The irresistible fragrance of dying vinegar flies
Bacterial pathogens cause infected flies to produce more sex pheromones and so expand their deadly reach
Vinegar flies should normally try to avoid their sick conspecifics to prevent becoming infected themselves. Nevertheless, as researchers from the Max Planck Institute for Chemical Ecology and Cornell University recently found out, they are irresistibly attracted to the smell given off by sick flies. A dramatic increase in the production of the sex pheromones responsible for the attractive odour of the infected flies is caused by pathogens: this perfidious strategy is used by the deadly germs to enable them to infect healthy flies and spread even further.
Markus Knaden and Bill Hansson, and their colleagues at the Department of Evolutionary Neuroethology, study ecologically relevant odours in the natural environment of insects, especially vinegar flies. In this new study they focused on a deadly smell: the odour of conspecifics which have a lethal bacterial infection.
“We had originally hoped to find a dedicated neuronal circuit in the flies which is specialized to detect and avoid sickness odors. Instead we observed that healthy flies were especially attracted to the smell of infected ones. When we realized that flies cannot avoid becoming infected, as sick flies produce particularly high amounts of pheromones, we were surprised but found that even more interesting,” says Markus Knaden, one of the leaders of the study.
State-of-the-art analytical methods enabled the researchers to identify and quantify the odours of single flies. Vinegar flies which suffered from bacterial infection and their faeces emitted dramatically increased amounts of the typical odours that attract other flies. The hypothesis that last-minute pheromone emission by sick insects would enhance their reproductive success turned out to be wrong, as mating assays demonstrated that sick flies were barely able to copulate.
Insect immunologist Nicolas Buchon from Cornell University and his team, who were also involved in the study, noticed that the increase in pheromone production matched the up-regulation of certain immune responses in the flies. Ian Keesey, the first author of the study, and his colleagues in Jena therefore tested mutant flies which lacked the ability to produce these responses and found that these flies emitted far fewer pheromones when they became infected in comparison to sick wild-type flies. Further analysis of the insects’ metabolism convinced the researchers that ongoing bacterial growth and the subsequent damages caused by the pathogens are necessary to induce increases in pheromone production.
The scientists observed similar results when they conducted experiments with other fly species. Seven other Drosophila species as well as the yellow fever mosquito Aedes aegyptii conspecifics dramatically changed their olfactory profile after infection with the pathogen. Manipulation of social communication in insects by pathogenic bacteria seems to be a more general phenomenon in nature than thought.
Markus Knaden hopes that the new insights can one day contribute to useful applications: “A well-established method to combat insect-transmitted diseases and to control agricultural pest insects is the use of pheromone traps. By infecting insects with bacteria we could generally increase their pheromone emission. This could enable us to identify novel pheromones in species that have not been investigated so far.”