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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

The Origin of Tooth Replacement : Three-dimensional Synchrotron Histology Visualizes the Dental Development of Silurian Stem Osteichthyans

Chen, Donglei January 2017 (has links)
Mechanisms of tooth replacement distribute incongruently among extant gnathostomes (jawed vertebrates): a permanent tooth-generating dental lamina exists in chondrichthyans (cartilaginous fish) and tetrapods but not teleosts, whereas tooth shedding by basal hard tissue resorption occurs in tetrapods and teleosts but not chondrichthyans. Theories about the evolution of tooth development have been biased towards the chondrichthyan conveyor-belt replacement, since there has been no fossil evidence for the origin of osteichthyan (bony fish and tetrapods) tooth replacement until now. 3D virtual dissections with submicron-scale resolution, based on propagation phase contrast synchrotron microtomography (PPC-SRµCT), reveal the growth history of the dentitions of Andreolepis and Lophosteus, 423-Myr-old Silurian stem osteichthyans close to the common ancestor of tetrapods and teleosts. Their marginal jawbones and “tooth cushions” (possible homologues of coronoids) shed teeth by in situ cyclic basal resorption, the earliest examples of osteichthyan-style tooth replacement. The replacement cycles were site-autonomic, and occurred in broad irregular multi-row tooth fields, including at sites separated from the margin of the bone by intervening teeth, showing that the production of replacement teeth did not occur in a single deep dental lamina, but in pockets associated with each tooth, as in many teleosts. It suggests that the functionally and anatomically similar laminae of chondrichthyans and tetrapods are convergent. The marginal jaw bones of both genera carry an initial non-shedding dentition arranged in alternate transverse files, labial to the shedding tooth field, overgrown by later dermal ornament and probably not belonging to the oral domain, but bearing in vivo biting damage showing that they functioned as teeth. The most lingual of these odontodes have been resorbed apically and are overlain by shedding teeth. The first-generation teeth on the tooth cushions display basal resorption in Andreolepis, but semi-basal resorption in Lophosteus. The latter leaves a basal dentine ring from each tooth, implying only odontoclasts are involved in the semi-basal resorption, which is probably the first step towards evolving a site-specific resorption. The polarized displacement of each generation of resorption surfaces reflects the fact that the cyclic replacement, as well as the sequential addition of tooth sites, is closely related to bone growth. Resorption surfaces and growth arrest surfaces also record the life history and the replacement rate. These data provide unique insights into the origin of osteichthyan tooth replacement.
2

Pollen morphology in Ephedra (Gnetales) and implications for understanding fossil ephedroid pollen from the Tibetan Plateau, using a phylogenetic approach

Norbäck Ivarsson, Lena January 2013 (has links)
No description available.
3

Reproduktiv morfologi hos Gnetum cuspidatum-gruppen och dess implikationer för evolution av pollinationsbiologi inom Gnetales

Jörgensen, Annelie January 2014 (has links)
No description available.
4

Hur pollineras kottepalmer?

Eliasson, Clara January 2013 (has links)
No description available.
5

CpG islands, but not their methylation level, are key in the regulation of meiotic recombination in chicken (Gallus gallus)

Westerberg, Ivar January 2019 (has links)
Meiotic recombination plays a fundamental role in many sexually reproducing species. Recombination shuffles the genetic material during the first meiotic cell division resulting in new combinations of alleles within each chromosome. In many organisms, the rate of recombination is not uniform across the genome but consists of so called hotspots where the recombination rate is remarkably higher than the genome average. In mammals, the regulation and location of recombination hotspots is regulated by a gene called PRDM9. Many nonmammalian animals, like birds, lack this gene and the precise mechanism for recombination rate regulation is still unknown. Previous findings in passerine birds have observed an association between recombination rate and a genomic feature known as CpG islands (CGIs). CGIs are often located in promoter regions of genes and depending on their methylation status constitute accessible chromatin regions. It has therefore been suggested that the proteins involved in the regulation of recombination have better access to less condense chromatin regions. In this study, I tested if the association between recombination rate and CGIs found in passerine birds is also true in chicken. I also tested if methylation levels of CGIs play a role in recombination rate regulation in chicken. To this end, I used previously published data for CGI locations and a methylation map in chicken, and unpublished data of recombination rate estimates. I found that the association between recombination rate and CGIs observed in passerine birds extends to chicken, suggesting that this is an ancestral trait in birds. I did not, however, find a negative association between methylation levels and recombination rate as hypothesised based on a relationship between methylation level and chromatin accessibility. This suggests that DNA methylation level at CGIs is not a strong determinant of recombination in chicken, although there may be some workflow artefacts or unknown factors remaining in my analysis obscuring the relationship between these two variables.
6

