In Pacific Northwest watersheds several species of Pacific salmonid fishes are hosts for the rhabdovirus infectious hematopoietic necrosis virus (IHNV). In this multi-host ecosystem specialist genogroups of IHNV have evolved with high fitness and high virulence in sockeye salmon, steelhead trout, or Chinook salmon. A generalist genogroup of IHNV also occurs with high frequency in both Chinook salmon and steelhead trout, but it exhibits low virulence in the field. In controlled wet laboratory infection studies we are investigating the specialist and generalist phenotypes of the four IHNV subgroups in all three salmonid fish host species. Results of virulence trials mimic the specialist and generalist trends observed in the field, confirming that for IHNV specialization leads to high virulence. We find that all viruses are able to infect all host species but there are differences in early replication and they vary dramatically in ability to persist. This suggests that the basis of host specificity involves ability to avoid the strong innate immune response of the host, rather than differing ability to gain entry. In addition, a new project is investigating the evolution of virulence after a historic host jump of IHNV from sockeye salmon to rainbow trout in the 1970s, leading to evolution of a major pathogen impacting rainbow trout farming world-wide. These controlled infection studies comprise tractable model systems for investigating the natural evolution of a generalist from a specialist ancestor, empirical testing of specialist-generalist theory predictions in vivo, and tracking of virulence evolution after a vertebrate virus host jump. In combination with genetic surveillance in the field and landscape virus transmission modeling of IHNV in salmonid fish, this system provides a broad understanding of evolution of virulence for an aquatic pathogen in natural and anthropogenically impacted environments.
1. Describe different possible "lifestyles" for viruses evolving in a multi-host ecosystem
2. Understand the differences between specialist, generalist, and non-adapted virus:host relationships
3. Consider different possible outcomes for the evolution of virulence after a virus jumps into a new host