Fishing for parasite genomes: host-parasite co-evolution of an emerging salmonid disease

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Changing climates and environments are linked with the emergence of diseases - with potentially severe consequences for ecosystem health and services. Epidemics may arise through range-shifts, or via temperature-sensitivity of host susceptibility and parasite virulence. This implies that parasites with low virulence under normal conditions, may emerge as major disease agents in their usual ranges when the environmental conditions change. Under such scenarios, the mechanisms leading to disease outbreaks crucially depend on the coevolutionary history of the pathogen. For example, parasite strains with higher infectivity or virulence may be favoured under warming conditions, or the degree of host specificity among parasite strains may pre-dispose certain host species or populations to higher mortality. This project aims to investigate whether there is a genomic basis to climate change driven disease emergence in an aquatic emerging disease system, or whether the dynamic between host susceptibility and tolerance under multiple stressors alone can drive disease outbreaks. We will focus on Proliferative Kidney Disease (PKD) - an emerging disease farmed and wild salmonids in Europe and in North America. PKD is predicted to increase in frequency and severity with climate change, as the development of the pathological alterations during infection is strongly temperature driven. All salmonids can be infected by the causative parasite Tetracapsuloides bryosalmonae (Phylum Cnidaria), but little is known about the genetic structure and strain specific virulence in this widespread, ecologically and economically important parasite. This project will 1) characterise the population genomic structure and divergence of T. bryosalmonae across its known and emerging Holarctic distribution, 2) evaluate patterns of host-specific adaptation by focusing on a broad range salmonid hosts, and 3) link patterns of parasite genetic differentiation with estimates of pathogenicity during a disease outbreak using a genome-wide association study.