Parasitic nematodes of grazing livestock represent an increasing economic and welfare problem for British agriculture. By investigating specific life-cycle stages of these parasites, it may be possible to identify key molecules or pathways that are required for the survival of the worms, and thus exploit these for future control strategies. It has been shown previously that the third larval stages (L3) of the ovine parasitic nematode Teladorsagia circumcincta produce high levels of transcript for the enzyme GTP-Cyclohydrolase relative to later developmental stages. As the ratelimiting factor in the production of tetrahydrobiopterin, GTP-Cyclohydrolase is required for a number of different biochemical pathways, including those involved in the production of serotonin and melanin. As the L3 do not feed, it can be hypothesised that, if finite resources are being used in the production of transcript encoding this enzyme, then it may be important for survival. In this thesis, a number of approaches were taken to explore the function of GTPCyclohydrolase in the life-cycle development of T. circumcincta. The closely related parasite, Dictyocaulus viviparus, was used as a model organism to explore the role of GTP-Cyclohydrolase and serotonin production with regards to larval arrest, or hypobiosis. This process occurs readily under experimental conditions in D. viviparus, which is not possible with T. circumcincta. Quantitative PCR was used to examine GTP-Cyclohydrolase transcript levels in two different strains of D. viviparus, one that enters larval arrest when exposed to cold conditions and one that does not. No differences were observed between the two strains suggesting that GTP-Cyclohydrolase was unlikely to be involved in hypobiosis. The model nematode, Caenorhabditis elegans, was used to perform functional complementation experiments to assess the role of GTP-Cyclohydrolase in the cuticle, as it has been shown previously that C. elegans GTP-Cyclohydrolase mutants have a ‘leaky cuticle’ and are killed by lower doses of anthelmintics and bleach than the wild-type worms. The T. circumcincta gene for GTP-Cyclohydrolase was able to restore cuticular integrity of C. elegans GTP-Cyclohydrolase-deletion mutants, suggesting that the role played by the protein in both species is similar. In vitro inhibition experiments using a chemical inhibitor of GTP-Cyclohydrolase showed that T. circumcincta larval development was disrupted in the presence of the inhibitor. It was also shown that T. circumcincta L3 that were exposed to sunlight produced melanin, suggesting that the levels of GTP-Cyclohydrolase observed in the preparasitic stages of T. circumcincta may be required for the synthesis of melanin. Together, these data suggest that GTP-Cyclohydrolase is required by the preparasitic stages to survive on pasture. Ultraviolet radiation has been shown previously to be harmful to T. circumcincta L3, so if the melanin production provides protection from this, then it would be crucial for the survival of the pre-parasitic stages.

Gossypium hirsutum, upland cotton, is one of the major crops grown in the United States and the world. Upland cotton is cultivated in areas that are ideal breeding grounds for the difficult to manage, southern root-knot nematode (RKN), Meloidogyne incognita. Host plant resistance is the most effective way to control RKN populations. However, resistance used in most breeding programs stems from a few related sources. Novel sources of resistance have been identified but have yet to be introduced into elite breeding lines or genetically studied. The objectives of this study are two-fold. The first is to develop elite germplasm by introgressing RKN resistance from primitive accessions into modern cotton genotypes via backcrossing. The second is to use simple sequence repeats (SSRs) to identify loci associated with RKN resistance in the primitive accessions. The genotypes used will be: 1) inoculated with M. incognita, 2) phenotypically analyzed by measuring the nematode reproduction as eggs per gram of fresh root and host response using a root gall index, 3) genetically evaluated by using SSR markers to detect polymorphisms between the RKN resistant TX accessions and DP90 (susceptible genotype), and 4) analyzed using linkage and mapping software. Elite germplasm that contains: 1) high yield potential and a high level of RKN- resistance or 2) high fiber quality and RKN-resistance was developed by performing two backcrosses based on phenotypic analyses. A third screen is currently underway to ensure the introgression of the RKN resistance genes. Agronomic tests will need to be done before the germplasm is released. Genetic analyses using SSR-based primer sets of the TX accessions did not yield expected results. Of the 508 primers sets tested, only 31 were polymorphic between the TX accessions and DP90. A bulked segregant analysis approach was used to test the 31 primer sets on the resistant and susceptible bulks of the F2 population but no polymorphisms were seen. However, analyses found that the TX accessions were more genetically similar to Mexico Wild Jack Jones than to Clevewilt 6-3-5. More work needs to be done to understand the mechanism of RKN resistance in the TX accessions.

Meiofauna are an abundant, diverse and important component of the marine biota, however, much of their ecology has been neglected. Despite their high densities, meiofaunal abundance is often patchy. Meiofauna present in high numbers at one site will often be less abundant in seemingly similar adjacent sites. What factors govern this variability? How readily do these animals colonize new patches? How do various biological and environmental factors affect meiofaunal colonization rate and resulting assemblage structure? The response of meiofauna to changes in abiotic factors, including sediment grain size, depth, exposure and distance from the ocean floor, was quite variable. Often one factor would affect certain taxa and not others. Even slight increases in depth resulted in drastic declines of harpacticoid copepods while nematodes were unaffected. Meiofauna were also fewer in sediments with large interstitial spaces. Some meiofauna were most abundant in sediments placed closer to the ocean floor. Other taxa colonized distant substrata as rapidly as they did substrate located closer to the ocean floor. This suggested differences between taxa in their rates of active dispersal. The effects of macrofauna on meiofauna have been debated. In particular, how do clams affect the colonization and assemblage structure of meiofauna? Certain characteristics of clams were isolated and evaluated: feeding behaviour, bioturbation rate/depth and metabolic byproducts. Clams that caused the greatest meiofauna declines were shallow burrowing deposit-feeders. Constant disturbance to the upper sediment by these clams was likely responsible for meiofaunal impact. Conversely, suspensionfeeding clams that passed quickly to deeper sediment and remained stationary had little impact on meiofauna. Finally, a survey of local marine nematodes added nine genera new to Canada and 24 genera new to British Columbia. A review was also compiled that shows nematodes and other meiofauna have been neglected for much of Canada. Although these small and abundant animals are quick to colonize even distant habitats they are quite sensitive to cues from the surrounding biotic and abiotic environment. This sensitivity combined with their ease of collection make meiofauna a valuable asset to any number of ecological investigations. / Ecology

Bibliography: 129-139 / iv, 139 leaves, [14] leaves of plates : ill ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Plant Pathology, 1984

Bibliography: leaves 96-112 / iv, 156 leaves : ill ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Agronomy, Waite Agricultural Research Institute, 1986

Diss. (sammanfattning) Uppsala : Sveriges lantbruksuniv., 2004. / Härtill 6 uppsatser.

Diss. (sammanfattning) Uppsala : Sveriges lantbruksuniversitet, 2005. / Härtill 5 uppsatser.

Thesis (Ph. D.)--University of Adelaide, Dept. of Plant Science, 1996? / Includes bibliographical references (leaves 307-329).

Thesis (M.Sc.(Agric.))--University of Western Australia, 2006.

Thesis (Ph. D.)--University of Missouri-Columbia, 1998. / Typescript. Vita. Includes bibliographical references. Also available on the Internet.