| Outcomes of encounters between hosts and pathogens are difficult to predict a priori because evolutionary forces such as trade-offs between life history traits, local selection pressures, and rates of gene flow shape host-pathogen interactions. Thus, even in situations where host and pathogen have shared considerable evolutionary history, outcomes of exposure are not generally predictable. The widespread tiger salamander (Ambystoma tigrinum)---Ambystoma tigrinum virus (ATV) system exhibits several features that theoretically promote localized host-pathogen dynamics. Multi-year common garden experiments suggested that ATV strains are locally adapted to their host populations whereas A. tigrinum populations appear to be responding to forces other than ATV. Further, outcomes of exposure in non-sympatric A. tigrinum-ATV strain pairings were not predictable based on outcomes in sympatric pairings, presumably as a result of complex evolutionary forces that shape this system on smaller spatial scales. Outcomes of exposure are also shaped by the frequency and ease with which pathogens are transmitted among species because inter-species transmission shapes key disease dynamics such as persistence and evolution of virulence. Host ranges of ATV and the closely related ranavirus Frog Virus 3 (FV3) were explored using molecular techniques as well as experimental challenge trials. FV3 appears to be especially lethal in multiple host species (particularly anurans), and molecular characterizations suggest that FV3, much more so than ATV, routinely infects multiple species. Non-genetic factors can also shape disease dynamics. The study area, the northern Great Plains, is dominated by diverse agricultural activities. Multiple approaches were used to test whether contamination from agricultural cropland might be affecting outcomes of exposure in the A. tigrinum-ATV system. No evidence was found in support of this hypothesis. However, results from the widely used Frog Embryo Teratogenesis Assay-Xenopus, or FETAX assay, produced data not usually expected from a standardized assay, raising concerns about its general applicability. The degree to which outcomes of exposure in this system appear to be shaped by local evolutionary pressures suggests the A. tigrinum-ATV system, and amphibian ranaviruses in general, are valuable model systems for exploring the evolutionary ecology of infectious diseases. |
