Sex-pheromone link to insect evolution

Cornell University entomologists have unlocked an evolutionary secret to how insects evolve into new species. The discovery has major implications for the control of insect populations through disruption of mating, suggesting that over time current eradication methods could become ineffective, similar to the way insects develop pesticide resistance.

The researchers, led by Wendell L. Roelofs, the Liberty Hyde Bailey Professor of Insect Biochemistry at Cornell, made the discovery while examining ways to keep European corn borers from mating, multiplying and then chewing up farmers’ fields. They discovered the existence of a previously undetected gene, the delta-14, that can regulate the attractant chemicals produced in sex-pheromone glands of female borers. The gene can be suddenly switched on, changing the pheromone components that females use to attract males for mating.

The entomologists have demonstrated that insects evolve chemical systems in leaps rather than in minute stages, as had been previously assumed. The researchers also discovered that there are rare males in the corn borer population — about 1 in 200 — capable of responding to chemicals produced by the delta-14 gene.

“This is one way that insects become new species,” says Roelofs, whose paper, “Evolution of moth sex pheromones via ancestral genes,” will be published on the web site of the Proceedings of the National Academy of Science (Sept. 9-15, 2002.) The Cornell co-authors on the paper are: Weitian Liu, research associate in entomology; Guixia Hao, postdoctoral researcher in entomology; Hongmei Jiao, laboratory technician in entomology; and Charles E. Linn Jr., senior research associate in entomology. Alejandro P. Rooney, Mississippi State University assistant professor in biological sciences also contributed to the paper. The research was funded by the National Science Foundation and will continue to be funded by the U.S. Department of Agriculture’s National Research Initiative.Roelofs explains that female insects attract males with specialized pheromones that he compares to radio frequencies. At major events with thousands of people, for instance, police might communicate on channel one, emergency medical personnel on channel two and administrators on channel three.

“With male and female borers, it’s the same thing,” says Roelofs. “Certain species communicate on channel one, others on channel two, others on channel three. But when a female has a mutated delta-14 gene — and by mutated I mean the gene is turned on — it changes her channel from three to five. That means that out of 200 male borers, 199 cannot respond to her. It’s the one male borer capable of responding to her very selective channel that sets out to mate.”

Soon other females with the delta-14 gene mate with other rare respondent males. Eventually, over time, the males and females stabilize their pheromone communication system, essentially isolating this new population from the parent species. “That’s one way species evolve,” Roelofs says.

Manipulation of insect chemistry is an effective pest control strategy in that it can be used to disrupt mating behavior. For more than 20 years, Roelofs’ research at Cornell’s New York State Agricultural Experiment Station in Geneva has focused on chemical analyses of the pheromone components. Agricultural researchers have identified pheromones in over 1,000 species of insects and use them to monitor pest populations in 250 species and to disrupt mating in more than 20 species, Roelofs says.

This new research has implications for pest control. In addition to explaining how pheromone evolution might have occurred in the past, the paper also demonstrates that the conditions required for dramatic shifts in pheromone blends could well be present today and in the future. Insect populations could be capable of shifting away from a pheromone blend being used for their control in the field, making such control ineffective.

“Based on the difficulty of generating even small changes in pheromone blends in the lab, we thought that such resistance could not develop because natural pressure would prevent the species from gradually shifting to a different blend,” says Roelofs. The presence of this kind of gene, capable of sudden activation, might provide a mechanism for resistance to occur, although no evidence for this has been found so far, he notes.

Roelofs expects this discovery to stimulate more research in this area, specifically to determine the breadth of the phenomenon and how it affects the evolution of many insect communication systems. His research team will be working on the genomes of fruit flies, mosquitoes, crickets and silkworms to detect if these kinds of genes are present.