Harnessing Nutritional Niches to Explore Fungus-Animal Symbioses.

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 optimised nutrient exchange, we lack a framework to explore whether animal-derived nutrients are optimal for fungal symbionts. This conceptual gap has constrained studies about the ecological success and evolutionary stability of fungus-animal symbioses. We use Nutritional Geometry (NG) to harness nutritional niche theory and identify the crucial nutritional niche dimensions of fungi that mediate symbiotic stability. We hypothesise that these fungal nutritional niche dimensions are governed by 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 explore the promise of integrating NG with advanced imaging and -omics approaches to test coevolutionary hypotheses at precise microscales where fungus and animal cells trade nutrients. We conclude that niche-based theory can advance studies of coevolutionary dynamics from arms races to the emergence of economically important pathogens.