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Media contact: Dr. Astrid Groot, research assistant professor in the N.C. State Department of Entomology, 919.515.1651 or astrid_groot@ncsu.edu
N.C. State researchers uncover evolutionary twist in moth pheromone communication
Birds, frogs, flies - all species, in fact, including humans - use pheromone (sexual) communication signals. Whether audio, visual or chemical, the signals work like a lock-and-key mechanism, ensuring that males and females of the same species are able to identify mates. Of particular interest to agricultural researchers in recent decades is pheromone communication in moths, many of which are significant agricultural pests.
A team of researchers in the North Carolina State University College of Agriculture and Life Sciences recently discovered that interference in moth pheromone communication between closely related species could be a strong enough selection force to alter these signals. These findings open doors to new thought on the evolution of moth pheromone communication, and they also could play a role in future applied research on controlling moth populations in agricultural crops.
The study, published in the Proceedings of the National Academy of Sciences, was conducted by Dr. Astrid Groot when she was a postdoctoral research associate in the laboratories of Dr. Fred Gould, William Neal Reynolds professor of entomology and the senior leader of the project; and Dr. Coby Schal, Blanton J. Whitmire professor of entomology. Research technicians Joy Horovitz, Jennifer Hamilton and Richard Santangelo were crucial contributors for the success of the project, which was funded by grants from the National Science Foundation, the W.M. Keck Center for Behavioral Biology and the Blanton J. Whitmire Endowment at N.C. State.
“This is the first experimental evidence that communication interference can play an important role in how moth communication pheromones evolve,” said Groot, now a research assistant professor in the Department of Entomology. “People have theorized about it for years, but it has not yet been shown, at least not in chemical communication, until now.”
The team worked with two closely related moth species: Heliothis subflexa (Hs), and Heliothis virescens (Hv). The latter is also known as the agricultural pest tobacco budworm. The questions they set out to answer: How can pheromone blends change, or evolve, from one species to the next? In other words, which selection forces play a role? And specifically, could communication interference from males of closely related species exert strong enough directional selection to cause evolution of these signals?
“If you have a couple of different species in the same area and they are similar in their pheromone blend, there's a chance they could attract the wrong mates,” Groot said. “So, closely related species can exert selection on each other. With moths, you have two forces at work: mate recognition and communication interference.”
To figure out which selection forces are strongest and how those forces can cause evolutionary changes in moth pheromone communication, the team took a unique approach. First, they crossed and backcrossed the two species of moths, using Hs females and Hv males, to create offspring females that had a different pheromone “blend” than a normal Hs female.
The pheromone blends of these genetically modified female offspring lacked three particular compounds typical to normal Hs female pheromone composition. These three compounds, or acetates, had been identified in previous research as critical to repelling males of the closely related Hv species.
Next, they tested the females in the field, which has never been done before, according to Groot, using traps baited with normal Hs females as well as traps with the genetically altered females, whose pheromone blends were missing the three acetates. The results were stunning: Ten times more Hv males were captured in traps baited with the genetically altered Hs females than in traps with the normal females.
“This shows that the three acetates actually are important in the attraction of their own males,” Groot says. “But what was surprising to us was the cross attraction, because we didn't expect that at all. We had thought that the subflexa [Hs] blend would stay species-specific even when you take out the acetates.”
Using data collected over the last 40 years, the team was able to estimate quantitatively that the directional selection force exerted by Hv males on Hs females to produce relatively high amounts of acetates can be as high as 23 percent.
“It really is important for the females not to attract mates from the opposite species, because doing so can reduce their reproductive output by 23 percent,” Groot said. “In the evolutionary sense, that's huge … it's incredible. One percent is already a lot.”
In prior studies, synthetic chemicals have been used to mimic pheromone blends. When tested in the same traps in the field, Hs males were attracted, but Hv males have never been found to be attracted. According to Groot, this shows that testing the attraction of live females is much more accurate than experiments using synthetic interpretations of the pheromone blend. In fact, she said, without testing live females, the team would not have made their discovery.
For Groot, these findings will lead to further study, from North Carolina to the west coast of Mexico, on how geographic variation affects the pheromone blends of the two species of moths.
“Our approach could be used with other moth species to determine the generality of this evolutionary pathway,” she said. “I'm very excited about the possibilities.”-Suzanne Stanard, 919-513-3126 or suzanne_stanard@ncsu.edu-
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