Not Extremely Plastic: Testing the Limits of Morphological Plasticity in Fungal Mycelia in Response to Soil Grazers

Phenotypic plasticity is widespread among living organisms, but exceptionally high levels of plasticity are rarely observed in nature. While plasticity can enhance fitness in fluctuating environments, it comes with substantial costs that limit both its expression and evolution. Most of what we know about these limits comes from studies on animals and plants, which typically have fixed body plans. In contrast, modular organisms such as filamentous fungi possess an undetermined, decentralized morphology that allows individuals to adjust their phenotype in response to local environmental conditions. This flexibility has led to speculation that fungi exhibit "extreme phenotypic plasticity," making them an ideal system for studying plasticity mechanisms—yet empirical tests of this assumption remain scarce. To address this gap, we analyzed grazer-induced morphological plasticity in four cord-forming fungal species exposed to soil microfauna with differing feeding behaviors, using a phenotypic trajectory analysis framework and tested alternative hypotheses: One hypothesis is that the flexibility of fungal network development would allow species to converge on a common "grazing-resistant-phenotype". Alternatively, the developmental plasticity for each species might be heavily constrained within species-specific limits. Our results showed that fungal species accounted for most of the total morphological variation, while grazer induced morphologies remained relatively aligned to non-grazed morphological trajectories. Instead of convergence, we found that grazed-phenotypes were more dissimilar across species, reflecting distinct developmental trajectories within species-specific limits. The extent of plasticity varied widely with a maximum of 30% of the morphological variation attributable to grazing pressure. Nevertheless, these findings suggest that morphological plasticity plays a relatively minor role in response to grazing for most species. The dominant species signature and constrained plasticity supports the use of morphological attributes of mycelia as functional traits in ecological studies. Our findings provide new evidence on the extent of plasticity of modular organisms and highlight the need to extend plasticity theories to include organisms with non-determinate, modular body types like fungi.