The influence of ergosterol on termites' defenses against Metarhizium infections

Termites respond to spores of pathogenic fungi that have adhered to the cuticle with brief bouts of rapid shaking, an alarm response that promotes allogrooming, which helps prevent fatal infections. The pathogen associated molecular pattern (PAMP) that elicits this proactive hygienic response is unknown. Ergosterol, an important component of the fungal cell membrane, has been identified as a potential PAMP in the subterranean termite Coptotermes formosansus. In this study, commercial or extracted ergosterol elicited a hygienic behavioral response in workers of the subterranean termite Reticulitermes flavipes. The ergosterol content of pathogenic spores of Metarhizium spp. were also found to vary among different strains and species of the fungus. However, there was a significant positive correlation between ergosterol content of different Metarhizium isolates and fatal infections caused by their spores (conidia). Alarm responses from challenges with conidia were not positively correlated with estimates of ergosterol content and instead were negatively correlated with ergosterol content although not significantly, which does not support the hypothesis that ergosterol is a critical recognition cue for the hygienic response. Termicin, an antifungal peptide in termite salivary secretions, was also most effective against the most lethal isolates, which had the highest ergosterol content. Ergosterol appears to influence spore membrane vulnerability to disruption by the antifungal peptide. Overall, our results indicate ergosterol influences both the vulnerability of Metarhizium conidia to termicin and the detection of an unidentified PAMP, which alerts termites to fungal contamination. This sets up a potential trade-off for the fungus between a shield for protection and signal for detection: reducing vulnerability to termicin by decreasing the content of cell membrane ergosterol results in an increase in a cell wall PAMP that termites can recognize and respond to with allogrooming. Reticulitermes spp. appear to have adaptively exploited this trade-off.