Fungus-animal symbioses have evolved countless times across the tree of life. While the stability of these mutualistic or parasitic interkingdom interactions often depends on optimized nutrient exchange, we lack a framework to explore whether animal-derived nutrients are optimal for fungal symbionts. We propose that this conceptual gap has constrained studies of how fungus-animal symbioses achieve ecological success as well as predictions about whether they will remain evolutionarily stable over time. We use Nutritional Geometry (NG) to harness nutritional niche theory and identify the crucial fundamental and realized nutritional niche dimensions of fungi that mediate symbiotic stability. We hypothesize that the dimensions of fungal nutritional niches are governed by their symbiotic role (mutualist vs. pathogen), degree of animal host control over nutritional competition (monoculture vs. polyculture), and breadth of host associations (specialist vs. generalist). We then show how these NG predictions can be rigorously tested integrating cleverly designed NG experiments with recent technological advances. We propose that this general theory can provide powerful niche-based insights into phenomena ranging from coevolutionary arms races to the potential emergence of economically important pathogens.

Some insect-pathogenic fungi have evolved the ability to behaviorally manipulate their insect hosts. This has required the fungi to develop intricate mechanisms of infection, proliferation, and behavioral hijacking, which has led to speculation that behaviorally manipulating fungi must only infect a narrow range of hosts. One well-known example is the insect-pathogenic fungus Entomophthora muscae, which infects dipterans. Here, we present the different stages of the life cycle of E. muscae, focusing on the unique adaptations that allows the fungus to enter and proliferate inside its hosts, the possible ways it manipulates behavior, how the fungus exits the killed host to seek new susceptible hosts, and the ecological implications of these adaptations for determining the host range and intra-specific variation of E. muscae. We address the biology of E. muscae from an evolutionary ecology perspective and discuss the capacity of the fungus for behavioral manipulation within an extended phenotype framework. We highlight areas where further research is needed to fully develop E. muscae as a model system for host-pathogen research, for example to address questions relating to fitness consequences of an infection.