Microevolution in pygmy grasshoppers

Tinnert, Jon January 2017 (has links)
Knowledge of how spatiotemporal environmental variation impacts ecological and evolutionary processes and contributes to genetic and phenotypic diversity of natural populations is key to understanding and protecting biological diversity. In this thesis I used pygmy grasshoppers to study how environmental conditions, population dynamics, dispersal and admixture may influence genetic structure and diversity, and to evaluate how functionally important variation may affect the ability of populations to cope with novel and changing habitats. Analyses of AFLP (Amplified Fragment Length Polymorphism) markers in Tetrix subulata individuals from 20 sampling locations in Sweden showed significant genetic structure and restricted gene flow among populations. Genetic diversity increased with population size and proportion of long-winged dispersive phenotypes on the island of Öland, but not on the mainland. A contrasting environment comparative approach (CECA) applied to 20 T. undulata populations suggested that processes associated with environmental change differently influence functional and neutral diversity. Long-winged phenotypes were more common in disturbed than in stable habitats, indicative of recent establishment. Color morph diversity was higher in disturbed environments consistent with the notion that polymorphism promotes establishment success. Conversely, neutral diversity (AFLP) was lower in disturbed habitats, pointing to a stronger eroding effect of genetic drift in disturbed compared to stable habitats. I compared genetic and morphological variation between sympatric populations of the two species. Populations of the generally dispersive T. subulata were genetically less differentiated compared with the more sedentary T. undulata, suggesting that the latter species has been less influenced by the homogenizing effects of gene flow. Non-parallel body size differences pointed to species-specific drivers of morphological change. Finally, comparisons of reproductive output of T. subulata females that had been experimentally mated with males from the same or from a different population suggested that responses to interbreeding and genetic admixture can differ in direction and magnitude even between populations within a species, and thus influence whether dispersal translates into gene flow. My thesis emphasizes the complexity of microevolution and illustrates how the effects of different ecological and evolutionary processes can vary according to disturbance regimes and geographic areas, and differ between closely related sympatric species.
7

Sympatric Divergence in Whitefish Dwarfs : An investigation of phenotype-environment correlations in whitefish dwarfs

Eriksson, Olivia January 2021 (has links)
Recent studies have found populations of European Whitefish (Coregonus lavaretus) to be under the process of divergence induced by a predator (pike, Esox lucius). This has caused whitefish to diverge into similar ecotypes but in separate lakes. Even though there are established ecotypes, these do display a high degree of variation, especially among whitefish dwarfs. The underlying mechanisms has not been fully understood and other determining ecological factors than predation are yet to be explored. In this study, I examine whether there are phenotype-environment correlation between whitefish dwarfs that could help identifying the selection pressures shaping this variation between whitefish dwarfs in different lakes. The morphological features of interest were body size and number of gill rakers since these are determining features of divergence. The environmental variables of interest were features of the lake in questions and its surrounding area. No significant correlations were found between the environmental variables and the body size of whitefish dwarfs. I was however through this study able to conclude significant results regarding phenotype-environment correlations between the number of gill rakers in whitefish dwarfs and the combined effects of lake percentage, lake area, temperature and number of (other fish) species in the lake from which they were sampled. It is probable that this is due to these variables affecting the niche space in which the whitefish dwarfs can diverge. Lakes with more potential refuges from pike will result in more divergence in whitefish dwarfs and vice versa. I can hereby propose a model containing these four environmental variables for how they affect the number of gill rakers in whitefish dwarfs.
8

