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Interplay of Two Insulin Receptors Underlies Crucial Switch Between Short Wings and Long Wings in Leaf Hopper Pests

Each year, rice in Asia faces a big threat from a sesame-seed-sized insect called the brown plant hopper. Now, a study reveals the molecular switch that enables some plant hoppers to develop short wings and others long -- a major factor in their ability to invade new rice fields. The findings were published online on March 18, 2015 in Nature. Lodged in the stalks of rice plants, plant hoppers use their sucking mouthparts to siphon sap. Eventually the plants turn yellow and dry up, a condition called "hopper burn." Each year, plant hopper outbreaks destroy hundreds of thousands of acres of rice, the staple crop for roughly half the world's population. The insects have a developmental strategy that makes them particularly effective pests. When conditions in a rice field are good, young plant hoppers develop into adults with stubby wings that barely reach their middles. These short-winged adults cannot fly, but they are prolific breeders. A single short-winged female can lay more than 700 eggs in her lifetime. "The short-winged ones have great big fat abdomens. They're basically designed to stay put and reproduce," said biologist Dr. Fred Nijhout of Duke University, who co-authored the study with colleagues at Zhejiang University in China. But in the fall, as days get shorter and temperatures begin to drop -- signs that the rice plants they're munching on will soon disappear -- more plant hopper nymphs develop into slender adults with long wings. These long-winged plant hoppers lay fewer eggs, but are built for travel, eventually flying away to invade new rice fields. Until now, scientists did not know exactly how the shorter days and cooler temperatures triggered the shift between short and long wings, or which hormones were involved.

To find out, the researchers used a technique called RNA interference (RNAi) to silence the genes for two different insulin receptors -- regions on the cell membrane that bind to the hormone insulin -- and measured the effects on the animals' wings.

"Previously it had been assumed that all insects only had a single insulin receptor gene. We discovered that brown plant hoppers have two," Dr. Nijhout said.

When the researchers silenced the first insulin receptor, short-winged adults emerged. Silencing the second receptor produced adults with long wings.

Further study revealed that long wings are the default design. But when plant hoppers secrete a particular type of insulin in response to changing temperatures or day length, the second insulin receptor deactivates the first receptor in the developing wings, leading to short-winged adults.

"The second insulin receptor acts by interfering with the first one, therefore shutting down the signal," Dr. Nijhout said.

It's too early to say whether the findings could lead to techniques to treat plant hopper populations so they are unable to invade new rice fields, Dr. Nijhout says.

But the researchers have found similar mechanisms in other plant hopper species, and are now trying to find out if insulin plays a similar role in other insect pests with flying and flightless forms, such as aphids.

One of the leading pests of rice, brown plant hoppers can grow up to have either short or long wings, depending on conditions such as day length and temperature in the rice fields where they suck sap. The hormone insulin controls the switch that tells young plant hoppers whether to develop into short- or long-winged adults, finds a new study. The image shows short-winged plant hoppers on the left, and long-winged plant hoppers on the right. (Photo by Chuan-Xi Zhang of Zhejiang University in China).

[Press release] [Nature abstract]