When a male wasp decides it's time to settle down and start a family, he releases a chemical calling card in the form of pheromones, broadcasting his location, his availability, and, most importantly, his identity. Most other kinds of insects will either ignore his signal or be repelled by it, but female wasps of his own species will buzz over and get down to business. But how and why did different pheromone blends—and the species that prefer them—evolve in the first place? A new study offers a possible solution to this long-standing evolutionary mystery, suggesting that new sex pheromones may evolve through genetic mutation before potential mates develop the ability to detect them.
Scientists have long been impressed by the perfect harmony of chemical communication among insects, especially when it comes to choosing mates by detecting and responding to the sex pheromones of only their own species. But scientists were puzzled by how such a delicate system evolved. If female wasps respond to only a specific blend of pheromones, males that produce even a subtly different blend shouldn't have much luck mating and passing on their mutant genes. It seemed that in order for males to evolve new pheromones, the female insects would need some preexisting adaptation that would cause them to prefer the new chemical blend. But how could they evolve a preference for something they had never encountered and should, logic suggests, find off-putting? In essence, the question is which came first, a new species or its sex pheromone?
In order to answer this question, a team of researchers in Germany turned to the Nasonia vitripennis wasp, a species famous for its propensity to lay its parasitic eggs on doomed fly pupae. When the scientists analyzed the N. vitripennis male sex pheromone, they found it contained two important chemicals, which they call RS and RR. RS also turns up in the male sex pheromones of another species of wasp, N. giraulti, whereas RR appears to be unique.
N. vitripennis  females preferred a blend of RS and RR, but a whiff of RS alone was enough to get them in the mood, the researchers report online today in Nature. RR on its own, however, did nothing for them, suggesting that it evolved later. N. giraulti females, meanwhile, did not discriminate between RS and the RS-RR blend. This suggests that the ability to produce the RS-RR mixture may have started off as a neutral genetic mutation among an earlier lineage of N. vitripennis males. As long as they were still emitting RS, they could continue to attract mates even as they also began to produce RR.
The new study shows that the responding female wasps "seem to be forgiving to some extent," says Christer Löfstedt, a chemical ecologist at Lund University in Sweden who was not involved in the research. "They are not turned off by the new compound that evolves," which gives it a chance to spread through the population of males without being selected against.
But not being selected against isn't the same thing as being selected for, other experts caution. If N. vitripennis females ultimately come to prefer the RS-RR pheromone blend, they must eventually start associating it with males of their own species and evolve a way to recognize it. "What are the forces that cause [the females] to incorporate this new compound into the communication system?" asks Ring Carde, an entomologist from the University of California, Riverside, who was not involved in the study. The researchers "sort of leave that hanging."