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.
