The molecular basis and evolution of host manipulation by nematomorph parasites

The Nematomorpha, commonly known as horsehair worms, comprise one of three fully parasitic animal phyla. Nematomorphs are parasitic worms of invertebrates, which infect and manipulate the behaviour of their definitive arthropod host (e.g. crickets) to 'commit suicide' by jumping into water. They do this to reach an aquatic environment, which they require to complete their life cycle.

Although observations of this behaviour are relatively common, the underlying mechanism of this behavioural manipulation at a molecular and genetic level remains understudied. Therefore, the overall aims of this project are to investigate the underlying molecular mechanism involved in this behavioural manipulation, how this mechanism evolved, and the diversity of nematomorphs within the UK, and potentially internationally, thus increasing our knowledge of this understudied area.

The initial objectives of this project involve; culturing the nematomorph life cycle in the laboratory whilst maintaining stocks of the definitive (cricket) and paratenic (freshwater snails) hosts, and designing a behavioural assay for analysis of experimental manipulation.

The methodology for this project involves field, laboratory and computational components. Field aspects will involve collecting free-living adult worms, cysts or larvae from aquatic environments, or collecting the host (e.g. crickets), which would be placed in water for the emergence of worms. Specimens will be collected from various locations across the UK, with potential collection from the US and Japan.

All specimens would be identified to species level, where applicable, via microscopy or molecular techniques. Genomic and genetic investigation, and phylogenetic analyses would be used to investigate the diversity of nematomorphs in the UK, with potential comparisons between locations.

Laboratory components will involve establishing and maintaining a culture of nematomorphs, and colonies of their definitive and paratenic hosts in the laboratory. This system will be used for collection of tissue or secretions from the various life cycle stages of the parasite, and the infected and uninfected host, which will enable RNA, protein and small RNA extraction and identification via mass spectrometry or sequencing.

Isolated secretions will be used in behavioural assays of the insect host to investigate the ability of each secretion to alter host behaviour.

Finally, this project will use bioinformatics to investigate which genes, proteins, and small RNAs are expressed and the level of expression. These expression profiles can then be compared between the life cycle stages of the worm and infection status of the host. This will aid in investigating how this behavioural manipulation evolved, e.g. if a given gene, protein or small RNA associated with host infection is present or expressed throughout the nematomorph life cycle, it could imply said gene, protein or small RNA is essential for nematomorph functioning and accidently had the given behavioural effect, which being beneficial for the worm, evolved to exploit this.

Conversely, if the given gene, protein or small RNA is only present or expressed during the stages of the worm associated with host infection, it could imply this mechanism evolved in response to being inside the host as a method to re-enter an aquatic environment to complete their life cycle.