Epichloë endophytes of Poa alsodes employ alternative mechanisms for host defense: insecticidal versus deterrence

Abstract

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.

Environmental factors affect the distribution of two Epichloë fungal endophyte species inhabiting a common host grove bluegrass (Poa alsodes)

“…This negative association may reflect the powerful insecticidal effects of NANL, a loline alkaloid. NANL alkaloid concentrations produced by E. alsodes in P. alsodes plant tissues are high enough to cause larval and adult mortality for various insect species (Jensen, Popay, & Tapper, ; Popay, Tapper, & Podmore, ; Shymanovich et al, ).…”

“…pennsylvanica infect this grass (Figure ). However, the two Epichloë species dramatically differ in their distributions, alkaloid genetic profiles, and the different strategies used to defend against insect herbivory (Shymanovich et al, ; Shymanovich, Musso, Cech, & Faeth, ). E. alsodes produces the toxic insecticidal alkaloid, N ‐acetylnorloline (NANL), while E. schardlii var.…”