Genomic Correlates of Virulence Attenuation in the Deadly Amphibian Chytrid Fungus, Batrachochytrium dendrobatidis.

Emerging infectious diseasespose a significant threat to global health, but predicting disease outcomes for particular species can be complicated when pathogen virulence varies across space, time, or hosts. The pathogenic chytrid fungus Batrachochytrium dendrobatidis (Bd) has caused worldwide declines in frog populations. Not only do Bd isolates from wild populations vary in virulence, but virulence shifts can occur over short timescales when Bd is maintained in the laboratory. We leveraged changes in Bd virulence over multiple generations of passage to better understand mechanisms of pathogen virulence. We conducted whole-genome resequencing of two samples of the same Bd isolate, differing only in passage history, to identify genomic processes associated with virulence attenuation. The isolate with shorter passage history (and greater virulence) had greater chromosome copy numbers than the isolate maintained in culture for longer, suggesting that virulence attenuation may be associated with loss of chromosome copies. Our results suggest that genomic processes proposed as mechanisms for rapid evolution in Bd are correlated with virulence attenuation in laboratory culture within a single lineage of Bd. Moreover, these genomic processes can occur over extremely short timescales. On a practical level, our results underscore the importance of immediately cryo-archiving new Bd isolates and using fresh isolates, rather than samples cultured in the laboratory for long periods, for laboratory infection experiments. Finally, when attempting to predict disease outcomes for this ecologically important pathogen, it is critical to consider existing variation in virulence among isolates and the potential for shifts in virulence over short timescales.