The Evolution and Genetics of Fungal Virulence

PROJECT SUMMARY The study of infectious disease often focuses on pathogenic microbes that either specialize on exploiting animal hosts or on commensals that switch to pathogenesis when the delicate balance between host and microbe is perturbed. These microbes are presumed to have co-evolved complex adaptations that allow

survival and reproduction in and on hosts. However, there exists a broad range of microbial organisms that live in the open environment that are capable of causing disease when the opportunity arises. Such microbes also have adaptations that allow host exploitation, but the origin of these adaptations is unclear, as growth and

survival in a host is not a required part of the lifecycle. The existence of virulence traits in environmentally- derived, opportunistic pathogens is likely due to selection favoring the traits for other uses in the non-host environment. This hypothesis is known as “coincidental selection” in the bacterial literature, and “accidental

virulence” in the fungal literature. The threat of emerging bacterial pathogens has received far more attention than the threat of fungal pathogens. In 2022, the World Health Organization issued its first-ever report prioritizing fungal pathogens; three of the top four were environmental fungi. Thus, understanding the evolution

of opportunistic pathogens is essential. Examples of ‘dual-use’ traits that are “accidentally selected” include multicellularity (e.g., biofilm formation, filamentous growth, aggregation), protective capsules, and toxin production. Selection for these traits can be imposed by the abiotic environment (e.g., temperature), but also

by the biotic environment. Indeed, amoeba predation has been hypothesized to be a major selective force favoring dual-use traits and preadapting microbes to be resistant to phagocytes in the immune system. The research proposed here will use three biomedical models to investigate the relationship between dual-use

traits, resistance to amoeba predation, and virulence. In Aim 1, a collection of 1000 isolates of the opportunistic budding yeast, Saccharomyces cerevisiae, will be investigated for the distribution and genetic basis of multicellular traits. To estimate predation resistance, the strains will also be subject to phagocytosis by the

amoeba, Dictyostelium discoideum, and a subset will be tested for virulence against the invertebrate model host, larvae of the wax moth, Galleria mellonella. Aims 2 and 3 will focus on specific clinical yeast isolates. First, the genetic basis of the three traits will be explored with a mapping panel to determine whether there are

overlapping variants among the traits. Then, to directly test the role of amoeba predation on fungal virulence, the yeast and amoeba will be co-cultured and co-evolved to determine whether predation selects for increased virulence. This research represents one of the largest phenotypic screens of dual-use and predation-resistance

related traits in an opportunistic fungal species, and will generate experimental techniques that may be used in other fungal species.