Extensive Local Geographic Variation in Locoweed Toxin Produced by a Fungal Endophyte

Area of Science:

• Ecology
• Evolutionary Biology
• Chemical Ecology

Background:

• Legumes engage in coevolutionary "arms races" using chemical defenses against herbivores, especially insect seed predators.
• Locoweeds (legumes) produce swainsonine, a toxic alkaloid affecting livestock, but its efficacy as an insect deterrent is largely unknown.
• Swainsonine is synthesized by the fungal endophyte Alternaria section Undifilum, with its chemical nature well-understood, yet ecological roles remain uncertain.

Purpose of the Study:

• To investigate the role of swainsonine in locoweeds as an evolved chemical defense against insect herbivores.
• To quantify swainsonine concentrations and herbivory levels in Astragalus lentiginosus varieties.
• To evaluate the relationship between swainsonine levels and herbivory by generalist and specialist insects.

Main Methods:

• Quantification of swainsonine concentrations across different varieties of Astragalus lentiginosus.
• Assessment of herbivory levels on fruits and seeds by both generalist and specialist insect herbivores.
• Statistical analysis to determine correlations between swainsonine concentration and herbivory rates.

Main Results:

• Astragalus lentiginosus exhibited significant variation in swainsonine concentrations among varieties, including presence/absence at population and geographic scales.
• Herbivory by generalist insects showed no correlation with swainsonine concentrations.
• Herbivory by specialist seed beetles (Acanthoscelides) increased with higher swainsonine concentrations, indicating a potential deterrent effect.

Conclusions:

• Endophyte-produced toxin systems in plants can display complex geographic variation and ecological roles, potentially deviating from classical expectations.
• The findings suggest a specialized coevolutionary relationship between Astragalus lentiginosus, its fungal endophyte producing swainsonine, and specialist seed beetles.
• Further research is crucial to fully understand the dynamics of plant-endophyte toxin systems and their broader implications for coevolutionary processes.