Abstract
Intracellular parasites, endosymbionts that are specialised to live in host cells, have evolved independently in many different lineages. Endosymbionts can be transmitted vertically and horizontally, and many of them maintain intimate associations with their hosts over space and time. For example, the mitochondrion, which is now an essential organelle in eukaryotes, is believed to have an endosymbiotic origin.
This thesis focuses on two intracellular parasite groups, Microsporidia and Rickettsia, in diverse amphipod hosts in New Zealand. Microsporidia and Rickettsia are phylogenetically distantly related, but both are ecologically and evolutionary successful in diverse groups of hosts. I aimed to understand their diversity, the factors that have shaped their current distribution, the various phylogenetic patterns they are involved in, their spatiotemporal variations and cooccurrence, and their evolutionary histories.
By starting with molecular screening of diverse amphipods collected throughout the country, I uncovered a diversity that was previously unknown in this region, a first for both groups in the Southern Hemisphere. For Microsporidia, by expanding their known geographical and host ranges, I applied phylogenetic and cophylogenetic methods to infer the evolutionary history of host-parasite associations. Based on the congruent phylogenetic and phylogeographical patterns, I provided evidence for their shared evolutionary histories.
Some vertically transmitted parasites can manipulate host reproduction, and this can have various ecological and evolutionary consequences. Certain endosymbionts are known to disrupt phylogenetic patterns of mitochondrial DNA mainly due to linkage disequilibrium. I discussed the direct and indirect impacts of Rickettsia infections on the use of mitochondrial DNA in barcoding, phylogenetic, and phylogeographical studies. Also, I explored the possible role of Rickettsia infections in accelerating host mitochondrial DNA evolution, which could result in mitonuclear discordance patterns appearing on deep time scales.
Because Microsporidia and Rickettsia share the same amphipod hosts and coexist in many populations, they may interact with each other within the same individual. I investigated spatiotemporal dynamics in the prevalence of both parasites, and tested whether infection by one parasite influences the probability of infection by the other. Although there were variations in prevalence among different sampling times and locations, there were no clear consistent patterns between the two parasites and their patterns of co-occurrence within the same individual hosts did not depart from random expectations.
Taken together, Microsporidia and Rickettsia are widespread in New Zealand amphipod hosts. It seems that both vertical and horizontal transmission have played important roles in their current distribution. The vertical transmission of endosymbionts seems to have great potential to cause profound effects on host mitochondrial DNA. However, demonstrating this will require more data from multiple systems and scales. In the future, population- to community-level studies will be especially valuable to understand the ecological interactions between hosts and parasites and among different strains/species/groups of parasites.