Microevolutionary traits and comparative population genomics of the emerging pathogenic fungus Cryptococcus gattii

Emerging fungal pathogens cause an expanding burden of disease across the animal kingdom, including a rise in morbidity and mortality in humans. Yet, we currently only have a limited repertoire of available therapeutic interventions. A greater understanding of the mechanisms of fungal virulence, and the emergence of hypervirulence within species are therefore needed for new treatments and mitigation efforts. For example, over the last decade, an unusual lineage of Cryptococcus gattii, which was first detected on Vancouver Island, has spread to the Canadian mainland and the Pacific Northwest infecting otherwise healthy individuals. The molecular changes that led to the development of this hypervirulent cryptococcal lineage remain unclear. To explore this, we traced the history of similar microevolutionary events that can lead to changes in host-range and pathogenicity. Here, we detail fine-resolution mapping of genetic differences between two highly-related Cryptococcus gattii VGIIc isolates that differ in their virulence traits (phagocytosis, vomocytosis, macrophage death, mitochondrial tubularisation, and intracellular proliferation). We identified a small number of single site variants within coding regions that potentially contribute to variations in virulence. We then extended our methods across multiple lineages of C. gattii to study how selection is acting on key virulence genes within different lineages.