Epichloë endophytes of Poa alsodes employ alternative mechanisms for host defense: insecticidal versus deterrence
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Some cool-season pooid grasses partner with symbiotic fungal endophytes in the Epichloë genus for defense against insect herbivores via fungal alkaloids. Poa alsodes, North American woodland grass, independently hosts two species of Epichloë that vary by produced alkaloids. E. alsodes produces insecticidal N-acetylnorloline. E. schardlii var. pennsylvanica (E. schardlii hereafter) has the gene for peramine, an insect-deterring alkaloid, production, but peramine was not detected. We tested the effects of the two endophytes on survival, feeding preference, and plant damage by the generalist herbivore, Spodoptera frugiperda. No larvae survived when feeding on plants harboring E. alsodes. In contrast, survival was only slightly reduced by plants harboring E. schardlii. However, larvae that fed on E. schardlii infected plants experienced delayed development and reduced pupal mass. Uninfected plants and plants infected with E. schardlii were damaged severely when single larvae fed upon them, whereas larvae fed negligibly on plants infected with E. alsodes. Preference did not match performance. Larvae strongly avoided feeding on E. schardlii but not E. alsodes-infected leaves where survival was zero. When E. schardlii was experimentally removed, larval leaf choices suggested that this endophyte is responsible for deterrence. High levels of N-acetylnorloline were detected from E. alsodes infected plants. Peramine was not detected in the experimental plants harboring E. schardlii, so it remains unclear what mechanisms caused avoidance and developmental delays. The two endophytes may protect their common host in different ways: E. alsodes by larval mortality and E. schardlii by deterring feeding and negative effects on development.
KeywordsEndophytic alkaloids Larval mortality Larval performance Feeding preference N-acetylnorloline
We thank the University of North Carolina at Greensboro Biology Department; Dr. Stanley Faeth National Science Foundation grant# DEB 0917741 for funding, Dr. Carolyn Young and Dr. Nikki Charlton from the Noble research Institute for identifying infections and alkaloid genes profiling of mother plants, Dr. Johnathan Scheerer from College of William & Mary, Williamsburg, VA for N-acetylnorloline standard synthesis, Anna and Anastasia Shymanovich for help with growing plants and growth estimates, Angie Larsen for help with statistics, and Daniel Foil and Dr. Daniel Todd from the Triad Mass Spectrometry Facility at the University of North Carolina at Greensboro for assistance with mass spectrometry analysis.
TS and SHF conceived and designed the experiments, TS performed the experiments and analyzed the data, NBC and AMM performed chemical analyses, TS and SHF wrote the manuscript; other authors provided editorial advice.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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