Abstract
A central aim in parasitology is to understand the mechanisms and processes that determine host-parasite associations. The quest to understand these mechanisms has led to the rise of the host-parasite co-speciation paradigm, postulated in the early 20th century. This paradigm proposed that the parasite phylogeny follows the host phylogeny such that whenever the host speciates, the parasite speciates with it. Through the work of several authors over the years, it has become apparent that other processes also occur in the co-evolution of hosts and parasites. The importance of host switching, a process where a parasite species switches to a new host species, was eventually recognised by parasitologists. In host switching, ecological parameters are paramount where the parasite phylogeny tracks the host ecology. Despite ecological parameters being important in several host-parasite systems, in many systems the role of ecology remains unexplored. This study aims to explore the importance of phylogeny and ecology in a marine skatecestode system. Mixed results on the importance of co-speciation in the co-evolution of skates and cestodes have been reported by previous authors and none have explored the role of host ecology and instances of host switching in this system.
There were two main aims of this study. Firstly, the individual host and parasite trees were reconstructed for the skates and parasites used in this study. Secondly, the importance of phylogenetic and ecological influences in the skate-cestode system were explored. A suite of laboratory and computational techniques were used to address the aims of this study. The parasite phylogenetic tree was constructed using the 28S gene marker. The host evolutionary tree was created using COI and NADH2 genes. These trees were created by generating host and parasite sequences which were then used in the MrBayes and RAxML programs to generate host and parasite phylogenies based on Bayesian inference and maximum likelihood methods. Both approaches revealed almost identical trees. Both skate and cestode trees were monophyletic. The findings also supported previous recommendations by other authors regarding taxonomic changes of some cestode species.
The host and parasite trees were then used to test for co-speciation using software programs PACo and Jane. Co-speciation was supported using both these programs. Jane indicated that host switching exists in this system. As host switching is mainly influenced by ecological factors, the next step was to test for important ecological parameters. This test was done by modifying PACo using a novel approach developed in this study. The analysis revealed that all four ecological parameters tested (diet, foraging depth, size and geographical location) were significant in combination. This study was the first to investigate the role of ecology in this system.
The key findings of this project are that some cestodes tend to co-speciate with their hosts and follow host phylogeny, whereas others are influenced more by ecological factors and are likely to switch hosts given the opportunity. However, those cestodes influenced by ecological parameters may be vulnerable to fluctuations in important ecological factors. Based on the findings of this study, cestodes of skates are expected to be vulnerable to changes in diet, foraging depth, size and the geographical location of their hosts. The effects of anthropogenic factors on these parasites are discussed. The study raised some new questions that can be tested by future research.
Given that most skate species are highly vulnerable to anthropogenic stressors threatening the marine ecosystem, such as climate change, over-fishing and pollution, it is essential to evaluate whether parasites will face co-extinction if their hosts become extinct. In this thesis some of these questions were asked with the goal being to understand the repercussions of anthropogenic stressors on parasite populations. Parasites are crucial for the stability of marine systems as they form a large part of marine biodiversity, and a tremendous amount of energy flows through them via trophic interactions.