Det riktade yrkesfiskets påverkan på torskens livshistoria längs Sveriges kust

Björlin Svozil, Magnus January 2022 (has links)
Sammandrag Enligt Förenta nationernas livsmedel och agrikulturorganisation (FAO) är 51% av världens fiskbestånd fullt exploaterade och 17% överexploaterade. Denna andelen har ökat kraftigt de senaste hundra åren och en art som varit hårt drabbad av detta är den populära matfisken torsk (Gadus morhua). Havet runt svenska kusten har tidigare ett av världens mest produktiva torskbestånd, men har de två senaste decennierna minskat drastiskt pga. överfisket. Den intressanta frågan blir då hur torsken påverkats av detta hårt riktade fisket? Den här sammanfattningsartikeln diskuteras först om livshistorieteorin, om torsken och hur fisket på Grand banks utanför Kanada fick hela beståndet att kollapsa. Där efter kommer mera ingående om hur det fisket kan ha påverkat torskens evolution och livshistoria. Torskens fenotypiska variation har minskat under det senaste århundradet. Tillväxten har gått ner pga. av denna anledningen. Regimskiften i ekosystemet, som klimat eller föda, har inte påverkat lika mycket som yrkesfisket. Konkurrensen mellan torskens och yrkesfisket efter samma bytesdjur har en inverkan på torsken genom direkta och fördröjda effekter. Skyddade marina områden kan ha gynnsamma effekter på torskbeståndet genom Allee-effekter som en hårt fiskad population kan besitta, som hämmar tillväxten. Slutligen en diskussion om hur hållbart fiske med en jämförelse mellan aktiva och passiva redskap. Sedan om hur dagens fiske i svenska vatten ser ut och en analys av de nya fiskereglerna för år 2022 i delar av Östersjön. Till sist en kort diskussion om skyddade marina områden kan gynna torskbestånden runt Sveriges kuster. Sammanfattningsvis behöver fisket efter torsk runt Sveriges kuster moderniseras och bedriva ett fiske som tar hänsyn till torskens livshistoria.
9

A functional candidate gene approach and the role of insulin-like receptor genes in wing polyphenism in Gerris buenoi

Narayanan, Shrinath January 2021 (has links)
No description available.
10

Proteomics and phylogenetics of the Gnetales

Hou, Chen January 2014 (has links)
A central point of Darwin’s theory of evolution is that accumulation of many small changes during the evolutionary process can result in significant change over time. In light of his theory, plant scientists seek for and compare different plant traits among species e.g., from morphology, DNA or proteins in order to discover the underlying evolutionary patterns and processes. The Gnetales, an intriguing family that comprises Ephedra, Gnetum and Welwitschia, have puzzled scientists for over a century. Their features are evolutionarily difficult to understand in comparison with other seed plants and this has hampered analyses of evolution and phylogeny regardless of whether morphological or molecular data has been utilized. In this thesis, I first attempt (Paper I) to seek for a new evolutionary indicator; a protein profile from pollination drops of Ephedra is compiled, and the results are compared with those from conifers and other seed plants. The aim of this proteomic study was also to investigate whether proteomic profiles vary among Ephedra species and are affected by different selection factors, e.g., pollination mode, ovule protection etc. The results indicate, however, that proteins are present only in very small amounts in pollination drops of Ephedra, and mainly as waste products from degrading cells. This is surprising since proteins are considered important for defense of the naked ovules of gymnosperms, e.g., against pathogens. Pollination drops of Ephedra have a very high sugar concentration and it is possible that carbohydrates are responsible for ovule defense in Ephedra. The second chapter of my thesis (Paper II) is devoted to Gnetum; a phylogenetic study based on genetic markers derived from both nuclear ribosomal regions and chloroplast regions is conducted. Previous studies have been hampered by difficulties with outgroup comparison and homology assessments of informative gene regions. A few attempts have been made to estimate the deepest splits in the genus, all with a limited ingroup sampling. We address the phylogeny of Gnetum and make a first assessment of the monophyly of species, using a denser sampling of taxa and a combination of faster and more slowly evolving molecular markers. The results are discussed in comparison with previous classification and morphology, and will provide a basis for further studies of taxonomy, ecology, and biogeography in Gnetum.

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