Saved by the Symbiont: Environmental Stress Intensity and Endosymbiont-Mediated Stress Response Determine Evolved Host Complexity

Kiara M. Johnson, Camila Mendoza, Daya Tucker, Anya Vostinar, Alexander Lalejini , Emily Dolson

Understanding how stress responses affect the trajectory of host–symbiont coevolution is central to predicting and managing species outcomes in the face of disturbances to ecosystems. Critically, it remains an open question how exactly we expect stressors to influence the coevolutionary dynamics of symbioses (on either end of the parasitism–mutualism continuum). In this work, we use in silico experiments to examine how stressor frequency and intensity affect host and symbiont diversity and complexity when mutualists defend their hosts from stress and parasites render their hosts stress-susceptible. We find that stress-protective mutualism can enable host survival where it would otherwise be impossible, but can also constrain host diversification and complexification. These effects are particularly strong when stressors are more destructive (and thus protection is more crucial). Meanwhile, stress-exploitative parasitism can increase the evolution of complex traits (to escape trait-matching parasites). Hosts cycle between complex, parasite-resistant and simpler, parasite-vulnerable states over time. These results better enable us to predict (and, potentially, influence) how natural symbioses respond to environmental instabilities such as climate change.