time between 1838 and 2007. A qualitative multiple case study approach is used based on 30 cases, 15 from each country. The abductive stance facilitates deductions of existing theoretical frameworks, whilst also allowing exploratory new themes to emerge. Structured and semi-structured interviews, combined with documentation, allow triangulation of findings and help minimise bias. Guided by the conceptual framework, data has been thematically coded, analysed and systematically explored, allowing several new themes to emerge. Not one single GHC was found pursuing the traditional Uppsala Model internationalisation path. Instead 43% were identified as Born Globals (BGs); 20% as Born-Again Globals (BAGs); but no less than 37% emerged as quite different hybrid internationalisers, which because of their quite distinctive traits, were denoted ‘Re- Born-Again Globals’ (Re-BAGs). The analysis involved innovative methodological analysis, which further clarified some partial overlap of BGs, BAGs, and re-BAGs with Uppsala Model features, yet decided differences in terms of tempo and direction. All three alternative paths led to top three global market leadership positions, depending on the GHCs context and historical circumstances. Nevertheless, recently established GHCs pursued BG paths, whereas more enduring GHCs almost all followed BAG and re-BAG paths. Younger British GHCs frequently embarked on early proactive paths, whilst their more enduring German equivalents pursued more belated internationalisation routes. History emerged as pivotal. BAGs and re-BAGs were both distinguished by critical incidents which, in all 17 cases, shifted strategies on to much more proactive internationalisation paths. German GHCs, though, shifted primarily in response to technology advances, whereas British GHCs typically responded to specific management appointments. Literature on accelerated internationalisation paths, such as in relation to BGs, remains unclear on commitments beyond trade. Yet, 90% of GHCs deployed substantial foreign direct investment (FDI), in addition to mere trade activity. Initially, German GHCs grew organically focusing on neighbouring markets. In contrast, British GHCs often used acquisitions and targeted more global, in particular, Commonwealth markets. FDI thus emerged as a crucial addition to export activity in all three alternative paths followed by BGs, BAGs and re-BAGs enabling them to sustain leading global market positions. Yet, such paths were in turn contingent upon competitive advantages. GHCs from both countries complied remarkably closely with 6 out of 8 of Simon’s (2009) identified theoretical HC Model traits: being ‘leadership with ambitious goals’; ‘innovation’; ‘high-performance employees’; ‘closeness to customer’; ‘globalisation’, and ‘focus’. More in depth, albeit exploratory, analysis further uncovered the critical role played by ‘visions and values’ and ‘brand’, leading to a new tentative theoretical GHC Model. This new Model, moreover, recognises a virtuous cycle of market leadership advantages from which more enduring GHCs particularly benefitted. It also integrates comfortably with Teece’s (2014) model of dynamic capabilities, extending his framework by including market leadership approaches and more precise competitive advantages of the GHC Model, alongside more specific concepts relating to entrepreneurial orientation. In summary, this study contributes to the knowledge of how both long-standing and newer German and British GHCs conquer global markets by unveiling their specific and successful internationalisation paths, market entry mode choices and their underlying competitive advantages.

Wild bird reservoirs of influenza A contribute to the overall genetic diversity of influenza, an increased range of endemic areas, as well as, transmission methods not commonly seen in human infections. These additions to influenza transmission increase the threat posed to human populations. Therefore, understanding the patterns of transmission of influenza A subtypes in avian hosts, as well as the environmental variables associated with transmission, is paramount to creating effective surveillance programs and forecasting potential areas of high genetic changes. Using a dataset of ~151,000 birds sample for avian influenza in the US and Canada from 1986-2017, we explore spatial patterns of influenza genotypes and model the environmental niches where certain types are found. Cluster analysis and niche modeling indicate overlap but also imperfect concordance between where each subtype of avian influenza was found and where each was predicted to circulate in wild bird populations. Overall, the Midwest and New England regions indicate higher risks of influenza A in wild birds across all flu types. In addition, the urban, wetland, and water land-cover types, as well as, low levels of human population density increase the likelihood of influenza presence in the avian populations. These results indicate that influenza transmission in wild birds is heavily affected by the activities of humans as well as the general characteristics of land cover types. Together, these results allow researchers to gain a better understanding of the spatial mechanisms of the broad scale patterns associated with influenza and the areas of particular risk associated with subtypes.

Diverse fish species consume fruits and seeds in the Neotropics, in particular in the lowland reaches of large rivers, such as the Amazon, Orinoco, and Parana in South America. Floodplains of the Amazon River and its lowland tributaries are characterized by marked hydrological seasonality and diverse assemblages of frugivorous fishes, including closely related and morphologically similar species of several characiform families. Here, I investigated whether or not these fishes are capable of detecting fluctuations in food availability and if they are, how they adjust their feeding strategies. I tested predictions of optimal foraging, limiting similarity and resource partitioning theories with regard to expansion or compression of niche breadth and reduction in trophic niche overlap among species in relation with fluctuations in the availability of alternative food resources. I monitored fruiting phenology patterns to assess food availability and conducted intensive fishing during the high-, falling-, and low-water seasons in an oligotrophic river and an adjacent oxbow lake in the Colombian Amazon. I combined analysis of stomach contents and stable isotope ratios to evaluate dietary patterns, niche breadth, and niche overlap. Diets of six characiform fish species (Brycon falcatus, B. melanopterus, Myloplus asterias, M. rubripinnis, and M. torquatus) changed in a manner that indicated responses to fluctuations in food availability. Feeding strategies during the peak of the flood pulse were consistent with predictions of optimal foraging theory. During times of high fruit abundance, fish preferentially consumed items to which their phenotype is best adapted, maximizing net energy gain and enhancing fitness. As the flood pulse subsided and the availability of forest food resources was reduced in aquatic habitats, there was not a consistent pattern of diet breadth expansion or compression, even though diet shifts occurred, suggesting interspecific differences in foraging efficiencies. Analyses of diets and isotopic ratios revealed a general pattern of increased dietary segregation as the water level receded. Although there never was complete niche segregation among these fishes, these dietary changes effectively reduced interspecific niche overlap. Implications of these results and contribution of allochthonous food resources to diversity maintenance of floodplain fishes are discussed.

Thesis (Ph. D.)--University of Texas at Austin, 2006. / Vita. Includes bibliographical references.

Thesis (M.Sc.). / Written for the Dept. of Biology. Title from title page of PDF (viewed 2008/07/24). Includes bibliographical references.

Thesis (M.S.)--Ohio University, June, 2008. / Title from PDF t.p. Includes bibliographical references.

Thesis (Ph. D.)--Oregon State University, 1999. / Typescript (photocopy). Includes bibliographical references. Also available on the World Wide Web.

Thesis (Ph. D.)--University of Missouri-Columbia, 1996. / Typescript. Vita. Includes bibliographical references (leaves 97-105). Also available on the Internet.

Thesis (Ph. D.)--University of Missouri-Columbia, 1996. / Typescript. Vita. Includes bibliographical references (leaves 97-105). Also available on the Internet.