Understanding the Role of Indole in Tritrophic Interactions
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Chapter 1: Indole and Its Impact on Ecosystems
Indole is a volatile compound emitted by corn, which is recognized by both caterpillars and wasps. This chemical plays a crucial role in the plant's defense mechanism, creating a unique interaction between these species. Researchers from Switzerland have delved into how such volatile compounds influence tritrophic systems, which are vital in agricultural contexts.
In this tritrophic interaction, moths (Spodoptera littoralis) deposit their eggs on corn leaves, leading to the emergence of caterpillars that feed on the plants. Female wasps (Microplitis rufiventris) then lay their eggs inside these caterpillars. Upon hatching, the wasp larvae consume their host from within. However, corn plants are not entirely defenseless; they have evolved mechanisms to counteract the threats posed by these caterpillars.
Section 1.1: The Defense Mechanism of Corn
Corn has developed a defense system that harnesses volatile organic compounds, known as herbivore-induced plant volatiles (HIPVs). These compounds can deter herbivorous insects or even act as toxins. In a fascinating twist of nature, these volatiles can also attract beneficial insects that prey on or parasitize harmful herbivores.
Indole is a specific HIPV produced by corn when the leaves experience damage or in response to substances in caterpillar saliva. The synthesis of indole requires the enzyme indole-3-glycerol phosphate lyase (IGL). Researchers sought to understand how indole mediates interactions among corn, caterpillars, and wasps by comparing normal corn plants with genetically modified IGL-deficient plants.
Subsection 1.1.1: Testing Wasp Preferences
To assess whether corn plants utilized indole to lure female wasps, the researchers employed an olfactometer—a device designed to measure the movement of wasps in response to different scents. Results indicated that female wasps were indeed attracted to the scent of indole.
As a follow-up, the scientists explored whether wasps preferred wild-type corn plants over IGL-deficient plants when both types were exposed to caterpillar secretions. The findings revealed that wasps favored the wild-type plants capable of producing indole.
Chapter 2: Surprising Findings on Indole's Effects
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The next experiment yielded unexpected insights. Instead of manually inflicting damage to the corn plants, researchers introduced caterpillars directly onto both wild-type and IGL-deficient plants. Surprisingly, more wasps gravitated towards the IGL-deficient plants when caterpillars were present. This contradicted earlier findings regarding the attraction of wasps to indole.
To further investigate this phenomenon, the researchers set up another experiment comparing wasp preferences for caterpillar-infested IGL-deficient plants, one chamber having an indole dispenser and the other without. Interestingly, fewer wasps opted for the chamber with the indole dispenser.
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The researchers posited that indole was not deterring wasps from approaching plants directly but was instead altering the caterpillars themselves. Further tests showed that wasps preferred caterpillars that had fed on IGL-deficient plants, indicating that indole was modifying the caterpillars' appeal to their natural predators.
Ultimately, while indole did kill approximately 10% of the caterpillars, wasps proved far more lethal, eliminating over 60%. However, caterpillars that had consumed wild-type corn exhibited an increased survival rate due to reduced wasp egg hatching, demonstrating the complex interplay of survival strategies in these tritrophic systems.
Caterpillars also displayed behavioral changes in response to indole. In olfactometer tests, they tended to avoid the indole scent, but this aversion dissipated after just 30 minutes of exposure to wasps, highlighting the intricate dynamics at play.
In summary, corn emits scent molecules like indole, which are detected by both wasps and caterpillars. This interaction alters the caterpillars' scent, making them less appealing hosts for wasp eggs. Conversely, wasps also emit substances that modify caterpillar behavior, ensuring a balance in this ecological relationship.
Highlighted Article
- Ye, N. Veyrat, H. Xu, L. Fu, T. C. J. Turlings, M. Erb, "An herbivore-induced plant volatile reduces parasitoid attraction by changing the smell of caterpillars." Sci. Adv. 4, eaar4767 (2018). DOI: 10.1126/sciadv.aar4767
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