Abstract
Parasitism is a widespread interspecific interaction in which parasites have adapted to treat their hosts as a niche to be exploited for survival and reproduction. Parasites with complex life cycles that require multiple hosts or egression in a particular environment depend on their hosts for the transition. Such parasites from all major phylogenetic lineages have evolved a significant life history strategy where they induce specific aberrant phenotypic changes in their hosts to increase the odds of a successful transition. This is referred to “host manipulation”. It is a classic example of the extended phenotype, where gene or gene products from one organism can have phenotypic effects on another organism.
A dramatic example of such host manipulation is the water-seeking behaviour induced by parasitic worms (Nematoda and Nematomorpha) in their normally hydrophobic arthropod host. Parasites induce this behaviour to egress in water or water-saturated substrate to continue their free-living life cycle. The fact that similar host manipulation is exhibited in multiple arthropod hosts and induced by parasites from distant phylogenetic clades makes it an excellent model to study the proximate and ultimate causes of manipulation. However, the underlying mechanisms of such host manipulation are not well understood due to the lack of suitable model systems and genetic resources.
To address these issues, this thesis project has a twofold aim: 1) to develop a host-parasite model system (the mermithid nematode Mermis nigrescens infecting the earwig Forficula auricularia) to study the water-seeking behaviour by generating genetic resources, tools, and approaches for a comprehensive understanding of host manipulation. 2) to elucidate the genetic and epigenetic mechanisms underlying the water-seeking behaviour.
To lay the theoretical foundation for a comprehensive research approach in host manipulation studies, I first examine adaptive host manipulation in light of Tinbergen’s four questions and Poulin’s criteria. The importance of both proximate and ultimate mechanisms in understanding the adaptiveness of host manipulation is discussed, and guidelines for a comprehensive research strategy questioning all fours aspects of causation, complexity, ontogeny, and evolution of host manipulation is emphasized. I also provide examples of how these questions have been used and how such strategies can be extrapolated to many different host-parasite systems.
A good quality reference genome is essential for many in-depth genetic analyses. Therefore, I assembled, annotated and analysed the whole genome sequences of both the earwig and the nematode. Using multiple sequencing approaches and bioinformatics pipelines, I generated very high- quality assemblies with BUSCO scores of over 90% for both and N50 of 12.55Mb and 2.42Mb for earwig and nematode genome assembly, respectively.
I used total RNA-seq and bisulfite seq analyses in time series during manipulation for both the host and the parasite and carried out differential gene expression and gene set enrichment analysis. Candidate genes with significantly high and low expression during manipulation time points were extracted and candidate genes and pathways with a potential role in water-seeking behaviour were discussed in detail. From the analysis, the transmembrane signalling in response to environmental cues through cyclic nucleotide phosphodiesterase activity and odorant binding is a potential candidate pathway for further investigation. In addition, bisulfite sequencing showed less than 1% and 7.3% methylation in the earwig and nematode genomes, respectively.
To develop this system as an effective model, a non-invasive and minimally stressful method to detect nematode infection in live earwig was developed. The method uses the TaqMan assay in host frass DNA and can confirm infection status with great accuracy. This technique will help study the host-parasite interaction using live animals.
Overall, this thesis has generated essential resources and provides future research avenues for the study of parasite-induced water-seeking behaviour. I believe that the knowledge and resources will encourage further investigation to understand the precise underlying mechanisms of this extended phenotype.