Insect gut-dwelling fungus Zancudomyces culisetae: A hidden player in mosquito development

Mosquitoes and their gut-dwelling fungi have been documented worldwide, yet their relationships remain poorly understood. Harpellales fungi (Kickxellomycotina, Zoopagomycota) have traditionally been considered commensals, but recent studies suggest they may exhibit parasitic or mutualistic characteristics under certain conditions. In this study, we explored these interactions using two well-established laboratory models: Aedes aegypti and Zancudomyces culisetae. Specifically, we investigated the impact of the gut-dwelling fungus Z. culisetae on A. aegypti larval development by measuring body size and development time under different nutritional conditions, with or without the fungus in the hindgut. Significant differences in body size and development time were observed during larval development in the presence of the gut fungus compared to the control group. Larvae colonized by the fungus exhibited larger body sizes and accelerated development. These effects were consistent under both nutrient-rich and nutrient-deficient conditions, underscoring the symbiotic roles of the gut-dwelling fungus. Interestingly, our results also revealed that even dead fungal spores enhanced mosquito larval development, suggesting previously unrecognized beneficial mechanisms associated with the fungal tissue. Transmission electron microscopy provided additional evidence of mosquito-fungus interactions, showing electron-dense particles within mosquito cells at sites of close contact with fungal cells, although further investigation is required to confirm their identity. Collectively, our findings challenge the traditional view of insect relationships with gut-dwelling fungi, providing evidence for a potential shift from commensalism to mutualism.