Disentangling a freshwater amphipod–acanthocephalan system from ecological and molecular perspectives

One of the major goals in ecological research is to understand factors that influence distribution, diversity and prevalence of parasites and their hosts. How hosts are distributed geographically clearly restricts the spatial distribution of associated obligatory parasites. This restriction is more complicated for multi-host parasites, because they are affected by the geographical distribution of both intermediate and final host species. Taxonomic and genetic diversity of parasites in a particular area can be influenced by the history of colonization, with time available for colonizing a new area being particularly relevant. Prevalence of parasites in final hosts is likely to be positively related to their prevalence in intermediate hosts, but what determines the prevalence of parasites in intermediate hosts? In this thesis I explore these questions using a host–parasite system widely distributed in freshwater bodies across the Holarctic: acanthocephalan worms that use aquatic birds and mammals as final hosts and the amphipod Gammarus lacustris Sars as an intermediate host. Both the intermediate host and the parasites can be transported long distances to new areas, including newly formed water bodies, by clinging to the feathers of birds (the amphipods) or by being transported inside the bodies of intermediate and final hosts (the acanthocephalans).I explore the mechanisms influencing acanthocephalan prevalence and the intraspecific genetic diversity of Polymorphus species in their intermediate host G. lacustris in water bodies in and near Edmonton, Alberta, Canada. Most of these were human-made water bodies with known ages of construction. To identify acanthocephalan larvae to species level, I tested the consistency and accuracy of the traditional method of morphological identification of waterfowl-associated cystacanths, which is based on proboscis hook arrays, using computer-based statistical simulations together with molecular and morphological techniques. I found high accuracy of species identification for waterfowl-associated acanthocephalans based on hook morphology of real and simulated adult specimens. By using both molecular and morphological approaches, I differentiated four putative species of Polymorphus based on larvae. In field research over three years, I found that waterbody age and the abundance of common final hosts were important factors for acanthocephalan prevalence in 36 water bodies in both 2015 and 2016. In additional sampling in 2017, I found that acanthocephalan prevalence was significantly higher in older water bodies than in young ones. Furthermore, I tested how waterbody age is related to mtDNA genetic diversity of G. lacustris and Polymorphus species from ten water bodies with various ages. After controlling for the species richness of known hosts and waterbody size, I found that the intraspecific genetic diversity of G. lacustris had a hump-shaped relationship with waterbody age, which suggests that certain genotypes might out compete others over time. In contrast, P. cf. paradoxus Connell & Corner showed a linear relationship between its intraspecific genetic diversity and waterbody age, which is predicted by the ‘pure’ colonization-time hypothesis. After conducting these studies at a fine geographical scale, I investigated whether mtDNA population structure of G. lacustris is related to waterfowl flyways in North America, Europe and Asia and whether the Rocky Mountains acted as a barrier to gene flow. I found that mtDNA population structure of G. lacustris is correlated with flyways but was not strongly influenced by the Rocky Mountains. This thesis research highlights the importance of habitat age and use by final hosts for parasite prevalence in intermediate hosts, and provides empirical evidence for differing relationships between habitat age and intraspecific genetic diversity for the host and parasite species. Furthermore, I show how waterfowl might influence invertebrate dispersal and genetic connectivity within and among continents. Future research could assess whether population genetic structure of Polymorphus species also matches host flyways. My research provides insight into the mechanisms by which host and parasites become distributed at different spatial scales that are broadly relevant for many other host–parasite systems.

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