Amoeba predation ofCryptococcus: A quantitative and population genomic evaluation of the Accidental Pathogen hypothesis

Abstract

The "Amoeboid Predator-Fungal Animal Virulence Hypothesis" posits that interactions with environmental phagocytes shape the evolution of virulence traits in fungal pathogens. In this hypothesis, selection to avoid predation by amoeba inadvertently selects for traits that contribute to fungal escape from phagocytic immune cells. Here, we investigate this hypothesis in the human fungal pathogens Cryptococcus neoformans and Cryptococcus deneoformans. Applying quantitative trait locus (QTL) mapping and comparative genomics, we discovered a cross-species QTL region that is responsible for variation in resistance to amoeba predation. In C. neoformans, this same QTL was found to have pleiotropic effects on melanization, an established virulence factor. Through fine mapping and population genomic comparisons, we identified the gene encoding the transcription factor BZP4 that underlies this pleiotropic QTL and we show that decreased expression of this gene reduces melanization and increases susceptibility to amoeba predation. Despite the joint effects of BZP4 on amoeba resistance and melanin production, we find no relationship between BZP4 genotype and escape from macrophages or virulence in murine models of disease. Our findings provide new perspectives on how microbial ecology shapes the genetic architecture of fungal virulence, and suggests the need for more nuanced models for the evolution of pathogenesis that account for the complexities of both microbe-microbe and microbe-host interactions.

Distinct pathways of adaptive evolution in Cryptococcus neoformans reveal a mutation in adenylyl cyclase with trade-offs for pathogenicity

“…An emerging body of work examining strain-level variation both in vitro and in vivo reveals that even closely related C. neoformans strains can have divergent phenotypes. 4 , 5 , 7 , 9 , 10 , 47 , 51 , 52 Our study provides a new perspective on phenotypic variation by directly comparing the growth of the same panel of clinicaland environmental isolates in host cells from two different species ( Figures 1B – 1E ). We noted a correlation between strains that were able to withstand killing by amoebae and those that replicated well in macrophages, which suggests similarities in the intracellular environments of diverse host species that favor the growth of certain C. neoformans isolates over others.…”

“…Parallel studies of amoeba resistance, also in the Ftc555-1 background, similarly showed a lack of correlation between growth success in different host cells. 52 , 53 Collectively, this suggests that although there may be some overlap in genetic pathways required for growth in evolutionarily diverse hosts, such overlap may be challenging to identify. The host-adapted isolates we recovered here along with future studies of non-laboratory strainswillhelpshed lighton conservation and divergence in host selective pressures that have shaped C. neoformans adaptive evolution.…”

Strain heterogeneity in a non-pathogenic fungus highlights factors contributing to virulence

“…Strain heterogeneity within pathogenic fungal species can be leveraged to identify key traits contributing to virulence and identify virulence-associated loci 27 . Comparative analyses between A. fumigatus strains have revealed insights into the relationship between hypoxia tolerance and pathogenicity 17, 26 and have shown how gene regulatory differences can result in variable responsiveness to fungistatic drugs 28 .…”

Isolation of a fungal calcineurin A mutant suggests that amoebae can counter-select virulence attributes of microbes

“…Recently, an analysis of Cryptococcus species and strains was unable to correlate the levels of resistance to predation by amoebae with their replication within macrophages or virulence in mice. 14 …”