Characterisation of endogenous and exogenous viruses in invasive amphibians and arthropods

Invasive species have a devastating global impact. Viruses may have diverse impacts on the invasion process; for example, an invader may be more successful in a new place because it lacks viruses, and therefore diseases, from its native region. Some invaders, like mosquitoes, can also transmit pathogenic viruses to humans causing devastating outbreaks of disease. Lastly, viruses make viable biological control agents to control the spread of invasive species. In this thesis, our overarching aim was to discover novel viruses capable of infecting invasive species, to both better understand the interplay between viruses and invasion, and to find invader control strategies. We used deep sequencing of both RNA (transcriptomics) and DNA (genomics) to find viruses, where we looked for both exogenous (infecting) and endogenous (ancestral) viruses. We began with a viral case study of the invasive Australian cane toad, Rhinella marina. The cane toad is fatal to Australian fauna, and it has caused population declines in native Australian animals. We collected and analysed over 160 cane toads from Australia, uncovering that one novel picornavirus (Rhimavirus-A) infects cane toads in Australia at an unusually high prevalence (9%). We then analysed the virome of cane toads in their native range, South America, and compared this with the Australian range. We identified a higher diversity of viruses in South American cane toads (n=7), all of which are absent from Australia and therefore represent possible biological control candidates. This suggests that Australian cane toads have largely been released from the viral infection they experienced in their native range. Lastly, we analysed the endogenous virome of eight arbovirus vectors, arthropods who can transmit disease to humans. Endogenous viruses – those which have integrated into the host genome – may confer antiviral immunity to arbovirus vectors, allowing them to transmit pathogenic viruses. We found a high diversity of arthropod viruses have endogenised in all arbovirus vector viromes. However, we identified distinct patterns of endogenisation in the deer tick (Ixodes scapularis) which transmits fatal and emerging viruses to humans. We also provide evidence that these endogenous viruses are processed into PIWI-interacting RNAs, potentially providing antiviral immunity to the tick and helping it to efficiently transmit viruses to humans. Overall, our findings are a valuable addition to the growing global virosphere