# Drosophila host defense mechanisms against filamentous fungal pathogens with diverse lifestyles

## TL;DR

This study explores how fruit flies defend against various fungi, revealing key immune pathways and fungal evasion strategies.

## Contribution

The study identifies the Toll pathway as central to antifungal immunity and reveals a novel fungal evasion strategy in Entomophthora muscae.

## Key findings

- The Toll pathway is the key immune mechanism against all tested fungal species by limiting their growth.
- Melanization, not phagocytosis or the Imd pathway, helps block fungal entry and proliferation.
- Fungal protease detection by Persephone is more critical than GNBP3 in activating the Toll pathway.

## Abstract

Entomopathogenic fungi serve as powerful regulators of insect populations in nature. However, how immune effectors combat fungal pathogens remains incompletely understood. We employ Drosophila melanogaster as a genetically tractable model to dissect immune defense mechanisms against diverse fungal pathogens. We show that the Toll pathway is the key determinant of immunity against all species tested regardless of their ecological strategy, primarily through resistance mechanisms that limit fungal proliferation. In addition, melanization, but not phagocytosis or the Imd pathway, also has a role in limiting fungal entry and proliferation. Additionally, we show that fungal protease detection by Persephone has a quantitatively more critical role than the glucan sensor GNBP3 in the activation of the Toll pathway upon fungal infection. Our study also reveals that the fly-obligate fungus Entomophthora muscae employs a vegetative development strategy to hide from the host immune response. These findings reveal that Drosophila immune mechanisms effectively defend against a broad range of fungal pathogens, while highlighting striking adaptations to overcome these defenses in highly specialized fungal pathogens such as E. muscae.

Fungal infections are widespread in nature, profoundly impacting agriculturally important plants, insects, and human health. Here, we used a genetic model, Drosophila melanogaster, to systematically dissect immune defense mechanisms against fungal pathogens with different lifestyles. We discovered that the Toll pathway serves as the central regulator of antifungal immunity across all fungal species tested, primarily by limiting fungal growth within the host. We also found that melanization—but not other immune modules like phagocytosis—contributes to blocking fungal entry and proliferation. Surprisingly, fungal protease detection by Persephone has a quantitatively more critical role than the glucan sensor GNBP3 in the activation of the Toll pathway. Equally important, we uncovered a remarkable evasion strategy used by the fly-specialist fungus Entomophthora muscae, which adopts a stealthy vegetative growth mode to avoid triggering the host immune response. Together, our findings reveal both the core immune mechanisms that protect hosts against diverse fungal threats and the sophisticated adaptations that highly specialized fungi have evolved to overcome these defenses. These advances provide insights towards developing future strategies for managing fungal infections.