A strain of the bacterium Wolbachia reduces aggression in male fruit flies. The discovery marks the first time bacterial infection has been shown to alter aggression. The finding could bolster the quest to wipe out insect-borne diseases such as malaria and dengue fever.
The Wolbachia genus of bacteria infects half the insect species on Earth, as well as many other invertebrates. Different strains have highly varied effects, with some carrying our hopes to control mosquito-born diseases.
Depending on the strain, Wolbachia may either interfere with dengue transmission or kill mosquitoes old enough to carry diseases, while leaving young ones unharmed to slow the development of resistance.
Numerous research labs have been working on the idea, seeking a strain of Wolbachia that has the required characteristics to wipe out these diseases and save millions of lives. Much of the research is done, not on mosquitoes, but on Drosophila melanogaster – the fruit fly used as an insect model organism.
In the course of lab work at Griffith University, Elizabeth Bondy, an undergraduate on exchange from the University of Arizona, noticed something odd. Fruit flies infected with one strain of Wolbachia seemed to fight less than those infected with other strains, or not infected at all.
Bondy and Ph.D. student Chelsie Rohrscheib filmed the flies' behavior to verify Bondy's impression, leading to a paper in Applied and Environmental Microbiology.
The pair found that flies infected with the wMelPop strain initiated only a third as many fights as uninfected flies, while the behavior of those infected with one of the two other strains did not change. However, once the wMelPop flies did battle, they fought as long as their peers. Dr. Jeremy Brownlie, head of the lab where the research was done, says this demonstrates that the wMelPop flies were not weakened – they could still fight if required, they just seldom felt like it.
The wMelPop strain was found to suppress two genes that lead to the production of octopamine, which Brownlie says, “Suggested that Wolbachia directly affects fruit fly gene function.”
Rohrscheib told IFLScience that it is not clear if the Wolbachia strain gains some benefit from turning its hosts into pacifists. “I'm currently studying mating success for infected males when competing for females,” she said.
This matters, Rohrscheib says, because the challenge is to find a strain of Wolbachia that both controls disease and spreads through the mosquito population. “With the Queensland dengue trials, one strain spread very successfully, did exactly what they wanted, the other that had the same viral protection effect could not get it to take hold in the population,” she said.
The spread of the bacteria is driven by cytoplasmic incompatibility, which stops infected males and uninfected females from breeding successfully. An understanding of how strains change male mating behavior might be the key to explaining why some strains spread in the wild and others don't.
Vertebrates are immune to Wolbachia, so you won't create world peace by infecting us all with an aggression-reducing version. However, Rohrscheib notes that there are parallels with Toxoplasma gondii, the parasite that makes people more rash and may reduce intelligence.