Assessing the Evolution of Virulence in the Salmon Louse (Lepeophtheirus salmonis) in the Bay of Fundy
An unintended consequence of fish farming is the production of an environment which may be conducive for the evolution of virulence for pathogens interacting with the host in production. Rearing conditions used in salmon aquaculture such as high densities, compressed rearing cycles, limited genetic diversity of broodstock as well as practices applied to control infectious disease, may all lead to increased selection pressures for the evolution of virulence of pathogens. These selection pressures may also be occurring without the balancing of negative fitness costs of virulence due to a constant supply of naïve hosts entering the system every 18-24 months. Changes in virulence of Canadian Lepeophtheirus salmonis populations have not yet been assessed, despite having been identified as a crucial knowledge gap to understand the interactions between farmed and wild hosts as well as the impact of increased disease prevalence at sites which may harbour highly virulent strains. We developed a virulence challenge model by rearing lice originating from wild and farm sources for 6 generations and compared differences in virulence on their host, testing the hypothesis that lice originating from farms were more virulent than those from wild sources. RNA sequencing was completed for copepodid, chalimus II and adult stages of the louse as well as their respective attachment sites on the host to determine responses at the host-parasite interface. We describe the first instance of elevated lesion severity for host Atlantic salmon when infected with farm-sourced lice compared to their wild counterparts. However, this increase did not appear to translate to meaningful differences in host growth rates (e.g. change in lengths or weights). Approximately 900 transcripts had differential expression for at least one life stage between wild and farm sources of lice. Many of these transcripts had functions related to chitin- and heme-binding, serine-endopeptidase, metallo-endopeptidase, and trypsin-like activities. Overall design: Investigations of virulence differences between wild- and farm-collected Lepeophtheirus salmonis on Atlantic salmon hosts. Wild- and farm-sources were collected and reared in laboratory conditions for multiple generations before Atlantic salmon were challenged with infections from each source of lice. Differential gene expression was measured between the two sources of L. salmonis and gene ontology, pathway enrichment and a pilot genome-wide association study were completed.