About half of all insects are herbivores, but they didn’t all start out that way. About one-third of insect orders switched over to a plant-based diet at some point during their evolutionary history, but the mechanism that causes this phenomenon has not been well understood. A new paper in the Proceedings of the National Academy of Sciences by senior author Noah Whiteman from the University of Arizona describes how adaptations in their sense of smell has made some fruit flies into herbivores.
This might come as a shock, but plants don’t always want to be eaten. Sure, some grow fruit that attracts other organisms in order to help with seed dispersal, but many plants have adapted defense mechanisms to preserve their stems and leaves, which they need in order to grow and live. Some plants might have tough outer layers that make them more difficult to eat, secrete substances that taste disgusting, or even grow thorns as a physical warning to back off. Because plants put up such a fight, it might explain why evolving toward herbivory is fairly rare.
"Most plant-eating insects are parasites,” Whiteman said in a press release. "They're not like elephants roaming the savannah and ripping off leaves here and there. Insects have evolved ways to overcome those defenses but at the cost of becoming highly specialized. Many herbivorous insect species are extremely specialized, to the point where the animals have to spend their entire life cycle on their host plants.”
Scaptomyza flava is a great example of this lifelong parasitization. The flies embed their eggs within the leaves of mustard plants, and dine on the fluid that comes out when the leaf is injured. As the larvae eat their way through the leaf after they hatch, they inflict substantial damage to the plant. However, the ancestors of these insects did not live in the same way, preferring rotting meat or rotting, fermenting fruit over live plants.
The researchers explored what adaptations would have been required to drive the flies toward this behavior. The olfactory response of the flies were tested in order to see which scents they were attracted to most.
"It's like recording from the population of receptor cells in your nose," co-author John Hildebrand explained. "We look for electrical signals indicating that the antennal receptor cells have recognized and responded to a certain scent stimulus. If we don't see a signal, it means that the antenna doesn't respond to that compound.”
The Scaptomyza flies didn’t respond well to the scent of yeast, which is abundant in rotting fruit and is a great attractor of other genera of fruit flies. Instead, they were more drawn to the compounds that come from freshly cut grass, which is similar to the mustard leaves they parasitize. Genetic analysis between Scaptomyza and the heavily-studied and closely-related Drosophila melanogaster revealed that the genes that influence olfaction in Scaptomyza had evolved much more extensively than would have been expected. The preference for live plants over rotten fruit appears to have emerged only within the last 20 million years, and it appears to have been a very beneficial adaptation.
Parasitization by Scaptomyza can be fatal to the plant, which makes it especially problematic for farmers whose canola or mustard crops are infested with these insects. Having a better understanding of these insect adaptations could lead to new preventative measures against the flies, as well as protecting other crops that might be vulnerable in the future.
