The ecology, and life history evolution of the parasitic nematode Marshallagia marshalli, and its adaptability to extreme conditions
Date
2019-07-25
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Abstract
Marshallagia marshalli is a very common, but relatively poorly studied, abomasal nematode infecting a variety of wild ungulates in North America. Despite its economic, social and ecological importance little is known about its impact, life history traits and adaptations to cope with environmental extremes. This parasite has a direct life cycle and the development and survival of its free-living stages are directly influenced by climatic factors, thus are likely to be affected by climate change. The overall aim of my thesis was twofold; i) to understand the development, ecology and impact of M. marshalli, and ii) to use M. marshalli as a model species to study the importance of phenotypic plasticity and local thermal adaptation in the interaction of helminths with climate change. I achieved this using a combination of approaches, including the collection of field data, laboratory experiments, and modelling approaches. Marshallagia marshalli was negatively associated with a variety of fitness indicators of Dall’s sheep, including pregnancy rate and body condition. This negative effect also extended to fetal development and sex allocation through a combination of direct and indirect effects on the fetus and the mother. Hatching of M. marshalli occurred primarily as first-stage larvae (L1) in an advanced stage of development. Less frequently, M. marshalli also hatched as third stage larvae (L3) directly from the egg, with this phenomenon being significantly more common at higher temperatures. Free-living larvae of M. marshalli did not feed nor grow as they matured to infective L3. Eggs and unhatched L1 were significantly more tolerant to freezing compared to free-living L1, suggesting that remaining in the egg as L1, and even to the L3 stage, is an adaptation to cope with the extreme environmental conditions that Marshallagia faces across its extensive latitudinal distribution in North America. Finally, temperature dependent development and mortality rates of free-living stages differed among M. marshalli populations, supporting the hypothesis of local thermal adaptation among populations of this species. When these differences were modelled using classic disease modelling approaches complemented with the Metabolic Theory of Ecology I demonstrated that similar temperature variations have differential impacts on the fitness of M. marshalli from different locations. My research has highlighted the impact of M. marshalli on host fitness at different life stages of the host and demonstrated how subtle differences in the life history traits and thermal tolerances of nematodes can have implications for their persistence in extreme environments and in their response to climate change.
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Keywords
Marshallagia marshalli, local thermal adaptation, phenotypic plasticity, Dall's sheep, host-parasites interactions, body condition
Citation
Aleuy Young, O. (2019). The ecology, and life history evolution of the parasitic nematode Marshallagia marshalli, and its adaptability to extreme conditions (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca.