Abstract
Oomycetes are a diverse group of fungal-like organisms, representing one of the eukaryotic groups which have the greatest impact on human health and ecosystem functioning. As parasites, oomycetes cause mortality and population declines in a variety of terrestrial and aquatic hosts. Oomycete parasites belonging to the genus Aphanomyces infecting the brood pouch of female copepods are the most commonly reported parasitic infections in copepods. Yet, we know very little about the ecology of this host-parasite system. The aims of this thesis were to investigate the phylogeography of Aphanomyces brood parasitism and to describe the development, transmission, and distribution of Aphanomyces infections.
Globally, reports of Aphanomyces infections in copepods are restricted to central Europe, Norway, and New Zealand, with a distribution that cannot be explained by the biogeography of their hosts. Genetic analysis revealed a high sequence similarity between New Zealand and European strains, indicating that they belong to the same species. The restricted distribution of Aphanomyces in Europe to lakes stocked with fish and the high sequence similarity to Aphanomyces invadans, a fish parasite, suggests that fish can serve as a host, or may even represent the primary target of the parasite. If so, Aphanomyces may have arrived in New Zealand (and stocked lakes in Europe) with the movement of infected fish. This conclusion warrants further investigation, particularly to further our understanding of how oomycetes can invade and spread in new ecosystems.
Environmental conditions can have major impacts on host-parasite dynamics through their influence on the survival and infectivity of free-living transmission stages. In laboratory experiments, transmission of Aphanomyces was reduced in the presence of Daphnia. Filtering of parasite zoospores by Daphnia has been shown in a number of other host-parasite systems, and may represent an important energy pathway in lakes with Aphanomyces. The role of temperature on transmission rates was unclear. Aphanomyces developed faster at higher temperatures, suggesting a positive impact on parasite transmission. Sporangial development and zoospore survival were significantly impaired, revealing an upper thermal limit for parasite persistence.
Although the distribution of parasites is constrained by that of their hosts, the occurrence of Aphanomyces across a series of ponds was also affected by habitat variables, namely habitat size and substrate type. Small, ephemeral habitats lacking silt substrates may not be able to support Aphanomyces infections due to reduced survival of resting stages necessary for persistence in temporary ponds. The absence of Aphanomyces in ephemeral ponds in Europe suggests that it is poorly adapted to long periods of drying. In New Zealand, ephemeral ponds with Aphanomyces infections have shorter dry seasons and silt substrates that may facilitate its persistence.
These small-scale findings were examined in a global context by evaluating the role of habitat variables and transmission stage ecology in influencing host-parasite dynamics worldwide. At a global scale, habitat size (particularly depth) had an effect on host-parasite dynamics, with parasite prevalence increasing in shallower waterbodies. Parasites with mobile transmission stages, like oomycetes, did not respond to the same habitat variables as parasites with immobile transmission stages.
The factors that affect the distribution and survival of parasites in their host populations are complex. This thesis provided evidence that the global distribution of Aphanomyces may result from the introduction of fish populations. At smaller spatial scales, environmental variables which affect the survivability of free-living stages limit the ability of Aphanomyces to persist in certain habitats. Overall, a greater knowledge of the ecology of parasite free-living stages and the mechanisms which influence the transmission of these stages to their hosts will be essential for understanding parasite spread in aquatic ecosystems.